Magnetochemistry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Magnetochemistry

 

Magnetochemistry is concerned with the magnetic properties of chemical compounds. Magnetic properties arise from the spin and orbital angular momentum of the electrons contained in a compound. Compounds are diamagnetic when they contain no unpaired electrons. Molecular compounds that contain one or more unpaired electrons are paramagnetic. The magnitude of the paramagnetism is expressed as an effective magnetic moment, μ_eff. For first-row transition metals the magnitude of μ_eff is, to a first approximation, a simple function of the number of unpaired electrons, the spin-only formula. In general, spin-orbit coupling causes μ_eff to deviate from the spin-only formula. For the heavier transition metals, lanthanides and actinides, spin-orbit coupling cannot be ignored. Exchange interaction can occur in clusters and infinite lattices, resulting in ferromagnetism, antiferromagnetism or ferrimagnetism depending on the relative orientations of the individual spins.

Magnetic susceptibility

The primary measurement in magnetochemistry is magnetic susceptibility. This measures the strength of interaction on placing the substance in a magnetic field. The volume magnetic susceptibility, represented by the symbol ${\displaystyle \chi _{v}}$ is defined by the relationship

${\displaystyle {\vec {M}}=\chi _{v}{\vec {H}}}$

where, ${\displaystyle {\vec {M}}}$ is the magnetization of the material (the magnetic dipole moment per unit volume), measured in amperes per meter ( SI units), and ${\displaystyle {\vec {H}}}$ is the magnetic field strength, also measured in amperes per meter. Susceptibility is a dimensionless quantity. For chemical applications the molar magnetic susceptibility (χ_mol) is the preferred quantity. It is measured in m³·mol⁻¹ (SI) or cm³·mol⁻¹ (CGS) and is defined as

${\displaystyle \chi _{\text{mol}}=M\chi _{v}/\rho }$

where ρ is the density in kg·m⁻³ (SI) or g·cm⁻³ (CGS) and M is molar mass in kg·mol⁻¹ (SI) or g·mol⁻¹ (CGS). 

Schematic diagram of Gouy balance

 

A variety of methods are available for the measurement of magnetic susceptibility.

• With the Gouy balance the weight change of the sample is measured with an analytical balance when the sample is placed in a homogeneous magnetic field. The measurements are calibrated against a known standard, such as mercury cobalt thiocyanate, HgCo(NCS)₄. Calibration removes the need to know the density of the sample. Variable temperature measurements can be made by placing the sample in a cryostat between the pole pieces of the magnet.
• The Evans balance is a torsion balance which uses a sample in a fixed position and a variable secondary magnet to bring the magnets back to their initial position. It, too, is calibrated against HgCo(NCS)₄.
• With a Faraday balance the sample is placed in a magnetic field of constant gradient, and weighed on a torsion balance. This method can yield information on magnetic anisotropy.
• SQUID is a very sensitive magnetometer.
• For substances in solution NMR may be used to measure susceptibility.
  

Types of magnetic behaviour

When an isolated atom is placed in a magnetic field there is an interaction because each electron in the atom behaves like a magnet, that is, the electron has a magnetic moment. There are two types of interaction.

1. Diamagnetism. When placed in a magnetic field the atom becomes magnetically polarized, that is, it develops an induced magnetic moment. The force of the interaction tends to push the atom out of the magnetic field. By convention diamagnetic susceptibility is given a negative sign. Very frequently diamagnetic atoms have no unpaired electrons ie each electron is paired with another electron in the same atomic orbital. The moments of the two electrons cancel each other out, so the atom has no net magnetic moment. However, for the ion Eu³⁺ which has six unpaired electrons, the orbital angular momentum cancels out the electron angular momentum, and this ion is diamagnetic at zero Kelvin.
2. Paramagnetism. At least one electron is not paired with another. The atom has a permanent magnetic moment. When placed into a magnetic field, the atom is attracted into the field. By convention paramagnetic susceptibility is given a positive sign.

When the atom is present in a chemical compound its magnetic behaviour is modified by its chemical environment. Measurement of the magnetic moment can give useful chemical information.

In certain crystalline materials individual magnetic moments may be aligned with each other (magnetic moment has both magnitude and direction). This gives rise to ferromagnetism, antiferromagnetism or ferrimagnetism. These are properties of the crystal as a whole, of little bearing on chemical properties.

Diamagnetism

Diamagnetism is a universal property of chemical compounds, because all chemical compounds contain electron pairs. A compound in which there are no unpaired electrons is said to be diamagnetic. The effect is weak because it depends on the magnitude of the induced magnetic moment. It depends on the number of electron pairs and the chemical nature of the atoms to which they belong. This means that the effects are additive, and a table of "diamagnetic contributions", or Pascal's constants, can be put together. With paramagnetic compounds the observed susceptibility can be adjusted by adding to it the so-called diamagnetic correction, which is the diamagnetic susceptibility calculated with the values from the table.

Paramagnetism

Mechanism and temperature dependence

Variation of magnetic susceptibility with temperature

 

A metal ion with a single unpaired electron, such as Cu²⁺, in a coordination complex provides the simplest illustration of the mechanism of paramagnetism. The individual metal ions are kept far apart by the ligands, so that there is no magnetic interaction between them. The system is said to be magnetically dilute. The magnetic dipoles of the atoms point in random directions. When a magnetic field is applied, first-order Zeeman splitting occurs. Atoms with spins aligned to the field slightly outnumber the atoms with non-aligned spins. In the first-order Zeeman effect the energy difference between the two states is proportional to the applied field strength. Denoting the energy difference as ΔE, the Boltzmann distribution gives the ratio of the two populations as ${\displaystyle e^{-\Delta E/kT}}$, where k is the Boltzmann constant and T is the temperature in kelvins. In most cases ΔE is much smaller than kT and the exponential can be expanded as 1 – ΔE/kT. It follows from the presence of 1/T in this expression that the susceptibility is inversely proportional to temperature.

${\displaystyle \chi ={C \over T}}$

This is known as the Curie law and the proportionality constant, C, is known as the Curie constant, whose value, for molar susceptibility, is calculated as

${\displaystyle C={\frac {Ng^{2}S(S+1)\mu _{B}^{2}}{3k}}}$

where N is the Avogadro constant, g is the Landé g-factor, and μ_B is the Bohr magneton. In this treatment it has been assumed that the electronic ground state is not degenerate, that the magnetic susceptibility is due only to electron spin and that only the ground state is thermally populated.

While some substances obey the Curie law, others obey the Curie-Weiss law.

${\displaystyle \chi ={\frac {C}{T-T_{c}}}}$

T_c is the Curie temperature. The Curie-Weiss law will apply only when the temperature is well above the Curie temperature. At temperatures below the Curie temperature the substance may become ferromagnetic. More complicated behaviour is observed with the heavier transition elements. 

Effective magnetic moment

When the Curie law is obeyed, the product of molar susceptibility and temperature is a constant. The effective magnetic moment, μ_eff is then defined as

${\displaystyle \mu _{\text{eff}}=\mathrm {constant} {\sqrt {T\chi }}}$

Where C has CGS units cm³ mol⁻¹ K, μ_eff is

${\displaystyle \mu _{\text{eff}}={\sqrt {3k \over N\mu _{B}^{2}}}{\sqrt {T\chi }}\approx 2.82787{\sqrt {T\chi }}}$

Where C has SI units m³ mol⁻¹ K, μ_eff is

${\displaystyle \mu _{\text{eff}}={\sqrt {3k \over N\mu _{0}\mu _{B}^{2}}}{\sqrt {T\chi }}\approx 797.727{\sqrt {T\chi }}}$

The quantity μ_eff is effectively dimensionless, but is often stated as in units of Bohr magneton (μ_B).

For substances that obey the Curie law, the effective magnetic moment is independent of temperature. For other substances μ_eff is temperature dependent, but the dependence is small if the Curie-Weiss law holds and the Curie temperature is low. 

Temperature independent paramagnetism

Compounds which are expected to be diamagnetic may exhibit this kind of weak paramagnetism. It arises from a second-order Zeeman effect in which additional splitting, proportional to the square of the field strength, occurs. It is difficult to observe as the compound inevitably also interacts with the magnetic field in the diamagnetic sense. Nevertheless, data are available for the permanganate ion. It is easier to observe in compounds of the heavier elements, such as uranyl compounds. 

Exchange interactions

Copper(II) acetate dihydrate

 

Ferrimagnetic ordering in 2 dimensions

 

Antiferromagnetic ordering in 2 dimensions

 

Exchange interactions occur when the substance is not magnetically dilute and there are interactions between individual magnetic centres. One of the simplest systems to exhibit the result of exchange interactions is crystalline copper(II) acetate, Cu₂(OAc)₄(H₂O)₂. As the formula indicates, it contains two copper(II) ions. The Cu²⁺ ions are held together by four acetate ligands, each of which binds to both copper ions. Each Cu²⁺ ion has a d⁹ electronic configuration, and so should have one unpaired electron. If there were a covalent bond between the copper ions, the electrons would pair up and the compound would be diamagnetic. Instead, there is an exchange interaction in which the spins of the unpaired electrons become partially aligned to each other. In fact two states are created, one with spins parallel and the other with spins opposed. The energy difference between the two states is so small their populations vary significantly with temperature. In consequence the magnetic moment varies with temperature in a sigmoidal pattern. The state with spins opposed has lower energy, so the interaction can be classed as antiferromagnetic in this case. It is believed that this is an example of superexchange, mediated by the oxygen and carbon atoms of the acetate ligands. Other dimers and clusters exhibit exchange behaviour.

Exchange interactions can act over infinite chains in one dimension, planes in two dimensions or over a whole crystal in three dimensions. These are examples of long-range magnetic ordering. They give rise to ferromagnetism, antiferromagnetism or ferrimagnetism, depending on the nature and relative orientations of the individual spins.

Compounds at temperatures below the Curie temperature exhibit long-range magnetic order in the form of ferromagnetism. Another critical temperature is the Néel temperature, below which antiferromagnetism occurs. The hexahydrate of nickel chloride, NiCl₂·6H₂O, has a Néel temperature of 8.3 K. The susceptibility is a maximum at this temperature. Below the Néel temperature the susceptibility decreases and the substance becomes antiferromagnetic.

Complexes of transition metal ions

The effective magnetic moment for a compound containing a transition metal ion with one or more unpaired electrons depends on the total orbital and spin angular momentum of the unpaired electrons, ${\displaystyle {\vec {L}}}$ and ${\displaystyle {\vec {S}}}$, respectively. "Total" in this context means "vector sum". In the approximation that the electronic states of the metal ions are determined by Russell-Saunders coupling and that spin-orbit coupling is negligible, the magnetic moment is given by

${\displaystyle \mu _{\text{eff}}={\sqrt {{\vec {L}}({\vec {L}}+1)+4{\vec {S}}({\vec {S}}+1)}}\mu _{B}}$

Spin-only formula

Orbital angular momentum is generated when an electron in an orbital of a degenerate set of orbitals is moved to another orbital in the set by rotation. In complexes of low symmetry certain rotations are not possible. In that case the orbital angular momentum is said to be "quenched" and ${\displaystyle {\vec {L}}}$ is smaller than might be expected (partial quenching), or zero (complete quenching). There is complete quenching in the following cases. Note that an electron in a degenerate pair of d_x²–y² or d_z² orbitals cannot rotate into the other orbital because of symmetry.

Quenched orbital angular momentum

dn	Octahedral			Tetrahedral
		high-spin	low-spin
d1				e1
d2				e2
d3	t2g3
d4		t2g3eg1
d5		t2g3eg2
d6			t2g6	e3t23
d7			t2g6eg1	e4t23
d8	t2g6eg2
d9	t2g6eg3

legend: t_2g, t₂ = (d_xy, d_xz, d_yz). e_g, e = (d_x²–y², d_z²).

When orbital angular momentum is completely quenched, ${\displaystyle {\vec {L}}=0}$ and the paramagnetism can be attributed to electron spin alone. The total spin angular momentum is simply half the number of unpaired electrons and the spin-only formula results.

${\displaystyle \mu _{\text{eff}}={\sqrt {n(n+2)}}\mu _{B}}$

where n is the number of unpaired electrons. The spin-only formula is a good first approximation for high-spin complexes of first-row transition metals.

Ion	Number of unpaired electrons	Spin-only moment /μB	observed moment /μB
Ti3+	1	1.73	1.73
V4+	1		1.68–1.78
Cu2+	1		1.70–2.20
V3+	2	2.83	2.75–2.85
Ni2+	2		2.8–3.5
V2+	3	3.87	3.80–3.90
Cr3+	3		3.70–3.90
Co2+	3		4.3–5.0
Mn4+	3		3.80–4.0
Cr2+	4	4.90	4.75–4.90
Fe2+	4		5.1–5.7
Mn2+	5	5.92	5.65–6.10
Fe3+	5		5.7–6.0

The small deviations from the spin-only formula may result from the neglect of orbital angular momentum or of spin-orbit coupling. For example, tetrahedral d³, d⁴, d⁸ and d⁹ complexes tend to show larger deviations from the spin-only formula than octahedral complexes of the same ion, because "quenching" of the orbital contribution is less effective in the tetrahedral case.

Low-spin complexes

Crystal field diagram for octahedral low-spin d⁵

 

Crystal field diagram for octahedral high-spin d⁵

 

According to crystal field theory, the d orbitals of a transition metal ion in an octahedal complex are split into two groups in a crystal field. If the splitting is large enough to overcome the energy needed to place electrons in the same orbital, with opposite spin, a low-spin complex will result.

High and low -spin octahedral complexes

d-count	Number of unpaired electrons		examples
	high-spin	low-spin
d4	4	2	Cr2+, Mn3+
d5	5	1	Mn2+, Fe3+
d6	4	0	Fe2+, Co3+
d7	3	1	Co2+

With one unpaired electron μ_eff values range from 1.8 to 2.5 μ_B and with two unpaired electrons the range is 3.18 to 3.3 μ_B. Note that low-spin complexes of Fe²⁺ and Co³⁺ are diamagnetic. Another group of complexes that are diamagnetic are square-planar complexes of d⁸ ions such as Ni²⁺ and Rh⁺ and Au³⁺.

Spin cross-over

When the energy difference between the high-spin and low-spin states is comparable to kT (k is the Boltzmann constant and T the temperature) an equilibrium is established between the spin states, involving what have been called "electronic isomers". Tris-dithiocarbamato iron(III), Fe(S₂CNR₂)₃, is a well-documented example. The effective moment varies from a typical d⁵ low-spin value of 2.25 μ_B at 80 K to more than 4 μ_B above 300 K.

2nd and 3rd row transition metals

Crystal field splitting is larger for complexes of the heavier transition metals than for the transition metals discussed above. A consequence of this is that low-spin complexes are much more common. Spin-orbit coupling constants, ζ, are also larger and cannot be ignored, even in elementary treatments. The magnetic behaviour has been summarized, as below, together with an extensive table of data.

d-count	kT/ζ=0.1 μeff	kT/ζ=0 μeff	Behaviour with large spin-orbit coupling constant, ζnd
d1	0.63	0	μeff varies with T1/2
d2	1.55	1.22	μeff varies with T, approximately
d3	3.88	3.88	Independent of temperature
d4	2.64	0	μeff varies with T1/2
d5	1.95	1.73	μeff varies with T, approximately

Lanthanides and actinides

Russell-Saunders coupling, LS coupling, applies to the lanthanide ions, crystal field effects can be ignored, but spin-orbit coupling is not negligible. Consequently, spin and orbital angular momenta have to be combined

${\displaystyle {\vec {L}}=\sum _{i}{\vec {l}}_{i}}$

${\displaystyle {\vec {S}}=\sum _{i}{\vec {s}}_{i}}$

${\displaystyle {\vec {J}}={\vec {L}}+{\vec {S}}}$

and the calculated magnetic moment is given by

${\displaystyle \mu _{\text{eff}}=g{\sqrt {{\vec {J}}({\vec {J}}+1)}};g={3 \over 2}+{\frac {{\vec {S}}({\vec {S}}+1)-{\vec {L}}({\vec {L}}+1)}{2{\vec {J}}({\vec {J}}+1)}}}$

Magnetic properties of trivalent lanthanide compounds

lanthanide	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Number of unpaired électrons	1	2	3	4	5	6	7	6	5	4	3	2	1	0
calculated moment /μB	2.54	3.58	3.62	2.68	0.85	0	7.94	9.72	10.65	10.6	9.58	7.56	4.54	0
observed moment /μB	2.3–2.5	3.4–3.6	3.5–3.6		1.4–1.7	3.3–3.5	7.9–8.0	9.5–9.8	10.4–10.6	10.4–10.7	9.4–9.6	7.1–7.5	4.3–4.9	0

In actinides spin-orbit coupling is strong and the coupling approximates to j j coupling.

${\displaystyle {\vec {J}}=\sum _{i}{\vec {j}}_{i}=\sum _{i}({\vec {l}}_{i}+{\vec {s}}_{i})}$

This means that it is difficult to calculate the effective moment. For example, uranium(IV), f², in the complex [UCl₆]²⁻ has a measured effective moment of 2.2 μ_B, which includes a contribution from temperature-independent paramagnetism.

Main group elements and organic compounds

Simulated EPR spectrum of the CH₃• radical

 

MSTL spin-label

 

Very few compounds of main group elements are paramagnetic. Notable examples include: oxygen, O₂; nitric oxide, NO; nitrogen dioxide, NO₂ and chlorine dioxide, ClO₂. In organic chemistry, compounds with an unpaired electron are said to be free radicals. Free radicals, with some exceptions, are short-lived because one free radical will react rapidly with another, so their magnetic properties are difficult to study. However, if the radicals are well separated from each other in a dilute solution in a solid matrix, at low temperature, they can be studied by electron paramagnetic resonance (EPR). Such radicals are generated by irradiation. Extensive EPR studies have revealed much about electron delocalization in free radicals. The simulated spectrum of the CH₃• radical shows hyperfine splitting due to the interaction of the electron with the 3 equivalent hydrogen nuclei, each of which has a spin of 1/2.

Spin labels are long-lived free radicals which can be inserted into organic molecules so that they can be studied by EPR. For example, the nitroxide MTSL, a functionalized derivative of TEtra Methyl Piperidine Oxide, TEMPO, is used in site-directed spin labeling.

Applications

The gadolinium ion, Gd³⁺, has the f⁷ electronic configuration, with all spins parallel. Compounds of the Gd³⁺ ion are the most suitable for use as a contrast agent for MRI scans. The magnetic moments of gadolinium compounds are larger than those of any transition metal ion. Gadolinium is preferred to other lanthanide ions, some of which have larger effective moments, due to its having a non-degenerate electronic ground state.

For many years the nature of oxyhemoglobin, Hb-O₂, was highly controversial. It was found experimentally to be diamagnetic. Deoxy-hemoglobin is generally accepted to be a complex of iron in the +2 oxidation state, that is a d⁶ system with a high-spin magnetic moment near to the spin-only value of 4.9 μ_B. It was proposed that the iron is oxidized and the oxygen reduced to superoxide.

Fe(II)Hb (high-spin) + O₂ ⇌ [Fe(III)Hb]O₂⁻

Pairing up of electrons from Fe³⁺ and O₂⁻ was then proposed to occur via an exchange mechanism. It has now been shown that in fact the iron(II) changes from high-spin to low-spin when an oxygen molecule donates a pair of electrons to the iron. Whereas in deoxy-hemoglobin the iron atom lies above the plane of the heme, in the low-spin complex the effective ionic radius is reduced and the iron atom lies in the heme plane.

Fe(II)Hb + O₂ ⇌ [Fe(II)Hb]O₂ (low-spin)

This information has an important bearing on research to find artificial oxygen carriers. 

Compounds of gallium(II) were unknown until quite recently. As the atomic number of gallium is an odd number (31), Ga²⁺ should have an unpaired electron. It was assumed that it would act as a free radical and have a very short lifetime. The non-existence of Ga(II) compounds was part of the so-called inert pair effect. When salts of the anion with empirical formula such as [GaCl₃]⁻ were synthesized they were found to be diamagnetic. This implied the formation of a Ga-Ga bond and a dimeric formula, [Ga₂Cl₆]²⁻.

History of mobile phones

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/History_of_mobile_phones

 

A man talks on his mobile phone while standing near a conventional telephone box, which stands empty. Enabling technology for mobile phones was first developed in the 1940s but it was not until the mid 1980s that they became widely available. By 2011, it was estimated in Britain that more calls were made using mobile phones than wired devices.

 

The history of mobile phones covers mobile communication devices that connect wirelessly to the public switched telephone network.

While the transmission of speech by radio has a long history, the first devices that were wireless, mobile, and also capable of connecting to the standard telephone network are much more recent. The fit such devices were barely portable compared to today's compact hand-held devices, and their use was clumsy. 

Along with the process of developing a more portable technology, and a better interconnections system, drastic changes have taken place in both the networking of wireless communication and the prevalence of its use, with smartphones becoming common globally and a growing proportion of Internet access now done via mobile broadband. 

Foundations

Predecessors

In 1908, a Professor Albert Jahnke and the Oakland Transcontinental Aerial Telephone and Power Company claimed to have developed a wireless telephone. They were accused of fraud and the charge was then dropped, but they do not seem to have proceeded with production. Beginning in 1918, the German railroad system tested wireless telephony on military trains between Berlin and Zossen. In 1924, public trials started with telephone connection on trains between Berlin and Hamburg. In 1925, the company Zugtelephonie AG was founded to supply train telephony equipment and, in 1926, telephone service in trains of the Deutsche Reichsbahn and the German mail service on the route between Hamburg and Berlin was approved and offered to first-class travelers.

Karl Arnold drawing of public use of mobile telephones

 

Fiction anticipated the development of real world mobile telephones. In 1906, the English caricaturist Lewis Baumer published a cartoon in Punch magazine entitled "Forecasts for 1907" in which he showed a man and a woman in London's Hyde Park each separately engaged in gambling and dating on wireless telephony equipment. Then, in 1926, the artist Karl Arnold created a visionary cartoon about the use of mobile phones in the street, in the picture "wireless telephony", published in the German satirical magazine Simplicissimus.

The Second World War made military use of radio telephony links. Hand-held radio transceivers have been available since the 1940s. Mobile telephones for automobiles became available from some telephone companies in the 1940s. Early devices were bulky, consumed large amounts of power, and the network supported only a few simultaneous conversations. Modern cellular networks allow automatic and pervasive use of mobile phones for voice and data communications.

In the United States, engineers from Bell Labs began work on a system to allow mobile users to place and receive telephone calls from automobiles, leading to the inauguration of mobile service on June 17, 1946 in St. Louis, Missouri. Shortly after, AT&T offered Mobile Telephone Service. A wide range of mostly incompatible mobile telephone services offered limited coverage area and only a few available channels in urban areas. As calls were transmitted as unencrypted analog signals, they could be eavesdropped on by anyone with radio equipment that could receive those frequencies. The introduction of cellular technology, which allowed re-use of frequencies many times in small adjacent areas covered by relatively low-powered transmitters, made widespread adoption of mobile telephones economically feasible. 

In the USSR, Leonid Kupriyanovich, an engineer from Moscow, in 1957-1961 developed and presented a number of experimental pocket-sized communications radio. The weight of one model, presented in 1961, was only 70 g and could fit on a palm. However, in the USSR the decision at first to develop the system of the automobile "Altai" phone was made.

In 1965, the Bulgarian company "Radioelektronika" presented a mobile automatic phone combined with a base station at the Inforga-65 international exhibition in Moscow. Solutions of this phone were based on a system developed by Leonid Kupriyanovich. One base station, connected to one telephone wire line, could serve up to 15 customers.

The advances in mobile telephony can be traced in successive generations from the early "0G" services like MTS and its successor Improved Mobile Telephone Service, to first-generation (1G) analog cellular network, second-generation (2G) digital cellular networks, third-generation (3G) broadband data services to the state-of-the-art, fourth-generation (4G) native-IP networks. 

Underlying technology

The development of metal-oxide-semiconductor (MOS) large-scale integration (LSI) technology, information theory and cellular networking led to the development of affordable mobile communications. There was a rapid growth of wireless telecommunications towards the end of the 20th century, primarily due to the introduction of digital signal processing in wireless communications, driven by the development of low-cost, very large-scale integration (VLSI) RF CMOS (radio-frequency complementary MOS) technology.

The development of cell phone technology was enabled by advances in MOSFET (metal-oxide-silicon field-effect transistor) semiconductor device fabrication. The MOSFET (MOS transistor), invented by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959, is the basic building block of modern cell phones. MOSFET scaling, where MOS transistors get smaller with decreasing power consumption, enabled very large-scale integration (VLSI) technology, with MOS transistor counts in integrated circuit chips increasing at an exponential pace, as predicted by Moore's law. Continuous MOSFET scaling eventually made it possible to build portable cell phones. A typical modern smartphone is built from billions of tiny MOSFETs as of 2019, used in integrated circuits such as microprocessors and memory chips, as power devices, and as thin-film transistors (TFTs) in mobile displays.

Advances in MOSFET power electronic technology also enabled the development of digital wireless mobile networks, which are essential to modern cell phones. The wide adoption of power MOSFET, LDMOS (lateral diffused MOS) and RF CMOS (radio frequency CMOS) devices led to the development and proliferation of digital wireless mobile networks by the 1990s, with further advances in MOSFET technology leading to increasing bandwidth during the 2000s. Most of the essential elements of wireless mobile networks are built from MOSFETs, including the mobile transceivers, base station modules, routers, RF power amplifiers, telecommunication circuits, RF circuits, and radio transceivers, in networks such as 2G, 3G, and 4G.

Another important enabling factor was the lithium-ion battery, which became indispensable as an energy source for cell phones. The lithium-ion battery was invented by John Goodenough, Rachid Yazami and Akira Yoshino in the 1980s, and commercialized by Sony and Asahi Kasei in 1991.

Early services

MTS

In 1949, AT&T commercialized Mobile Telephone Service. From its start in St. Louis, Missouri, in 1946, AT&T introduced Mobile Telephone Service to one hundred towns and highway corridors by 1948. Mobile Telephone Service was a rarity with only 5,000 customers placing about 30,000 calls each week. Calls were set up manually by an operator and the user had to depress a button on the handset to talk and release the button to listen. The call subscriber equipment weighed about 80 pounds (36 kg).

Subscriber growth and revenue generation were hampered by the constraints of the technology. Because only three radio channels were available, only three customers in any given city could make mobile telephone calls at one time. Mobile Telephone Service was expensive, costing US$15 per month, plus $0.30–0.40 per local call, equivalent to (in 2012 US dollars) about $176 per month and $3.50–4.75 per call.

In the UK, there was also a vehicle-based system called "Post Office Radiophone Service," which was launched around the city of Manchester in 1959, and although it required callers to speak to an operator, it was possible to be put through to any subscriber in Great Britain. The service was extended to London in 1965 and other major cities in 1972. 

IMTS

AT&T introduced the first major improvement to mobile telephony in 1965, giving the improved service the obvious name of Improved Mobile Telephone Service. IMTS used additional radio channels, allowing more simultaneous calls in a given geographic area, introduced customer dialing, eliminating manual call setup by an operator, and reduced the size and weight of the subscriber equipment.

Despite the capacity improvement offered by IMTS, demand outstripped capacity. In agreement with state regulatory agencies, AT&T limited the service to just 40,000 customers system wide. In New York City, for example, 2,000 customers shared just 12 radio channels and typically had to wait 30 minutes to place a call.

Radio Common Carrier

A mobile radio telephone

 

Radio Common Carrier or RCC was a service introduced in the 1960s by independent telephone companies to compete against AT&T's IMTS. RCC systems used paired UHF 454/459 MHz and VHF 152/158 MHz frequencies near those used by IMTS. RCC based services were provided until the 1980s when cellular AMPS systems made RCC equipment obsolete.

Some RCC systems were designed to allow customers of adjacent carriers to use their facilities, but equipment used by RCCs did not allow the equivalent of modern "roaming" because technical standards were not uniform. For example, the phone of an Omaha, Nebraska–based RCC service would not be likely to work in Phoenix, Arizona. Roaming was not encouraged, in part, because there was no centralized industry billing database for RCCs. Signaling formats were not standardized. For example, some systems used two-tone sequential paging to alert a mobile of an incoming call. Other systems used DTMF. Some used Secode 2805, which transmitted an interrupted 2805 Hz tone (similar to IMTS signaling) to alert mobiles of an offered call. Some radio equipment used with RCC systems was half-duplex, push-to-talk LOMO equipment such as Motorola hand-helds or RCA 700-series conventional two-way radios. Other vehicular equipment had telephone handsets and rotary dials or pushbutton pads, and operated full duplex like a conventional wired telephone. A few users had full-duplex briefcase telephones (radically advanced for their day).

At the end of RCC's existence, industry associations were working on a technical standard that would have allowed roaming, and some mobile users had multiple decoders to enable operation with more than one of the common signaling formats (600/1500, 2805, and Reach). Manual operation was often a fallback for RCC roamers. 

Other services

In 1969 Penn Central Railroad equipped commuter trains along the 360 kilometres (220 mi) New York-Washington route with special pay phones that allowed passengers to place telephone calls while the train was moving. The system re-used six frequencies in the 450 MHz band in nine sites.

In the UK, Channel Islands and elsewhere the "Rabbit" phone system was briefly used, being a hybrid of "cell" base stations and handsets. One major limitation was that you had to be less than 300 feet (closer with buildings) from a base due to power limitations on a portable device.  With modern technology a similar variant is being considered for Apple's new 4G "smart watch" so they can be used in large events in a broadly similar way to a femtocell.

European mobile radio networks

In Europe, several mutually incompatible mobile radio services were developed.

In 1966 Norway had a system called OLT which was manually controlled. Finland's ARP, launched in 1971, was also manual as was the Swedish MTD. All were replaced by the automatic NMT, (Nordic Mobile Telephone) system in the early 1980s. 

In July 1971 Readycall was introduced in London by Burndept after obtaining a special concession to break the Post Office monopoly to allow selective calling to mobiles of calls from the public telephone system. This system was available to the public for a subscription of £16 month. A year later the service was extended to two other UK towns.

West Germany had a network called A-Netz launched in 1952 as the country's first public commercial mobile phone network. In 1972 this was displaced by B-Netz which connected calls automatically. 

The cellular concept

A multi-directional, cellular network antenna array ("cell tower")

 

In December 1947, Douglas H. Ring and W. Rae Young, Bell Labs engineers, proposed hexagonal cells for mobile phones in vehicles. At this stage, the technology to implement these ideas did not exist, nor had the frequencies been allocated. Two decades would pass before Richard H. Frenkiel, Joel S. Engel and Philip T. Porter of Bell Labs expanded the early proposals into a much more detailed system plan. It was Porter who first proposed that the cell towers use the now-familiar directional antennas to reduce interference and increase channel reuse (see picture at right) Porter also invented the dial-then-send method used by all cell phones to reduce wasted channel time.

In all these early examples, a mobile phone had to stay within the coverage area serviced by one base station throughout the phone call, i.e. there was no continuity of service as the phones moved through several cell areas. The concepts of frequency reuse and handoff, as well as a number of other concepts that formed the basis of modern cell phone technology, were described in the late 1960s, in papers by Frenkiel and Porter. In 1970 Amos E. Joel, Jr., a Bell Labs engineer, invented a "three-sided trunk circuit" to aid in the "call handoff" process from one cell to another. His patent contained an early description of the Bell Labs cellular concept, but as switching systems became faster, such a circuit became unnecessary and was never implemented in a system. 

A cellular telephone switching plan was described by Fluhr and Nussbaum in 1973, and a cellular telephone data signaling system was described in 1977 by Hachenburg et al.

Emergence of automated services

The first fully automated mobile phone system for vehicles was launched in Sweden in 1956. Named MTA (Mobiltelefonisystem A), it allowed calls to be made and received in the car using a rotary dial. The car phone could also be paged. Calls from the car were direct dial, whereas incoming calls required an operator to locate the nearest base station to the car. It was developed by Sture Laurén and other engineers at Televerket network operator. Ericsson provided the switchboard while Svenska Radioaktiebolaget (SRA) and Marconi provided the telephones and base station equipment. MTA phones consisted of vacuum tubes and relays, and weighed 40 kilograms (88 lb). In 1962, an upgraded version called Mobile System B (MTB) was introduced. This was a push-button telephone, and used transistors and DTMF signaling to improve its operational reliability. In 1971 the MTD version was launched, opening for several different brands of equipment and gaining commercial success.^[34][35] The network remained open until 1983 and still had 600 customers when it closed. 

In 1958 development began on a similar system for motorists in the USSR. The "Altay" national civil mobile phone service was based on Soviet MRT-1327 standard. The main developers of the Altay system were the Voronezh Science Research Institute of Communications (VNIIS) and the State Specialized Project Institute (GSPI). In 1963 the service started in Moscow, and by 1970 was deployed in 30 cities across the USSR. Versions of the Altay system are still in use today as a trunking system in some parts of Russia. 

In 1959 a private telephone company in Brewster, Kansas, USA, the S&T Telephone Company, (still in business today) with the use of Motorola Radio Telephone equipment and a private tower facility, offered to the public mobile telephone services in that local area of NW Kansas. This system was a direct dial up service through their local switchboard, and was installed in many private vehicles including grain combines, trucks, and automobiles. For some as yet unknown reason, the system, after being placed online and operated for a very brief time period, was shut down. The management of the company was immediately changed, and the fully operable system and related equipment was immediately dismantled in early 1960, not to be seen again.

In 1966, Bulgaria presented the pocket mobile automatic phone RAT-0,5 combined with a base station RATZ-10 (RATC-10) on Interorgtechnika-66 international exhibition. One base station, connected to one telephone wire line, could serve up to six customers ("Radio" magazine, 2, 1967; "Novosti dnya" newsreel, 37, 1966).

One of the first successful public commercial mobile phone networks was the ARP network in Finland, launched in 1971. Posthumously, ARP is sometimes viewed as a zero generation (0G) cellular network, being slightly above previous proprietary and limited coverage networks.

Handheld mobile phone

Martin Cooper photographed in 2007 with his 1973 handheld mobile phone prototype

 

Prior to 1973, mobile telephony was limited to phones installed in cars and other vehicles. Motorola was the first company to produce a handheld mobile phone. On April 3, 1973, Martin Cooper, a Motorola researcher and executive, made the first mobile telephone call from handheld subscriber equipment, placing a call to Dr. Joel S. Engel of Bell Labs, his rival. The prototype handheld phone used by Dr. Cooper weighed 1.1 kilograms (2.4 lb) and measured 23 by 13 by 4.5 centimetres (9.1 by 5.1 by 1.8 in). The prototype offered a talk time of just 30 minutes and took 10 hours to re-charge.

John F. Mitchell, Motorola's chief of portable communication products and Cooper's boss in 1973, played a key role in advancing the development of handheld mobile telephone equipment. Mitchell failed to push Motorola into developing wireless communication products that would be small enough to use anywhere and participated in the design of the cellular phone.

The early generations

Newer technology has been developed and rolled out in a series of waves or generations. The "generation" terminology only became widely used when 3G was launched, but is now used retroactively when referring to the earlier systems. 

1G – Analog cellular

First automatic analog cellular systems deployed were NTT's system first used in Tokyo in 1979, later spreading to the whole of Japan, and NMT in the Nordic countries in 1981.

The first analog cellular system widely deployed in North America was the Advanced Mobile Phone System (AMPS). It was commercially introduced in the Americas in 13 October 1983, Israel in 1986, and Australia in 1987. AMPS was a pioneering technology that helped drive mass market usage of cellular technology, but it had several serious issues by modern standards. It was unencrypted and easily vulnerable to eavesdropping via a scanner; it was susceptible to cell phone "cloning" and it used a Frequency-division multiple access (FDMA) scheme and required significant amounts of wireless spectrum to support. 

On 6 March 1983, the DynaTAC 8000X mobile phone launched on the first US 1G network by Ameritech. It cost $100M to develop, and took over a decade to reach the market. The phone had a talk time of just thirty-five minutes and took ten hours to charge. Consumer demand was strong despite the battery life, weight, and low talk time, and waiting lists were in the thousands.

Many of the iconic early commercial cell phones such as the Motorola DynaTAC Analog AMPS were eventually superseded by Digital AMPS (D-AMPS) in 1990, and AMPS service was shut down by most North American carriers by 2008. 

In February 1986 Australia launched its Cellular Telephone System by Telecom Australia. Peter Reedman was the first Telecom Customer to be connected on 6 January 1986 along with five other subscribers as test customers prior to the official launch date of 28 February. 

2G – Digital cellular

Two 1991 GSM mobile phones with several AC adapters

 

In the 1990s, the 'second generation' mobile phone systems emerged. Two systems competed for supremacy in the global market: the European developed GSM standard and the U.S. developed CDMA standard. These differed from the previous generation by using digital instead of analog transmission, and also fast out-of-band phone-to-network signaling. The rise in mobile phone usage as a result of 2G was explosive and this era also saw the advent of prepaid mobile phones. 

In 1991 the first GSM network (Radiolinja) launched in Finland. In general the frequencies used by 2G systems in Europe were higher than those in America, though with some overlap. For example, the 900 MHz frequency range was used for both 1G and 2G systems in Europe, so the 1G systems were rapidly closed down to make space for the 2G systems. In America the IS-54 standard was deployed in the same band as AMPS and displaced some of the existing analog channels. 

In 1993, IBM Simon was introduced. This was possibly the world's first smartphone. It was a mobile phone, pager, fax machine, and PDA all rolled into one. It included a calendar, address book, clock, calculator, notepad, email, and a touchscreen with a QWERTY keyboard. The IBM Simon had a stylus, used to tap the touch screen. It featured predictive typing that would guess the next characters as you tapped. It had applications, or at least a way to deliver more features by plugging a PCMCIA 1.8 MB memory card into the phone. Coinciding with the introduction of 2G systems was a trend away from the larger "brick" phones toward tiny 100–200 grams (3.5–7.1 oz) hand-held devices. This change was possible not only through technological improvements such as more advanced batteries and more energy-efficient electronics, but also because of the higher density of cell sites to accommodate increasing usage. The latter meant that the average distance transmission from phone to the base station shortened, leading to increased battery life while on the move. 

Personal Handy-phone System mobiles and modems, 1997–2003

 

The second generation introduced a new variant of communication called SMS or text messaging. It was initially available only on GSM networks but spread eventually on all digital networks. The first machine-generated SMS message was sent in the UK on 3 December 1992 followed in 1993 by the first person-to-person SMS sent in Finland. The advent of prepaid services in the late 1990s soon made SMS the communication method of choice among the young, a trend which spread across all ages. 

2G also introduced the ability to access media content on mobile phones. In 1998 the first downloadable content sold to mobile phones was the ring tone, launched by Finland's Radiolinja (now Elisa). Advertising on the mobile phone first appeared in Finland when a free daily SMS news headline service was launched in 2000, sponsored by advertising. 

Mobile payments were trialed in 1998 in Finland and Sweden where a mobile phone was used to pay for a Coca-Cola vending machine and car parking. Commercial launches followed in 1999 in Norway. The first commercial payment system to mimic banks and credit cards was launched in the Philippines in 1999 simultaneously by mobile operators Globe and Smart. 

The first full internet service on mobile phones was introduced by NTT DoCoMo in Japan in 1999. 

3G – Mobile broadband

As the use of 2G phones became more widespread and people began to use mobile phones in their daily lives, it became clear that demand for data (such as access to browse the internet) was growing. Further, experience from fixed broadband services showed there would also be an ever-increasing demand for greater data speeds. The 2G technology was nowhere near up to the job, so the industry began to work on the next generation of technology known as 3G. The main technological difference that distinguishes 3G technology from 2G technology is the use of packet switching rather than circuit switching for data transmission. In addition, the standardization process focused on requirements more than technology (2 Mbit/s maximum data rate indoors, 384 kbit/s outdoors, for example). 

Inevitably this led to many competing standards with different contenders pushing their own technologies, and the vision of a single unified worldwide standard looked far from reality. The standard 2G CDMA networks became 3G compliant with the adoption of Revision A to EV-DO, which made several additions to the protocol while retaining backwards compatibility:

• Introduction of several new forward link data rates that increase the maximum burst rate from 2.45 Mbit/s to 3.1 Mbit/s
• Protocols that would decrease connection establishment time
• Ability for more than one mobile to share the same time slot
• Introduction of QoS flags

All these were put in place to allow for low latency, low bit rate communications such as VoIP.

The first pre-commercial trial network with 3G was launched by NTT DoCoMo in Japan in the Tokyo region in May 2001. NTT DoCoMo launched the first commercial 3G network on 1 October 2001, using the WCDMA technology. In 2002 the first 3G networks on the rival CDMA2000 1xEV-DO technology were launched by SK Telecom and KTF in South Korea, and Monet in the US. Monet has since gone bankrupt. By the end of 2002, the second WCDMA network was launched in Japan by Vodafone KK (now Softbank). European launches of 3G were in Italy and the UK by Three/Hutchison group, on WCDMA. 2003 saw a further eight commercial launches of 3G, six more on WCDMA and two more on the EV-DO standard.

During the development of 3G systems, 2.5G systems such as CDMA2000 1x and GPRS were developed as extensions to existing 2G networks. These provide some of the features of 3G without fulfilling the promised high data rates or full range of multimedia services. CDMA2000-1X delivers theoretical maximum data speeds of up to 307 kbit/s. Just beyond these is the EDGE system which in theory covers the requirements for 3G system, but is so narrowly above these that any practical system would be sure to fall short. 

The high connection speeds of 3G technology enabled a transformation in the industry: for the first time, media streaming of radio (and even television) content to 3G handsets became possible, with companies such as RealNetworks and Disney among the early pioneers in this type of offering. 

In the mid-2000s, an evolution of 3G technology began to be implemented, namely High-Speed Downlink Packet Access (HSDPA). It is an enhanced 3G (third generation) mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family, also coined 3.5G, 3G+ or turbo 3G, which allows networks based on Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. Current HSDPA deployments support down-link speeds of 1.8, 3.6, 7.2 and 14.0 Mbit/s.

By the end of 2007, there were 295 million subscribers on 3G networks worldwide, which reflected 9% of the total worldwide subscriber base. About two thirds of these were on the WCDMA standard and one third on the EV-DO standard. The 3G telecoms services generated over $120 billion of revenues during 2007 and at many markets the majority of new phones activated were 3G phones. In Japan and South Korea the market no longer supplies phones of the second generation.

Although mobile phones had long had the ability to access data networks such as the Internet, it was not until the widespread availability of good quality 3G coverage in the mid-2000s (decade) that specialized devices appeared to access the mobile web. The first such devices, known as "dongles", plugged directly into a computer through the USB port. Another new class of device appeared subsequently, the so-called "compact wireless router" such as the Novatel MiFi, which makes 3G Internet connectivity available to multiple computers simultaneously over Wi-Fi, rather than just to a single computer via a USB plug-in.

Such devices became especially popular for use with laptop computers due to the added portability they bestow. Consequently, some computer manufacturers started to embed the mobile data function directly into the laptop so a dongle or MiFi wasn't needed. Instead, the SIM card could be inserted directly into the device itself to access the mobile data services. Such 3G-capable laptops became commonly known as "netbooks". Other types of data-aware devices followed in the netbook's footsteps. By the beginning of 2010, E-readers, such as the Amazon Kindle and the Nook from Barnes & Noble, had already become available with embedded wireless Internet, and Apple had announced plans for embedded wireless Internet on its iPad tablet devices later that year. 

4G – Native IP networks

By 2009, it had become clear that, at some point, 3G networks would be overwhelmed by the growth of bandwidth-intensive applications like streaming media. Consequently, the industry began looking to data-optimized 4th-generation technologies, with the promise of speed improvements up to 10-fold over existing 3G technologies. The first two commercially available technologies billed as 4G were the WiMAX standard (offered in the U.S. by Sprint) and the LTE standard, first offered in Scandinavia by TeliaSonera. 

One of the main ways in which 4G differed technologically from 3G was in its elimination of circuit switching, instead employing an all-IP network. Thus, 4G ushered in a treatment of voice calls just like any other type of streaming audio media, using packet switching over Internet, LAN or WAN networks via VoIP.

5G - Cellular Mobile Communications

Mobile device charger standards

USB power standards for mobile charger

Port	Current	Voltage	Power (max)
Micro-USB	500 mA	5 V	2.5 W
	1 A	5 V	5 W
	2 A	5 V	10 W
USB-C	100 mA to 3 A	5 V	15 W
	1.7 A to 3 A	9 V	27 W
	1.8 A to 3 A	15 V	45 W
	2.25 A to 5 A	20 V	100 W

The Micro-USB interface is found on chargers for feature phones and the lower end smartphones.

 

The USB-C interface is increasingly found on (chargers for) smartphones.

 

Before a universal charger standard was agreed upon in the late 2000s users needed an adapter which was often proprietary by brand or manufacturer to charge their battery. Later, mobile phones from major brands typically used a USB cable with a mini or micro-USB or, since the mid-2010s, USB-C interface. Apple's iPhone is the sole major brand to retain its own interface (30-pin dock connector replaced by Lightning in 2012). 

In China

As of 14 June 2007, all new mobile phones applying for a license in China are required to use a USB port as a power port for battery charging. This was the first standard to use the convention of shorting D+ and D−.

OMTP/GSMA Universal Charging Solution

In September 2007, the Open Mobile Terminal Platform group (a forum of mobile network operators and manufacturers such as Nokia, Samsung, Motorola, Sony Ericsson, and LG) announced that its members had agreed on Micro-USB as the future common connector for mobile devices.

The GSM Association (GSMA) followed suit on 17 February 2009, and on 22 April 2009, this was further endorsed by the CTIA – The Wireless Association, with the International Telecommunication Union (ITU) announcing on 22 October 2009 that it had also embraced the Universal Charging Solution as its "energy-efficient one-charger-fits-all new mobile phone solution," and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."

EU smartphone power supply standard

In June 2009, many of the world's largest mobile phone manufacturers signed an EC-sponsored Memorandum of Understanding (MoU), agreeing to make most data-enabled mobile phones marketed in the European Union compatible with a common External Power Supply (common EPS). The EU's common EPS specification (EN 62684:2010) references the USB Battery Charging Specification and is similar to the GSMA/OMTP and Chinese charging solutions. In January 2011, the International Electrotechnical Commission (IEC) released its version of the (EU's) common EPS standard as IEC 62684:2011.

Satellite mobile

As well as the now-common cellular phone, there is also the very different approach of connecting directly from the handset to an Earth-orbiting satellite. Such mobile phones can be used in remote areas out of reach of wired networks or where construction of a cellular network is uneconomic. 

The Inmarsat system is the oldest, originally developed in 1979 for safety of life at sea, and uses a series of satellites in geostationary orbits to cover the majority of the globe. Several smaller operators use the same approach with just one or two satellites to provide a regional service. An alternative approach is to use a series of low Earth orbit satellites much closer to Earth. This is the basis of the Iridium and Globalstar satellite phone services.