Metal-nonmetal transition

Ti by V up to 10% makes the system metallic, the c/a ratio of the solid solution corresponding to that of the high-temperature metallic phase of Ti203- 

The alloy system (Vj ,MJ203 deserves special mention because of its spectacular M-NM transitions. The present situation with regard to these transitions may be summarized as follows (Honig, 1982). M-NM transitions in this system are shown schematically in Fig. 6.30 where the roman numerals correspond to the different regimes. 

005(M = Cr or Al) At low temperatures, the oxide is an antiferromagnetic insulator (AFI) and shows the major M-NM transition around 167 K. Above 350 K the resistivity increases by a factor of 3 and becomes constant above 800 K. 

005 X 0.018 These compositions show three transformations. The first is from the AFI phase to a metallic (M) phase around 170 K while the second is from the M phase to a paramagnetic insulating (I) phase at T = in the range 

The (Vj jjTij203 system is similar to I, except that the temperature of the AFI-M transition is reduced for x 0.055, the M-AFI discontinuity disappears. 

It is possible to induce metallic transport behavior at some finite temperature in solids with band gaps by doping the conduction band with charge carriers. This is appropriately termed a filling-control metal-nonmetal transition, since one is filling a formerly vacant conduction band with charge carriers. This is a topic that will be expanded upon later in 

Principles of Inorganic Materials Design, Second Edition. By John N. Lalena and David A. Cleary Copyright 2010 John Wiley Sons, Inc. 

About one decade after the development of band theory, two Dutch industrial scientists at the NV Philips Corporation, Jan Hendrik de Boer (1899-1971) (de Boer was later associated with the Technological University, Delft) and Evert Johaimes Willem Verwey (1905-1981), reported that many transition metal oxides, with partially filled bands that band theory predicted to be metallic, were poor conductors and some were even insulating (de Boer and Verwey, 1937). Rudolph Peierls (1907-1995) first pointed out the possible importance of electron correlation in controlling the electrical behavior of these oxides (Peierls, 1937). Electron correlation is the term applied to the interaction between electrons via Coulombs law. 

The intrasite Coulomb gap is completely unaccounted for in the Hartree-Fock theory (e.g. tight-binding calculations), since electron correlation is neglected in the 

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Flensel F and Franok E U 1968 Metal-nonmetal transition in dense meroury vapor Rev. Mod. Phys. 40 697... 

Given the efficiency of VASP, electronic structure calculations with or without a static optimization of the atomic structure can now be performed on fast workstations for systems with a few hundred inequivalent atoms per cell (including transition-metais and first row elements). Molecular dynamics simulationsextending over several picoseconds are feasible (at tolerable computational effort) for systems with 1000 or more valence electrons. As an example we refer to the recent work on the metal/nonmetal transition in expanded fluid mercury[31]. 

Ito H, Ishiguro T, Kuhota M, Saito G (1996) Metal-nonmetal transition and superconductivity localization in the two-dimensional conductor /c-(BEDT-TTE)2Cu[N(CN)2]Cl under pressure. J Phys Soc Jpn 65 2987-2993... 

Ito H, Kubota M, Ishiguro T, Saito G (1997) Metal-nonmetal transition of hydrogenated and deuterated k-(BEDT-TTF)2Cu[N(CN)2]X under pressure. Synth Met 85 1517-1518... 

Figure 6.29 Temperature dependence of electrical resistivity of Ti203, VO2 and V2O3 through the metal-nonmetal transition.

Figure 6.30 Metal-nonmetal transition in (Vi Mj203 (M = Cr, A1 or Ti) and V2,i-j,)03 systems. (After Honig, 1982.) See text for explanation of the various curves.

Figure 6.31 Composition-controlled metal-nonmetal transition in Agj+ Se system. (After Shukla et ai, 1981.)...

U. Even Prof. Gerber, could you follow the metal-nonmetal transition in mercury clusters ... 

The nature of these two phases helps to throw light on the metal-nonmetal transition. For example there has been much speculation that hydrogen molecules at sufficiently high pressure, such as those occurring on the planet Jupiter, might undergo a transition to un alkali metal The fundamental transition is one of a dramatic change of the van der Waals interactions of H, molecules into metallic cohesion. ... 

Edwards, Lusis, and Sienko have recently reported an ESR study (60) of frozen lithium-methylamine solutions which suggests the existence of a compound tetramethylaminelithium(O), Li(CH3NH2)4, bearing all the traits (60) of a highly expanded metal lying extremely close to the metal-nonmetal transition. Specifically, both the nuclear-spin and electron-spin relaxation characteristics of the compound, although nominally metallic, cannot be described in terms of the conventional theories of conduction ESR (6,15, 71) and NMR in pure metals (60, 96, 169). 

Ohana, I. 1989. Metal-nonmetal transition induced by reorientation of the fluorine molecules in stage-2 graphite-fluorine compounds. Phys. Rev. B 39 1914-1918. 

Since the first preparation of potassium-ammonia solution (Sir Humphrey Davy, in 1808) alkali metal-ammonia solutions have been at the centre of much theoretical and experimental interest. Novel properties include low density, high electrical conductivity, liquid-liquid phase separation, and a concentration driven metal-nonmetal transition [35]. 

The relevance to small particles and indeed massive surfaces now becomes clear, because the preponderance of low CN atoms increases as particle size goes down, and this may turn out to be the most important factor in determining reactivity14 — more important than quantum size effects (i.e. the metal —> nonmetal transition), surface mobility or any of the other properties that are characteristic of very small assemblies of atoms (see Section 3.4). It becomes possible to imagine that activity in catalytic oxidations is solely due... 

Among the phase transitions where electronic factors play a major role, the most well-known are the metal-insulator transitions exhibited by transition-metal oxides, sulfides, and so on. This subject has been discussed at length.2,23,24 A recent observation26 of some interest is that the metal-nonmetal transition occurs at a critical electron concentration as given by the particular form of the Mott criterion, = 0.26 ... 

Diverse Systems Exhibiting Metal-Nonmetal Transitions... 

Figure 4. Compositionally controlled metal—nonmetal transitions in (a) Lai-jSr CoOj and (b) LaNi, Mn,Oj (from Rao and Ganguly20).

Figure 5. Compositionally controlled metal—nonmetal transition in superconducting cuprates (a) B S Cai-.NdrCujOs and (b) TlCa,. NthS CuO (after Rao ).

There appears to be little doubt that the Mott criterion given by eq 3 is an effective indicator of the critical condition at the M-NM transition itself. At the least, this simple criterion provides a numerical prediction for the metal-nonmetal transition in many situations. Figure 10 summarizes some of the experimental data.34-36 Interestingly, besides doped semiconductors, metal—ammonia and metal—noble gas systems and superconducting cuprates all follow the linear relation given by eq 3. This is truly remarkable. 

Minimum Metallic Conductivity at the Metal - Nonmetal Transition... 

Figure 9. A schematic representation of the situation (F = 0 K) for P doped Si at both high and low donor densities. Also shown are two scenarios for the composition dependence of the electrical conductivity, showing the metal—nonmetal transition.

Figure 11. Schematic illustration of the two possibilities of a continuous or discontinuous metal—nonmetal transition at T = 0 K. The minimum metallic conductivity, aln,n, at the transition is also shown (from Lee and Ramakrishnan39). The conductivity at zero temperature (ordinate) and Fermi energy (abscissa) are shown. The discontinuous conductivity transition suggested by Mott is the full curve, with aTOln occuning as Ef crosses the mobility edge energy c. The dotted curve is the continuous conductivity transition predicted by the scaling theory...

Figure 12. Electrical conductivity as a function of relative electron (donor) concentration in P doped Si to illustrate the metal-nonmetal transition at 0 K. The relative P density is varied by changing nc with uniaxial stress on a single sample. The open circles are extrapolated to T = 0 K (from Thomas33).

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