Metal-semiconductor phase transition

Sok] Sokolovich, V.V., Smyk, A.A., Loseva, G.V., Metal-Semiconductor Phase Transitions in the System FeCr2S4-Fe (in Russian), Niz. Tverd. Tela, 27(9), 2851-2853 (1985) (Experimental, Phase Relations, Crys. Stracture, 6)... 

We have studied the resistance and thermopower behavior of the p -phase [9]. From the temperature-dependent resistance curve, one can see clearly a metal-semiconductor phase transition at about 140 K (Fig. 2), whereas on the temperature-dependent thermopower curve, only a very small (but distinct) kink appears at the same temperature (Fig. 3). To interpret these seemingly conflicting phenomena, a two-energy band model was used [9]. In this model, the conductivity is due to a combination of the two bands, and the thermopower is due to a competition of the two bands. From the room-temperature X-ray diffraction, which shows the iodine atoms arranged randomly, we speculate that, at room temperature, there should be an energy gap at the Fermi surface, but that the random arrangement of iodine atoms smears the gap. Thus, the crystal stays metallic at room temperature. 

Many beam lattice images obtained at 200 kV from thin regions (5nm) in V2O3 crystals for showing the importance of the defocus value and crystal thickness to interpret the images in crystals of relatively small unit cells (two molecule rhomboedral cell with ur = 5.473nm and a = 53.79°). This crystal depicts also a metal-semiconductor phase transition at about 150 K, which can be easily observed by HREM [16]. 

Metal-semiconductor phase transition (MSPT) in VO2 reflected in the interfacial behavior of bound water because of changes in the absorptivity of the materials with respect to water and efficiency of nuclear magnetic relaxation (Turov et al. 1995a). Therefore, the signal width and intensity of bound water strongly changed. However, these materials were not structurally characterized using adsorption or/and cryoporometry methods. 

Turov, V.V, Gorbik, P.P., Ogenko, V.M., Shulga, O.V., and Chuiko, A.A. 2001. Characteristic properties of metal-semiconductor phase transitions of vanadium dioxide in a polyethylene glycol medium containing tetraethylammonium bromide. Colloids Surf. A Physicochem. Eng. Aspects 178 105-112. 

C. Constantin, M. B. Haider, D. Ingram and A. R. Smith Metal/semiconductor Phase Transition in Chromium Nitride(OOl) Grown by Rf-plasma-assisted Molecular-beam Epitaxy ,/Ipp/. Phys. Lett., 85, 6371-6373 (2004)... 

The Peierls169 metal-to-semiconductor phase transition in TTFP TCNQ p was detected in an oscillation camera these streaks became bona fide X-ray spots only below the phase transition temperature of 55 K this transition is incommensurate with the room-temperature crystal structure, due to its partial ionicity p 0.59, and the "freezing" of the concomitant itinerant charge density waves (this effect was missed by four-circle diffractometer experiments, which had been set to interrogate only the intense Bragg peaks of either the commensurate room-temperature metallic structure, or the commensurate low-temperature semiconducting structure). 

In elemental semiconductors and the polar faces of compound semiconductors, an odd number of electrons is formed per surface atom by the creation of a surface. The solid therefore undergoes a metal—insulator phase transition [82] to produce an even number of electrons per surface unit cell, thus reducing its symmetry in the plane of the surface. For non-polar faces of compound semiconductors, the simple truncated bulk geometry is already insulating in character because anionic and cationic species are electronically inequivalent. No distortions which reduce the symmetry are therefore necessary to provide stability, but the unbalanced ionic forces and unsaturated covalencies can produce quite large ( 0.5 A) atomic movements ( surface relaxation ). 

Phase transitions are involved in critical temperature thermistors. Vanadium, VO2, and vanadium trioxide [1314-34-7] V2O3, have semiconductors—metal transitions in which the conductivity decreases by several orders of magnitude on cooling. Electronic phase transitions are also observed in superconducting ceramics like YBa2Cu30y but here the conductivity increases sharply on cooling through the phase transition. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

The dependence of Tc on pressure is studied for a variety of reasons. In a chemical sense, bond lengths are shortened, and orbital interactions are increased. The volume decrease leads in principle to a rise in carrier density. In reality, however, not only do vibrational frequencies change, but crystal structure and symmetry are often affected by high pressure. Numerous materials undergo semiconductor to metal phase transitions as a function of pressure. Increasing pressure can often be considered analogous to a decrease in temperature. 

Uchida, S., Kitazawa, K. and Tanaka, S., Superconductivity and Metal-Semiconductor Transition in BaPbj.xBix03. Phase Transitions 8 95 (1987). 

Khan, Y., Nahm, K., Rosenberg, M. and Willner, H., Superconductivity and Semiconductor-Metal Phase Transition in the System BaPbj.xBixOs. Phys. Stat. Sol.(a) 39 79 (1977). 

The electrical conductivity of CoOP as a function of temperature is shown in Figure 6. Above room temperature the compound exhibits metallic behaviour but coincidental with the development of the superstructure the conductivity falls rapidly with decreasing temperature. Below 250 K CoOp behaves as a semiconductor with an activation energy of meV.74 The conduction has been shown to be frequency dependent below 250 K.75 Thermopower studies have also clearly demonstrated the changeover from metallic behaviour above 300 K. to semiconductor behaviour below 250 K.72 The behaviour of ZnOP is very similar to that of CoOp, with the phase transition from the Cccm to Pccn space group occurring at 278 K. Superstructure formation is complete by about 260 K.77... 

Some important observations, which should apply de facto to many nematic systems containing dispersed nanoparticles, particularly those with metal or semiconductor cores, were reported in 2006 by Prasad et al. [297]. The authors found that gold nanoparticles stabilized with dodecanethiol decreased the isotropic to nematic phase transition of 4-pentyl-4 -cyanobiphenyl (5CB) almost linearly with increasing nanoparticle concentration (x p) and increased the overall conductivity of these mixtures by about two orders of magnitude. However, the anisotropy of the electric conductivity (Act = [Pg.349]

The existence of a solid itself, the solid surfaces, the phenomena of adsorption and absorption of gases are due to the interactions between different components of a system. The nature of the interaction between the particles of a gas-solid system is quite diverse. It depends on the nature of the solid s atoms and the gas-phase molecules. The theory of particle interactions is studied by quantum chemistry [4,5]. To date, one can consider that the prospective trends in the development of this theory for metals and semiconductors [6,7] and alloys [8] have been formulated. They enable one to describe the thermodynamic characteristics of solids, particularly of phase equilibria, the conditions of stability of systems, and the nature of phase transitions [9,10]. Lately, methods of calculating the interactions of adsorbed particles with a surface and between adsorbed particles have been developing intensively [11-13]. But the practical use of quantum-chemical methods for describing physico-chemical processes is hampered by mathematical difficulties. This makes one employ rougher models of particle interaction - model or empirical potentials. Their choice depends on the problems being considered. 

As has been pointed out previously, ionic compounds are characterized by a Fermi level EF that is located within an s-p-state energy gap Ef. It is for this reason that ionic compounds are usually insulators. However, if the ionic compound contains transition element cations, electrical conductivity can take place via the d electrons. Two situations have been distinguished the case where Ru > Rc(n,d) and that where Rlt < Rc(n,d). Compounds corresponding to the first alternative have been discussed in Chapter III, Section I, where it was pointed out that the presence of similar atoms on similar lattice sites, but in different valence states, leads to low or intermediate mobility semiconduction via a hopping of d electrons over a lattice-polarization barrier from cations of lower valence to cations of higher valence. In this section it is shown how compounds that illustrate the second alternative, Rtt < 72c(n,d), may lead to intermediate mobility, metallic conduction and to martensitic semiconductor metallic phase transitions. 

---