Electrical properties of the high-pressure phases of gallium and indium tellurides

A study has been made of the resistance ρ, the thermopower S, and magnetoresistance MR of Ga₂Te₃ and α-In₂Te₃ single crystals at pressures P up to 25 GPa. It is found that the resistance ρ and |S| sharply decrease at ∼0–5 and 1.5–3 GPa, respectively. The semiconductor-metal phase transitions in the temperature range from 77 to 300 K are established from the sign reversal of the temperature coefficient of ρ to occur at P>4.4 and >1.9 GPa. The values S ≈+(10–20)μ V/K for the metallic phases with a Bi₂Te₃-type structure agree with those for liquid In₂Te₃ and Ga₂Te₃. Negative MR is revealed in In₂Te₃ at P≈1.9 GPa. No MR is observed in Ga₂Te₃ up to 25 GPa. The variation of the electronic structure of In₂Te₃ and Ga₂Te₃ under pressure is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1004–1008. Original Russian Text Copyright ? 2000 by Shchennikov, Savchenko, Popova.     

Optical functions of ternary Li-containing tellurides

First-principle calculations of electronic structures of new nonlinear nondirect-gap LiInTe ₂ and direct-gap LiGaTe ₂ chalcogenides are made within the method of density functional. The special features of the structure of valence band top and conduction band bottom are determined. The spectra of fundamental optical functions are calculated and studied using the model of direct band-to-band transitions. The pecularities of optical functions are discussed by the example of ε₂(E). __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 56–60, December, 2007.     

Thermoelectric properties of high-pressure silicon phases

The thermo emf in Czochralski-grown silicon single crystals (Cz-Si) was experimentally studied in a range of pressures up to 20 GPa. The pressure dependences revealed phase transitions in the metallic phase of silicon, which passed from tetragonal to orthorhombic and then to hexagonal lattice. The high-pressure silicon phases, as well as the metallic high-pressure phases in A_NB_8?N semiconductors, possess conductivity of the hole type. As the pressure decreases, the emf behavior reveals transitions to the metastable phases Si-XII and Si-III. Preliminary thermobaric treatment of the samples at a pressure of up to 1.5 GPa and a temperature of T=50–650°C influences the thermoelectric properties of Cz-Si at high pressures.     

Predicted novel high-pressure phases of lithium

Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.     

Superconductivities of high-pressure phases in the metal-hydrogen systems

Abstract

The superconducting properties of the d-metal hydrides rank among the most interesting and less studied of their characteristics. Correct and complete data have only been obtained for one superconducting hydride, the palladium hydride'. The long studies on phases forming in the d-metal-hydrogen system in the well- mastered pressure range up to tens of atmospheres have not exhibited any new superconductors, and the scope of systems which could be of interest from this viewpoint has been mainly exhausted'.     

Determination and calculation of high-pressure ternary phase diagrams

Abstract

By the improved high-pressure DTA, we have determined the high-pressure vertical phase diagrams of the Cd-Pb-Sn ternary system up to 2.5 GPa.

A complete method of calculating high-pressure phase diagrams has been devised, in which the special attention has been paid to the effecs of high prssure on the partial molar free energy of each component in equilibrium phases. The high-pressure phase diagrams of Cd-Pb-Sn and Cd-Sn-Zn ternary systems up to 2.5 GPa were calculated successfully.

The agreement between the observed and the calculated resuts is satisfactory.     

A Raman study of high-pressure phases of lead chalcogenides PbX (X=S,Se, Te)

Pressure-induced phase transitions of lead chalcogenides were studied by Raman spectroscopy. The number of Raman modes observed for the first high-pressure phases was found to be significantly less than expected from selection rules for the proposed GeS-type orthorhombic structure. The observations are consistent with the alternative assignment as CrB-type. In the case of PbS, the disappearance of the intermediate-phase Raman lines in the pressure range of 19–25 GPa, i.e., in the vicinity of the orthorhombic to CsCl-type phase transition, is accompanied by a strong increase in a structureless background. The Raman background is attributed to light scattering by charge carriers in the metallic CsCl-type phase.     

Stabilization of high-pressure phases in nanocrystalline metals and alloys

It is shown experimentally that the formation of submicrocrystalline and nanocrystalline states significantly affects the stability of the high pressure hcp phase in iron-based alloys. It is established that the manganese segregation from the bulk of ɛ-phase grain to the grain boundary under high hydrostatic pressure can influence the stability of this phase in nanocrystalline alloys.     

Theoretical calculations of the high-pressure phases of ZnF2 and CdF2

First-principles calculations based on density functional theory were used to study the high-pressure phases of both ZnF₂ and CdF₂. We found that the sequence of the pressure-induced phase transitions is: Rutile (P4₂/mnm) ↦ CaCl₂ (Pnnm) ↦ PdF₂ (Pa-3) and CaF₂ (Fm3m) ↦ PbCl₂ (Pnma) ↦ Ni₂In (P6₃/mmc) for ZnF₂ and CdF₂ respectively. In ZnF₂ the behavior of the ground-state total energy, of the Gibbs free energy and of the lattice constant vs. pressure shown that the phase transition at 4 GPa from the rutile-type phase to the CaCl₂-type phase is a second-order phase transition. The mechanism of the structural change was also revealed by the transition from the PbCl₂-type phase to the Ni₂In-type phase in CdF₂. Moreover, the high-pressure behavior of divalent metal fluorides was compared and discussed.     

Electronic structure of low-pressure and high-pressure phases of silicon disulfide

Self-consistent density functional theory calculations of band spectra, densities of states as well as the spatial distribution of valence electron charge density were carried out for the low-pressure α-phase and the high-pressure β-phase of SiS₂. Group-theoretical analysis performed for both phases enabled the symmetry of wave functions in a number of high-symmetry points of the Brillouin zone as well as the structure of valence band representations to be found. Based on the calculations of the band structure, the orthorhombic α-phase of SiS₂ was determined to be an indirect-gap semiconductor with the band gap E _gi = 2.44 eV (T ₁ → X ₈ transition), while the β-phase was shown to be direct gap with E _gd = 2.95 eV (Г ₃ → Г ₂ transition). The calculated energy distribution of the total density of states in the valence band of α-SiS₂ qualitatively and quantitatively correlates with the main experimental features of the X-ray photoelectron spectrum.     

On the absence of intermediate phases in the two-dimensional Coulomb gas

For a simple, continuum two-dimensional Coulomb gas (with soft cutoff), Gallavotti and Nicoló [J. Stat. Phys. 38:133–156 (1985)] have proved the existence of finite coefficients in the Mayer activity expansion up to order 2n below a series of temperature thresholdsT _n =T [1+(2n–1)^–1] (n=1, 2,...). With this in mind they conjectured that an infinite sequence of intermediate, multipole phases appears between the exponentially screened plasma phase aboveT ₁ and the full, unscreened Kosterilitz-Thouless phase belowT T _KT. We demonstrate that Debye-Hückel-Bjerrum theory, as recently investigated ford=2 dimensions, provides a natural and quite probably correct explanation of the pattern of finite Mayer coefficients while indicating the totalabsence of any intermediate phases at nonzero density ; only the KT phase extends to >0.     

Neutron scattering study of high-pressure phases obtained by thermobaric treatment

Abstract

A technique of thermobaric quenching has been developed at the Institute of Solid State Physics RAS, which made it possible to prepare and study bulk samples of high-pressure phases in a metastable state at ambient pressure. The paper briefly discusses results of recent neutron scattering investigations of the crystal structure and lattice dynamics of high-pressure phases and forms of various alloys and compounds, such as the bulk amorphous alloys Ga-Sb, GaSb-Ge, Zn-Sb, Al-Ge, amorphous ice and metal hydrides TiH, Mn-H, PdH and NiH.     

Aggregation and intermediate phases in dilute spin systems

We study a variety of dilute annealed lattice spin systems. For site diluted problems with many internal spin states, we uncover a new phase characterized by the occupation and vacancy of staggered sublattices. In cases where the uniform system has a low temperature phase, the staggered states represent an intermediate phase. Furthermore, in many of these cases, we show that (at least part of) the phase boundary separating the low-temperature and staggered phases is a line of phase coexistence-i.e. the transition is first order. We also study the phenomenon of aggregation (phase separation) in bond diluted models. Such transitions are known, trivially, to occur in the large-q Potts models. However, it turns out that phase separation is typical in bond diluted spin systems with many internal states. (In particular, a bond aggregation transition is not tied to a discontinuous transition in the uniform system.) Along the portions of the phase boundary where any of these phenomena occur, the prospects for a Fisher renormalization effect are deemed to be highly unlikely or are ruled out altogether.Partly supported by the NSF grant DMS-93-02023 (L.C.), the grants GAR 202/93/0449 and GAUK 376 (R.K.), and the NSF grant DMS-92-08029 and the Russian Fund of Fundamental Investigations grant 93-01-01470 (S.B.S.).