V. V. Shchennikov

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.

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe1−xSx (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe1−xSx with x≥0.27 and HgSe are similar in properties.

Thermoelectric properties of hydrogen ion-irradiated silicon crystals under ultrahigh pressures of up to 20 GPa

The thermopower S of p-Si samples containing a thin hydrogenated layer was studied at high pressures P of up to 20 GPa. Near the phase transition to a white-tin lattice (P ≤ 10 GPa), the thermopower of samples with a hydrogenated layer was found to decrease as compared to that in the starting p-Si samples. However, the values of S of high-pressure metallic phases with a β-Sn structure and orthorhombic (above 12 GPa) and simple hexagonal (above 16 GPa) structures are approximately the same for different sample groups. The variation in crystal structure induced by pressure treatment was studied by ultrasoft x-ray spectroscopy. The observed decrease in S induced by the pressure treatment is tentatively assigned to the formation of an amorphous phase in addition to the Si-III metastable phase.

Magnetoresistance and thermopower of tellurium at high pressures up to 30 GPa

Measurements are reported of the transverse magnetoresistance MR and of the thermopower S, carried out at high pressures P on Te single crystals in synthetic-diamond chambers. The MR is found to increase with decreasing gas width under a pressure up to 4 GPa as one approaches the semiconductor-metal phase-transition point, to fall off subsequently in the high-pressure metallic phase. The behavior of S(P) correlates with the pressure dependences of the measured MR. A negative MR at T=77 K was found within a narrow interval P=1.5–2 GPa, where the valence band of Te is assumed to undergo rearrangement. Above the point of the phase transition to the β-Po structure, MR is established to increase with pressure for P>12 GPa. The MR data are used to estimate the hole mobility μ for various Te phases. A comparison is made of the mobilities in Te, Se, and high-pressure phases of mercury chalcogenides, which are their structural and electronic analogs, for pressures of up to 30 GPa.

Electrical properties of (PbS)0.59TiS2 crystals at high pressure up to 20 GPa

The electrical resistance ρ and thermopower S of the (PbS)0.59TiS2 single-crystal compound with mismatched layers and the TiS2 crystals have been investigated at room temperature in high-pressure chambers with synthetic diamond dies. The decrease in ρ and |S| observed in (PbS)0.59TiS2 under the pressure P≈2 GPa is associated with the structural transformation of PbS from the cubic phase into the orthorhombic phase. The jumps of ρ and |S| are presumably caused by the increase in the electron concentration in the TiS2 layers. For P≥4 GPa, at which the gap is absent in the electronic spectrum of TiS2, a decrease in ρ(P) is observed for the (PbS)0.59TiS2 samples.

Magnetoresistance of HgSeS crystals at hydrostatic pressures of up to 1 GPa

Magnetoresistance (MR) of HgSe1−xSx crystals has been studied in the temperature range 4.2–300 K and in magnetic fields up to 12 T under hydrostatic pressures P exceeding the threshold Pt for the structural phase transition. Shubnikov-de Haas quantum oscillations in longitudinal and transverse MR were observed to occur in the original samples at T=4.2 K. For P>Pt, HgSeS crystals transferred to metastable states, which presumably incorporate the high-and low-pressure phases, and in which the oscillations vanished. At the same time the monotonic behavior of MR with magnetic field B, as well as the temperature dependences of resistivity ρ retained the shape characteristic of the original phases. The observed weakening of the dependences of ρ on B and T is attributed to an increase of the temperature independent contribution to ρ caused by phase inclusions and structural defects in the metastable states. It is the corresponding decrease of electron mobility that suppresses the oscillations.

Anisotropy of the transport properties of single-crystal Bi2 Te3 disordered by electron bombardment

Using single-crystal samples of Bi2Te3 bombarded by 5-MeV electrons at a temperature of 250 K, we study the electrical resistivity and the Hall effect in the temperature range 1.7–370 K and the Shubnikov-de Haas effect at T=4.2 K in magnetic fields up to 14 T. We find that electron bombardment of Bi2Te3 crystals results in a transition from the metallic p-type state to the metallic state with a Fermi surface. Annealing at 350 K eliminates the radiation defects and restores the p-type metallic conductance.

Neutron diffraction study of the pressure-driven structural transition in the HgTe0.85S0.15 ternary compound

The structure of the HgTe0.85S0.15 ternary mercury compound was studied by neutron diffraction at high pressures of up to 40 kbar. A phase transition from the cubic (sphalerite-type) to the hexagonal (cinnabar-type) structure was established to occur with increasing pressure and to be accompanied by an abrupt change in the unit-cell volume and interatomic distances. The unit cell parameters, the positions of the Hg and Te/S atoms in the hexagonal cinnabar phase, and their pressure dependences were found.

Neutron diffraction investigation of the structural transition in HgSe1−xSx ternary mercury chalcogenide systems at high pressures

The structure of HgSe1−xSx ternary mercury chalcogenides at high pressures up to 35 kbar is investigated by neutron diffraction. It is found under pressure, that the HgSe1−xSx compounds undergo, a phase transition from the cubic sphalerite-type to the hexagonal cinnabar-type structure, which is accompanied by a jump-wise change in the unit cell volume and interatomic distances. The unit cell parameters and the positional parameters of Hg and Se (S) atoms in the high-pressure hexagonal phase are determined. A two-phase state is revealed in the phase transition region.

Conductivity anisotropy of the (PbS)0.59TiS2 misfit compound

A study is reported on the temperature dependence of the resistivity of the (PbS)0.59TiS2 misfit-layer compound in the commensurate and incommensurate basal-plane directions. The effect of the lattice incommensurability is shown to be equivalent to additional one-dimensional scattering from a neutral impurity.