Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ～10⁴ Ω^?1 cm^?1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.     

Phase transitions investigation in ZnTe by thermoelectric power measurements at high pressure

The pressure-induced phase transitions were studied in ZnTe by the thermoelectric power (S) technique. For the high-pressure trigonal phase P3₁21 cinnabar the large thermopower values S≈+400 correspond to semiconductor hole conductivity. During a transition into the orthorhombic structure Cmcm the value of S dropped by 40-50 times indicating metallic hole conductivity, like in the high pressure phases of other chalcogenides of II Group (HgSe, HgTe, CdTe) with Cmcm structure. In a transient region between the trigonal and orthorhombic phase (especially under decreasing pressure) a novel phase has been observed with a negative value of S. By analogy with other Zn and Cd chalcogenides whose NaCl phases have an electron type of conductivity the phase observed may have a NaCl structure.     

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.     

Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures P ～ 5–15 GPa. Data on the thermopower of calcium in the pressure range up to 9 GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at 5–6 GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower (5–12 μV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of 5–7.5 GPa is fairly noticeable (～10%). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.     

Thermoelectric properties of the Pr0.8Na0.2MnO3 manganite at ultrahigh pressures of up to 20 GPa

The pressure dependences of the thermopower and electrical resistance of the Pr_0.8Na_0.2MnO₃ manganite are measured in the pressure range 0–20 GPa at room temperature. The thermopower varies nonmonotonically with pressure: the magnitude of the thermopower increases in the range of comparatively low pressures P < 5 GPa and decreases at higher pressures, whereas the electrical resistance decreases throughout the pressure range studied. The pressure at which the pressure coefficient dS/dP reverses sign is close to the value at which the Pr_0.8Na_0.2MnO₃ manganite undergoes a magnetic phase transition in the low-temperature range. The correlation between the specific features in the behavior of the thermopower with variations in pressure and the transformations of the magnetic and crystal structures of Pr_0.8Na_0.2MnO₃ at high pressures is discussed.     

Electrical transport properties of polyacetylene tetrachloroferrate - [CH(FeCl4)y]x]

Conductivity and thermopower measurements of polyacetylene doped with FeCl₄ are reported. The conductivity changes over 10 orders of magnitude and reaches the maximum value of 200 Ω^-1 cm^-1 at y = 0.07. The thermopower reveals the semiconductor to metal transition at y ? 0.002, with high and temperature independent Seebeck coefficient in the dilute limit and a metallic dependence in the heavily doped samples.     

High-pressure thermoelectric characteristics of Bi2Te3 semiconductor with different charge carrier densities

The measurements of the absolute values of the thermopower and of the relative electrical resistance have been performed for n type Bi₂Te₃ under hydrostatic pressure up to 9 GPa at room temperature. Under pressures exceeding 5 GPa and up to the phase transition (at 7 GPa), the samples with the charge carrier density below 10^?19 cm^?3 exhibit an anomalous growth of the thermopower. For the purest sample (n = 10^?18 cm^?3), the thermopower is as high as +150 μV/K. The pressure dependence of the electrical resistance for n-Bi₂Te₃ does not exhibit any anomalies up to the pressure corresponding to the phase transition (7 GPa). Thus, the state with the giant thermoelectric efficiency is found in Bi2Te3 under pressure before the phase transition.     

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.     

Semiconductor-metal transition in doped (CH)x: Thermoelectric power

Thermoelectric power studies of polyacetylene have been carried out as a function of dopant concentration and temperature. The thermopower of pure trans-(CH)_x is large $\text{(S =}\text{+850 μ V}\text{°K}\text{)}$ and positive consistent with p-type material. With iodine doping, (CHI_y)_x, the thermopower remains positive over the full range of concentration 0 < y < 0.22. The semiconductor-metal transition is clearly observed at n_c ? 3 mole %; S falls dramatically from $\text{S =}\text{+850 μ V}\text{°K}$ at y = 0.003 to $\text{S =}\text{+30 μ V}\text{°K}$ at y = 0.03. At higher concentrations, S remains nearly constant saturating at $\text{+18 μ V}\text{°K}$ in the heavily doped metallic polymer. Temperature dependences are consistent with metallic behavior at the highest dopant concentrations and hopping transport in the undoped and lightly doped polymer.     

Electron and lattice stages in the valence transition in SmTe under a high hydrostatic pressure

The results of precision measurements of the resistivity, thermopower, volume, and thermal conductivity of the compound SmTe under truly hydrostatic pressure conditions at room temperature are reported. High quality stoichiometric and doped (n-type, n ≈ 8 × 10¹⁸ cm^?3) single crystals are studied. It is found that the valence transition occurs as consecutive stages of rearrangement of the electron subsystem and the crystal lattice, which take place under different pressures. At the initial stage of the transition, metallization is observed, which is accompanied by anomalies in kinetic coefficients; the curve describing the pressure dependence of the volume deviates from the curve corresponding to the initial semiconductor phase only slightly. The next stage is accompanied by a substantial change in the sample volume (lattice collapse); in this pressure range, however, the resistivity and thermopower become independent of pressure. At the final stage of the transition, the sample compressibility decreases; the resistivity and thermopower become again functions of pressure; and a state emerging in the sample in this case corresponds to the “golden” phase of SmS in all the properties being measured.     

Photoinduced instability and semiconductor-metal phase transition in a Peierls system

A microscopic theory of the appearance of electron-phonon instability and a semiconductor-metal phase transition away from thermodynamic equilibrium in a Peierls system upon the optical excitation of electron-hole pairs is devised. An equation which specifies the dependence of the order parameter of the phase transition and the uniquely related gap width in the electron spectrum, on the concentration n of conduction-band electrons is obtained. The critical concentration n=n _c, above which the semiconductor phase of the system is unstable toward the transition to the metallic state, is calculated. A comparison with an experiment on the irradiation of a substrate-supported vanadium dioxide film by a laser pulse is made. Fiz. Tverd. Tela (St. Petersburg) 40, 2113–2118 (November 1998)     

Electrical conductivity and thermopower of lead titanate crystals

The temperature dependences of the thermopower ?? and volume conductivity ?? _V of lead titanate crystals have been studied in the temperature range 400?C550 K, in which there are anomalies of the permittivity ? that are not related to the phase transition. It has been found that, in this temperature range, the thermopower ?? has a maximum and changes sign in many crystals; in this case, ?? _V decreases. It has been assumed that the anomalies of ?? and ?? _V , which correlate with anomalies of ?, are caused by thermal activation of electron traps, which leads to compensation of the p-type conductivity. The concentrations of free charge carriers and their drift mobility in the paraphase near the Curie point have been determined. These data indicate that the possibility exists of occurring the total internal screening of the bound charge P _s .     

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (?1.9 × 10^?2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.     

Thermoelectric behaviour of SmS and Sm0.84Gd0.16S at high pressures and temperatures

The 4f→ 5d electronic phase transition in SmS has been studied using thermoelectric power as a probe. The variation of thermopower with pressure in Sm_0.84Gd_0.16S and in the high pressure phase of SmS is anomalous, characterized by a rather large pressure coefficient. The temperature coefficient of thermopower in Sm_0.84Gd_0.16S is large and negative at low pressures leading to a change of sign at higher temperatures. The anomalies can be understood on the basis of the ICF model.     

Electrical properties of manganese doped ferrous oxide at high temperatures

Electrical conductivity and thermoelectric power of Mn-doped ferrous oxide have been measured as a function of temperature (1070–1570 K) and oxygen pressure in the whole range of its stability. It has been shown that manganese changes the concentration of ionic defects in ferrous oxide and changes its electrical properties toward classic semiconductor; the complex defect structure characteristic for pure wustite, being observed up to 20–30% Mn. The metallic behavior found for highly defected pure wustite is also observed for Fe_{1 − x}Mn_xO, containing 15–20% Mn, although the corresponding pressure range narrows and shifts toward the higher pressures.     

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.     

Semiconductor-metal phase diagram of Co-doped NiS2

The phase diagram of the semiconductor-metal and antiferromagnetic transitions in 5 at.% Co- and 7 at.% Co-doped NiS₂ is determined from the electrical resistance measurements below room temperature to 77 K at pressures up to 35 kbar. It is indicated that the antiferromagnetic transition occurs in both semiconducting and metallic phases.     

Electron transport properties of lithium and phase transitions at high pressures

The electric resistivity and thermopower of lithium have been precisely measured at high pressures (up to 8 GPa) and temperatures from room temperature to 100°C. Transition to the fcc phase of lithium has been analyzed. The hysteresis of the direct and inverse transitions is 0.3 GPa at room temperature, decreases slightly with an increase in the temperature, and is almost independent of the prehistory of the sample. The phase transition line on the P-T diagram has a positive slope of dP/dT = 0.03 GPa/K. It is assumed that the fcc phase of lithium, which is stable at a high pressure, can appear for kinetic regions from the 9R phase, which is intermediate in energy between the bcc and fcc modifications.     

Transport properties in single phase superconductor Ba2YCu3O9-δ

The electrical resistivity and the thermopower are measured on the single phase superconductor Ba₂YCu₃O₉-δ (δ=2.1). The results indicate that the temperature dependences of the resistance and thermopower exhibit typical metallic behaviour, and the sample conducts via electrons at high temperatures. The behaviour of the thermopower can be described with Mott's semi-classical model. The specific heat of electrons in normal state has been estimated 780mJ/K·mole at 200K, i.e. γ=3.9mJ/K²·mole. Unusual phonon-drag effect is observed above the superconducting transition temperature T_c. Below T_c, the electrical resistivity and the thermopower all drop to zero corresponding to a superconducting ground state.     

The structure,conductivity, and thermopower of HMTTF-TCNQ

The structure and some transport properties of a new organic conductor, HMTTF-TCNQ, are reported. A metallic conductivity and thermopower are found between room temperature and 50°K, with insulating behavior at lower temperatures. The conductivity data suggest that two phase transitions occur, one at 43±1°K and the other at 50±°K. Comparison with the properties of TTF-TCNQ and HMTSeF-TCNQ is made.