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.     

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.     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.     

Thermoelectric power of cerium at high pressures

The electronic phase transition in cerium occurring near 7 kbar pressure at room temperature which is attributed to the 4f–5d electron promotion has been studied using thermoelectric power as a tool. The important results that have emerged out of this work are: (a) the relatively large variation in the absolute thermoelectric power ofγ-cerium (normal fcc phase) with pressure prior to the phase transition (in contrast to the rather small resistivity change with pressure in this region); (b) a sharp decrease in the thermoelectric power accompanying the iso-structuralγ-α phase transition; and (c) the continuous decrease in the thermoelectric power ofα-cerium (collapsed fcc phase) with pressure, ultimately changing sign at higher pressures. An explanation based on the “virtual bound state” model is proposed to account for these results.     

Structural phase transitions in yttrium under ultrahigh pressures

X-ray diffraction studies were carried out on the rare earth metal yttrium up to 177?GPa in a diamond anvil cell at room temperature. Yttrium was compressed to 37% of its initial volume at the highest pressure. The rare earth crystal structure sequence hcp?→?Sm?type?→?dhcp?→?mixed(dhcp?+?fcc)?→?distorted fcc (dfcc) is observed in yttrium below 50?GPa. The dfcc (hR24) phase has been observed to persist in the pressure range of 50-95?GPa. A structural transition from dfcc to a low symmetry phase has been observed in yttrium at 99?±?4?GPa with a volume change of -?2.6%. This low symmetry phase has been identified as a monoclinic C2/m phase, which has also been observed in other rare earth elements under high pressures. The appearance of this low symmetry monoclinic phase in yttrium shows that its electronic structure under extreme conditions resembles that of heavy rare earth metals, with a significant increase in d-band character of the valence electrons and possibly some f-electron states near the Fermi level.     

Thermoelectric power of cerium up to 6 GPa

The thermoelectric power (TEP) of cerium has been measured up to 6 GPa. The results have been interpreted using the theories developed by Blandin et. al. and Hirst.     

Phase transitions in titanium diselenide intercalated with cobaltocene at high pressures of up to 20 GPa

The pressure dependences of the kinetic properties of TiSe₂ intercalated with cobaltocene (biscyclopentadienyl-cobalt, CoCp ₂) and cobaltocene itself were measured. Anomalies are revealed in the pressure dependences of the resistance and thermopower (for the intercalated material), which are interpreted as pressure-induced structural phase transitions. The main effect of pressure on the molecules of cobaltocene as an individual compound and in the intercalated state is suppression of the intramolecular motion in the form of rotation of the cyclopentadienyl rings. The general character of the pressure effect on the kinetic properties of (CoCp ₂) _x TiSe₂ agrees well with the assumption that the bonding of the organic molecules to the host lattice is covalent.     

Structural phase transitions in Si and Ge under pressures up to 50 GPa

The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa. Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa, and to hcp (Si-VII) around 40 GPa. In contrast, Ge remains in the β-tin structure up to at least 51 GPa. The pseudopotential method reproduces these differences in the high-pressure behavior of Si and Ge.     

Structural transitions in hydrogen and deuterium at ultrahigh pressures

Abstract

Raman measurements indicate that normal hydrogen and deuterium compressed in a diamond-anvil cell at 77 K undergo phase transformations at 145 (±l5) GPa and 190 (±l20) GPa, respectively. The new observations reported for deuterium indicate that the transition in both isotopes is a structural change not associated with a simple rotational ordering mechanism. The possibility of a structural transition driven by band-gap closure is discussed.     

Shock-induced phase transitions in the MgO-FeO system to 200 GPa

We report new shock-compression data for single-crystal MgO at 114 and 192 GPa. Our data together with the existing shock-wave data revealed a volume discontinuity at 170±10 GPa along with the MgO Hugoniot. The discontinuity gives a volume increase of 1.9%, indicating a possible phase transition from a rock-salt structure (B1) to a high-temperature phase along with the MgO Hugoniot. We re-examined the Hugoniot data on polycrystalline sample (Mg_0.6, Fe_0.4)O up to 200 GPa [M.S. Vassiliou, T.J. Ahrens, The equation of state of Mg_0.6Fe_0.4O to 200 GPa, Geophys. Res. Lett. 9 (1982) 127-130], which showed similar discontinuity with a 2.2% volume increase at 135±10 GPa. Our results add to fundamental understandings of the behavior of MgO and the lower mantle mineral magnesiowüstite (Mg, Fe)O at ultrahigh pressure and temperature.     

Structure phase transformation and equation of state of cerium metal under pressures up to 51 GPa

This study presents high pressure phase transitions and equation of states of cerium under pressures up to 51 GPa at room temperature. The angle-dispersive x-ray diffraction experiments are carried out using a high energy synchrotron x-ray source. The bulk moduli of high pressure phases of cerium are calculated using the Birch–Murnaghan equation. We discuss and correct several previous controversial conclusions, which are caused by the measurement accuracy or personal explanation. The c/a axial ratio of ε-Ce has a maximum value at about 29 GPa, i.e., c/a ≈ 1.690.     

Thermo-and galvanomagnetic properties of lead chalcogenides at high pressures up to 20 GPa

The Nernst-Ettingshausen (NE) effect in the initial NaCl and high-pressure GeS phases was studied at a high pressure P for n-PdTe, p-PbSe, and p-PbS to estimate the mobility µ and the charge-carrier scattering parameter r. It was found that the transverse and longitudinal NE effects in PbTe and PbSe increase with pressure, indicating the transition to the gapless state near P≈3 GPa. The sign of the transverse NE effect changes because of the change in the electron scattering mechanism in the GeS phase. The experimentally observed weakening of the NE and magnetoresistance effects at high P gives evidence for the indirect energy gap E_g in the high pressure phases with GeS structure.     

Electrical properties of (PBS)0·59TiS2 crystals at high pressures up to 20GPa

Abstract

The measurements of thermoelectric power S and resistance p at high pressure synthetic diamond anvils cell were performed for (PbS)0·59TiS2 and TiS2 crystals. The phase transition was found at P?;2GPa accompanied by descend of ρ and |S| for (PbS)o·59TiS2. This transition is connected with structural change of PbS fragment from pseudocubic cell to orthorombic one and as consequence, with change of the electron concentration in Tis2-layers. From the electronic structure calculations for TiS2, the semiconductor-metal transition occurs at pressure P ≥ 4 GPa. Experimentally at this pressure range the decrease of ρ(P) was observed for (PbS)0·59TiS2 crystals.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Thermoelectric properties of TmTe under pressure up to 20 GPa

The effect of quasi-hydrostatic pressure up to 20 GPa on thermopower of single-crystal samples of thulium monotelluride and monosulfide was studied. The pressure dependences of the thermopower of thulium monotelluride were found to have features in pressure regions of 2 and 6 GPa, which can be associated with phase transitions to the cation mixed-valence state and tetragonal structure, respectively. The upper pressure boundary of stability of the thulium ion mixed-valence state in monotelluride was evaluated by comparing the pressure dependences of the thermopower of TmTe and TmS.     

Raman and mid-infrared spectroscopic study of the phase transitions in octafluoronaphthalene at elevated pressures

Raman and mid-infrared spectra of C₁₀F₈ have been obtained under hydrostatic pressures up to 17 kbar in a diamond anvil cell. The C₁₀F₈ I–II phase change, previously observed by neutron diffraction at about 0.8 kbar, has been confirmed. No evidence was found to support the existence of a furtt ier phase change between 4 and 6 kbar indicated by the neutron work, although this is certainly not precluded as the extra spectral features expected in this case are extremely small. The mode Grüneisen parameters, γ_i, allow a clear distinction between internal and external molecular modes, and scale roughly in accord with Zallen's relation γ_i$\text{\$}\text{?}$v_i^?2.