High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

High pressure effects on the electrical resistivity behavior of the Kondo lattice, YbPd2Si2

We report the influence of external high-pressure (P up to 8 GPa) on the temperature (T) dependence of electrical resistivity (ρ) of a Yb-based Kondo lattice, YbPd₂Si₂, which does not undergo magnetic ordering under ambient pressure condition. There are qualitative changes in the ρ(T) behavior due to the application of external pressure. While ρ is found to vary quadratically below 15 K (down to 45 mK) characteristic of Fermi-liquids, a drop is observed below 0.5 K for P=1 GPa, signaling the onset of magnetic ordering of Yb ions with the application of P. The T at which this fall occurs goes through a peak as a function of P (8 K for P=2 GPa and about 5 K at high pressures), mimicking Doniach's magnetic phase diagram. We infer that this compound is one of the very few Yb-based stoichiometric materials, in which one can traverse from valence fluctuation to magnetic ordering by the application of external pressure.     

Structural stability of tin clathrates under high pressure

High pressure Raman scattering experiments have been performed for Rb₈Sn₄₄□₂ in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization.     

Heat capacity of CeIrSi3 under pressure

We measured the heat capacity of CeIrSi₃ (100 mK<T<6 K) under high pressure up to P=1.38 GPa. The measurements have been used a quasiadiabatic method utilizing a CuBe piston-cylinder pressure cell in a dilution refrigerator. At 0 GPa, a sharp anomaly which indicates the antiferromagnetically transition is observed at T_N=5 K. T_N decreases monotonically with increasing pressure up to P=1.38 GPa. The magnetic entropy is released below T_N only 19% of R ln 2 at 0 GPa. And the magnetic entropy decreases with increasing pressure up to 1.38 GPa, 64% compared to that at 0 GPa.     

Electrical properties of TlGaTe2 single crystals under hydrostatic pressure

The effect of hydrostatic pressure (up to 0.82 GPa) on the electric properties of chain TlGaTe₂ single crystals has been investigated in the temperature range 77-296 K. It has been shown that pressure leads to a considerable increase of conductivity (σ_⊥) across the chains of TlGaTe₂ single crystals. Parameters of localized states in the band gap of TlGaTe₂ single crystal according to the low-temperature electrical measurements were obtained at various pressures.     

Structural and electronic properties of type-I and type-VIII Ba8Ga16Sn30 clathrates under compression

The total energy and electronic structures for type-I (β phase) and type-VIII (α phase) Ba₈Ga₁₆Sn₃₀ clathrates under hydrostatic pressure have been investigated using density functional theory (DFT) calculations. It was found that the type-VIII phase is more stable than the type-I one at ambient conditions and that β→α phase transition can not occur under hydrostatic pressure. The band structures show that the type-I and type-VIII Ba₈Ga₁₆Sn₃₀ are indirect semiconductors with band gaps of 0.24 eV and 0.19 eV, respectively. The results suggested that type-I clathrate Ba₈Ga₁₆Sn₃₀ has a larger value of the thermoelectric (TE) power than that of type-VIII clathrate. We found that pressure tuning changes the k-point of conduction band minimum (CBM) in the Brillouin zone for β-phase, but it is not the case for α-phase. Furthermore, the results show that the pressure can change the interaction between guest atoms and the host lattice, and consequently results in the decrease of the band gap of β-phase and the increase of the band gap of α-phase, indicating that the pressure effect can play an important role in the magnitude of the TE power.     

In-situ control of different oxygen fugacity experimental study on the electrical conductivity of lherzolite at high temperature and high pressure

At pressure 1.0-4.0 GPa and temperature 1073-1423 K and under the control of oxygen fugacity (Mo+MoO₂, Fe+FeO and Ni+NiO), a YJ-3000t multi-anvil solid high-temperature and high-pressure apparatus and Solartron-1260 impedance/Gain-Phase analyzer were employed to analyze the electrical conductivity of lherzolite. The experimental results showed that: (1) within the range of the selected frequencies (10³-10⁶ Hz), either as viewed from the relationship between the real or imaginary part of complex impedance and the frequency, or from the relationship between modulus, phase angle and frequency, it can be seen clearly that the complex impedance has a strong dependence on frequency; (2) with the rise of temperature (T), the electrical conductivity (σ) increased, and Lg σ and 1/T follows the Arrhenius relationship; (3) with the rise of pressure, the electrical conductivity decreased, and activation enthalpy and temperature-independent pre-exponential factor decreased as well. And the activation energy and activation bulk volume of the main charge carrier in the lherzolite have been obtained for the first time, which are 1.68±0.02 eV and 0.04±0.01 cm³/mol, respectively; (4) under the given pressure and temperature, the electrical conductivity tends to increase with increasing oxygen fugacity, and under the given pressure, the activation enthalpy and pre-exponential factor tend to decrease with the rise of oxygen fugacity; (5) at 2.0 GPa and the control of the three solid buffers, Mo+MoO₂, Fe+FeO and Ni+NiO, the exponential factors of electrical conductivity variation range with oxygen fugacity are , and the theoretical model for the relationship between the electrical conductivity of lherzolite and the oxygen fugacity under high pressure has been established for the first time; (6) the electrical conduction mechanism of small polarons provides a reasonable explanation to the variation of conductivity of lherzolite with oxygen fugacity.     

Pressure-induced transition in a heavy fermion YbPd2Si2

We report the results of a room-temperature investigation of the thermoelectric and the dilatometric properties of a heavy fermion system YbPd₂Si₂ (itterbium-palladium-silicon, 1-2-2) at high pressure P up to 22 GPa; YbPd₂Si₂ is a less-studied representative of the RM₂X₂ family (R=Ce, Yb, U; M=transition metal; X=Si, Ge) with the tetragonal ThCr₂Si₂-type structure of the I4/mmm space group. Around P∼6±0.5 GPa, a phase transition in Yb-Pd-Si was registered by the drastic changes in the pressure dependencies of the electrical resistance R, the thermopower (Seebeck effect) S, a temperature difference along a sample ΔT, and a sample's thickness Δx (related to compressibility). Both a nature of the found phase transition and a presumable P-T phase diagram of YbPd₂Si₂ are discussed.     

High pressure X-ray diffraction and electrical resistance study of the quasi-one-dimensional sulfide InV6S8

The ambient structural details and the results of room temperature high pressure angle dispersive X-ray diffraction and electrical resistance measurements on the quasi-one-dimensional sulfide, InV₆S₈, to a pressure of 25 GPa are reported. The material does not undergo a phase transition in this pressure range, though an anomaly in the c/a ratio has been observed around 10 Gpa. A fit of the Murnaghan equation of state to the V/V₀ versus pressure data, with the value of the derivative of B₀ with respect to pressure, B′₀, fixed at 4 has yielded a value of the bulk modulus, B₀, of 110 GPa. We also present data of the pressure dependence of the lattice constants, a and c, the ratio c/a, and the resistance at room temperature.     

Manifestation of MnCl2 fillers incorporated into the polymer matrices in their dielectric properties

Polyethyl methacrylate (PEMA) films filled with different mass fractions of MnCl₂ were prepared using a casting method. The structural and electrical properties were studied. The filling content dependence of certain IR absorption bands was correlated with the obtained physical parameter characterizing the other properties. DC electrical resistivity (ρ) was measured in the temperature range 340-420 K for PEMA films filled with MnCl₂ fillers. An intrachain one-dimensional interpolaron hopping mechanism was assumed to interpret the electrical conduction. AC conductivity behavior of all the prepared samples was investigated over the frequency range (42-5M) Hz and under different isothermal stablilization in the temperature range 300-423 K. It suggested that the hopping mechanism might be playing an important role in the conduction process, in low temperature regime. The values of σ₀, A, and S satisfying the suitable fit of the conductivity data, as well as the corresponding (σ_DC).     

Structural,mechanical and thermal properties of some Holmium Pnictides under pressure: A theoretical approach

The high pressure structural, elastic and thermal properties of holmium pnictides HoX (X=N, P, As and Bi) were investigated theoretically by using an inter-ionic potential theory with modified ionic charge parameter. We have predicted a structural phase transition from NaCl (B₁) to CsCl (B₂)-type structure at pressure of 139 GPa for HoN, 52 GPa for HoP, 44 GPa for HoAs and 26 GPa for HoBi. Other properties, such as lattice constant, bulk modulus, cohesive energy, second and third-order elastic constants were calculated and compared with the available experimental and theoretical data. In order to gain further information the brittle behaviour of these compounds was observed. Some other properties like Shear modulus (G), Young's modulus (E), Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θ_D) were calculated. The variation of elastic constants (C₁₁ and C₄₄) and Debye temperature (θ_D) with pressure was also presented.     

Phase transition of cadmium fluoride under high pressure

We investigated the high pressure phases of CdF₂ by a joint theoretical and experimental study. The structural and electronic properties of CdF₂ were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF₂ under high pressure.     

Single-crystal elastic constants of magnesium difluoride (MgF2) to 7.4 GPa

The six independent elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, and C₆₆) of single-crystal MgF₂ in the rutile structure have been measured by Brillouin spectroscopy at room temperature from ambient conditions to 7.4 GPa. Measurements were performed on two monocrystals with perpendicular faces, (001) and (100). A quasi-linear fit from finite strain theory was applied to the experimental data revealing the pressure dependence of the six elastic constants of MgF₂. The shear modulus C_S=1/2(C₁₁−C₁₂), and the aggregate shear (Voigt–Reuss–Hill) modulus G show a softening with increasing pressure, indicating the approach of the rutile-to-CaCl₂-type structural phase transition at P~9 GPa. The adiabatic bulk modulus (Reuss average) and its pressure derivative have been determined: K_0S=105.1±0.3 GPa, (∂K_0S/∂P)_T=4.14±0.05. The pressure–volume equation of state of MgF₂ was computed self-consistently from the Brillouin data. Our results are in good agreement with X-ray diffraction data. As the phase transition is approached, MgF₂ becomes strongly anisotropic and develops partially auxetic behavior (a negative Poisson's ratio in certain directions).     

The optical properties of HgS under high pressures

The DOS (density of states) and the optical properties of HgS under high pressure are studied with the first-principle computations. The change of the imaginary part, ε₂(ω), of the dielectric function shows that HgS tends to metallization with increasing pressure, and this well agrees with the band gap calculations and the conductivities measurement results in the previous work. Under the pressures below 15 GPa, ε₂(ω) is relatively anisotropic and tends to be more anisotropic with increasing pressure; while under the pressures above 15 GPa, the anisotropy decreases and finally becomes almost absolutely isotropic after the phase transition. The behavior of ε₂(ω) is strongly related to the structure change in the cinnabar to rocksalt phase transition process under high pressure.     

In situ high-pressure X-ray diffraction study of densification of a molecular chalcogenide glass

Structural mechanisms of densification of a molecular chalcogenide glass of composition Ge_2.5As_51.25S_46.25 have been studied in situ at pressures ranging from 1 atm to 11 GPa at ambient temperature as well as ex situ on a sample quenched from 12 GPa and ambient temperature using high-energy X-ray diffraction. The X-ray structure factors display a reduction in height of the first sharp diffraction peak and a growth of the principal diffraction peak with a concomitant shift to higher Q-values with increasing pressure. At low pressures of at least up to 5 GPa the densification of the structure primarily involves an increase in the packing of the As₄S₃ molecules. At higher pressures the As₄S₃ molecules break up and reconnect to form a high-density network with increased extended-range ordering at the highest pressure of 11 GPa indicating a structural transition. This high-density network structure relaxes only slightly on decompression indicating that the pressure-induced structural changes are quenchable.     

Chemical diffusion in molybdenum disulphide

Ceramic molybdenum disulphide (MoS₂) was equilibrated at an ambient sulphur vapour partial pressure p(S₂), 10 Pa<p(S₂)<1000 Pa. After the step change of p(S₂) to a new value, the equilibration kinetics was monitored by measuring electrical conductivity. The application of the solution of Fick's second law (with the initial condition: no concentration gradient in specimen and the boundary condition: surface concentration constant) to the kinetic data gave the chemical diffusion coefficient. The chemical diffusion coefficient, D_chem, determined at 1273 K, was D_chem=(3.20±0.32)*10⁻⁷ cm² s⁻¹ and was found to be independent of sulphur vapour partial pressure. The usefulness of transient electrical conductivity method for determining real values of diffusion data was discussed in terms of defect structure of the studied material.     

Optical and electrical properties of thermally evaporated In49Se48Sn3 films

Nearly stoichiometric thin films of In₄₉Se₄₈Sn₃ were deposited at room temperature, by conventional thermal evaporation of the presynthesized materials, onto precleaned glass substrates. The microstructural studies on the as-deposited and annealed films, using transmission electron microscopy and diffraction (TEMD), revealed that the as-deposited films are amorphous in nature, while those annealed at 498 K are crystalline. The optical properties of the investigated films were determined from the transmittance and reflectance data, in the spectral range 650-2500 nm. An analysis of the optical absorption spectra revealed a non-direct energy gap characterizing the amorphous films, while both allowed and forbidden direct energy gaps characterized the crystalline films. The electrical resistance of the deposited films was carried out during heating and cooling cycles in the temperature range 300-600 K. The results show an irreproducible behavior, while after crystallization the results become reproducible. The analysis of the temperature dependence of the resistance (ln(R) vs. 1000/T) for crystalline films shows two straight lines corresponding to both extrinsic and intrinsic conduction. The room temperature I-V characteristics of the as-deposited films sandwiched between similar Ag metal electrodes shows an ohmic behavior, while non-ohmic behavior attributed to space charge limited conduction has been observed when the films are sandwiched between dissimilar Ag/Al metal electrodes.     

Epitaxial growth of Ba8Ga16Ge30 clathrate film on Si substrate by RF helicon magnetron sputtering with evaluation on thermoelectric properties

Epitaxial Ba₈Ga₁₆Ge₃₀ clathrate thin films were successfully grown on Si substrate by using helicon magnetron sputtering. The (1 0 0) lattice of Ba₈Ga₁₆Ge₃₀ was identified grown on four Si(2 0 0) lattices in small mismatch (0.1%). Both the color of samples and XRD results suggest 600 °C is the optimal substrate temperature for the growth of high quality Ba-Ga-Ge clathrate film on Si substrates. High Seebeck coefficients and electrical resistivities for the deposited clathrate thin films in comparison with those of bulk are obtained. The high crystal quality and thermionic effects in heterostructures may contribute to the larger Seebeck coefficients, while the increasing of interface scattering for electrons probably is the reason for large electrical resistivities. Although the thermoelectric (TE) results are not ideal as designed, our results are significant due to the first successful work on epitaxial growth of Ba₈Ga₁₆Ge₃₀ clathrate thin films on Si substrate with large Seebeck coefficient.     

The band gap and electrical resistivity of FeS2-pyrite at high pressures

The indirect energy gap and electrical resistivity of FeS₂-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS₂ is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10⁻² eV/GPa. If this linear trend continues, FeS₂ is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.