High pressure studies on the electrical resistivity of As–Te–bond Si glasses and the effect of network topological thresholds

The variation of resistivity in an amorphous As₃₀Te_70?x Si _x system of glasses with high pressure has been studied for pressures up to 8 GPa. It is found that the electrical resistivity and the conduction activation energy decrease continuously with increase in pressure, and samples become metallic in the pressure range 1.0–2.0 GPa. Temperature variation studies carried out at a pressure of 0.92 GPa show that the activation energies lie in the range 0.16–0.18 eV. Studies on the composition/average co-ordination number ? r? dependence of normalized electrical resistivity at different pressures indicate that rigidity percolation is extended, the onset of the intermediate phase is around ? r?=2.44, and completion at ? r?=2.56, respectively, while the chemical threshold is at ? r?=2.67. These results compare favorably with those obtained from electrical switching and differential scanning calorimetric studies.     

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D₂O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν₁ (A_1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D₂O ice VII.     

Magnetic ordering in Fe<Subscript>1.087</Subscript>Te under pressure

The crystal and magnetic structures of Fe_1.087Te have been studied by neutron powder diffraction in the temperature range from 1.7 to 80 K at pressures of  ≈0.4 and ≈1.2 GPa. No symmetry change of the tetragonal paramagnetic ambient pressure phase (space group P4/nmm) was observed for temperatures above 60 K and pressures up to  ≈1.2 GPa. A novel pressure-induced phase of Fe_1.087Te having orthorhombic symmetry (space group Pmmn) and incommensurate antiferromagneticbicollinear order was observed in the temperature range from 50 to 60 K at  ≈1.2 GPa. The known monoclinic ambient pressure phase of Fe_1.087Te (space group P2 ₁/n) with commensurate antiferromagnetic order was found to be stable up to at least  ≈1.2 GPa at low temperature.     

In situ observation of pressure-induced electrical resistance changes in zirconium: pressure calibration points for the large volume press at 8 and 35 GPa

Simultaneous in situ pressure–resistance measurements were carried out up to 40 GPa using a multianvil apparatus with synchrotron-based X-ray diffraction (XRD) measurements. Pressure-induced electrical resistance changes in zirconium were measured at ambient temperatures and two discontinuities were observed around the α–ω and ω–β structural phase transitions. The transition pressures were strictly determined from simultaneous measurements of the electrical resistance and in situ XRD as 7.96±0.16 and 34.5±0.3 GPa, respectively, using an equation of state for gold as the pressure scale. The precisely determined transition pressures are available for room temperature pressure calibration points for large volume presses installed at offline laboratories.     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Distortion of alpha-uranium structure in praseodymium metal to 311 GPA

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V/V ₀?=?0.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P2₁2₁2₁ phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b-axis and the c-axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures.     

Magnetic ordering temperatures in rare earth metal dysprosium under ultrahigh pressures

Magnetic ordering temperatures in heavy rare earth metal dysprosium (Dy) have been studied using an ultrasensitive electrical transport measurement technique in a designer diamond anvil cell to a pressure of 69 GPa and a temperature of 10 K. Previous studies using magnetic susceptibility measurements at high pressures were able to track magnetic ordering temperature only till 7 GPa in the hexagonal close packed (hcp) phase of Dy. Our studies indicate that the magnetic ordering temperature shows an abrupt drop of 80 K at the hcp-Sm phase transition followed by a gradual decrease that continues till 17 GPa. This is followed by a rapid increase in the magnetic ordering temperatures in the double hcp phase and finally leveling off in the distorted face centered cubic phase of Dy. Our studies reaffirm that 4f-shell remains localized in Dy and there is no loss of magnetic moment or 4f-shell delocalization for pressures up to 69 GPa.     

Compressibility determination by electrical resistance measurement: a universal method for both crystalline and amorphous solids

A new method is introduced for investigating the compressibility of solids under high pressure by in situ electrical resistance measurement of a manganin wire, which is wrapped around the sample. This method does not rely on the lattice parameters measurement, and the continuous volume change of the sample versus pressure can be obtained. Therefore, it is convenient to look at the compressibility of solids, especially for the X-ray diffraction amorphous materials. The I–II and II–III phase transition of Bi accompanying with volume change of 4.5% and 3.5% has been detected using the method, respectively, while the volume change for the phase transition of Tl occurring at 3.67 GPa is determined as 0.5%. The fit of the third-order Birch–Murnaghan equation of state to our data yields a zero-pressure bulk modulus K ₀=28.98±0.03 GPa for NaCl and 6.97±0.02 GPa for amorphous red phosphorus.     

Mechanical anisotropy and thermoacoustic properties of SnO2 under high pressures: an ab initio approach

The effects of hydrostatic pressures on the electronic, thermoacoustic and elastic anisotropies of SnO₂ in the rutile structure is analyzed up to 18 GPa. It is found that the polycrystalline bulk modulus B increases from 227 to 312 GPa between 0 and 18 GPa while the Young and shear moduli slightly decrease with pressures. The resulting polycrystalline ductility increases with pressures. The speed of the sound for longitudinal waves increases with pressure, while the transverse polarizations and the Debye temperature decrease. Large crystal anisotropy for the shear planes {001} between ? 110? and ? 010? directions under pressures, associated with the phase transition to the Cl₂Ca, is found.     

Structural and electrical transport properties of FeH x under high pressures and low temperatures

We present the first successful in situ simultaneous measurement of the electrical resistance and X-ray diffraction of FeH _x (x～ 1) under high-pressure H₂ up to 25.5 GPa and low temperatures down to 9 K. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeH _x (x～ 1) at 3.5 GPa. The ?′-phase of FeH _x was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T ⁵ was found to be better fitting at 25.5 GPa. This change can be considered to be related to the previously reported ferromagnetism collapse at corresponding pressure.     

First-principles study of structural and elastic properties of CrO2 at high pressure

The structural and elastic properties of CrO₂ in the rutile phase under high pressures have been investigated using pseudopotential plane-wave method based on density functional theory. The optimized lattice parameters and the bulk modulus at zero pressure agree well with available experimental and theoretical data. The elastic constants C ₁₁, C ₁₂, C ₄₄, C ₃₃, C ₁₃, and C ₆₆ at zero pressure are calculated to be 359.91, 264.69, 143.28, 309.45, 218.45, and 260.74 GPa, respectively. Elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio under pressures are obtained. Our results indicate that the rutile phase is mechanically stable below 11.99 GPa. The elastic anisotropy of rutile phase under pressures has also been predicted.     

Pressure-induced electronic and structural transformations in bulk GeSe2 glass

The pressure dependence of the electrical resistivity of bulk GeSe₂ glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure havinga = 4·06 A. The electrical conductivity has also been studied as a function of temperature at various pressures.     

Phase transformations in calcite from electrical impedance measurements

The phase transformation in calcite I-IV-V and calcite ? aragonite have been characterized by electrical impedance measurements at temperatures 600–1200°C and pressures 0.5–2.5?GPa in a piston cylinder apparatus. The bulk conductivity σ has been measured from Argand plots in the frequency range 10⁵–10^?2?Hz in an electric cell representing a coaxial cylindrical capacitor. The synthetic polycrystalline powder of CaCO₃ and natural crystals of calcite were used as starting materials. The transformation temperature T_c was identified from resistivity-temperature curves as a kink point of the activation energy. At pressure above 2?GPa in ordered phase calcite I, the activation energy E _σ is c. 1.05?eV, and in disordered phase calcite V E _σ is c. 0.75?eV. The pressure dependence of T_c for the rotational order–disorder transformation in calcite is positive for pressures <1?GPa and negative for pressures >1?GPa. The transformation boundary of calcite 1–IV is observed only during first heating in samples after a long annealing at low temperatures. The activation energy of calcite I???IV decreases gradually from 1.8 to 1.05?eV with the pressure increase from 0.5 to 2?GPa. The kinetics of calcite ? aragonite transformation has been monitored by measuring a time-variation of the electrical resistance of a calcite sample at 10³?Hz in the stability P-T field of aragonite. The variation of the impedance correlates with the degree of phase transformation, estimated from X-ray powder diffraction studies on quenched products of experiments. The kinetics of calcite ? aragonite transformation may be fitted to the Avrami kinetics with the exponent m???1–1.5.     

Electrical property of nanocrystalline γ-Fe2O3 under high pressure

Using a microcircuit fabricated on a diamond anvil cell, in situ conductivity measurements on nanophase (NP) γ-Fe₂O₃ are obtained under high pressure. For NP γ-Fe₂O₃, the abrupt increase in electrical conductivity occurs at a pressure of 21.3 GPa, corresponding to a transition from maghemite to hematite. Above 26.4 GPa, conductivity increases smoothly with increasing pressure. No distinct abnormal change is observed during decompression, indicating that transformation is irreversible. The temperature-dependence of the conductivity of NP γ-Fe₂O₃ was investigated at several pressures, indicating the electrical conductivity of the sample increases with increasing pressure and temperature, and that a remarkable phenomenon of discontinuity occurs at 400 K. The abnormal change is attributed to the electronic phase transitions of NP γ-Fe₂O₃ due to the variation of inherent cation vacancies. Besides, the temperature-dependence of the electrical conductivity displays semiconductor-like behavior before 33.0 GPa.     

Structural stability and electronic properties of AgInS<Subscript>2</Subscript> under pressure

We employ state-of-the-art ab initio density functional theory techniques to investigatethe structural, dynamical, mechanical stability and electronic properties of the ternaryAgInS₂ compoundsunder pressure. Using cohesive energy and enthalpy, we found that from the six potentialphases explored, the chalcopyrite and the orthorhombic structures were very competitive aszero pressure phases. A pressure-induced phase transition occurs around 1.78 GPa from the low pressure chalcopyritephase to a rhombohedral RH-AgInS₂ phase. The pressure phase transition around 1.78 GPa isaccompanied by notable changes in the volume and bulk modulus. The calculations of thephonon dispersions and elastic constants at different pressures showed that thechalcopyrite and the orthorhombic structures remained stable at all the selected pressure(0, 1.78 and 2.5 GPa), where detailed calculations were performed, while the rhombohedralstructure is only stable from the transition pressure 1.78 GPa. Pressure effect on thebandgap is minimal due to the small range of pressure considered in this study. Themeta-GGA MBJ functional predicts bandgaps which are in good agreement with availableexperimental values.     

Structure and magnetism in compressed iron–cobalt alloys

Combined Co K-edge XANES-XMCD and XRD measurements were used to shed light on the magnetic and structural phase diagram of the Fe_1?x Co _x alloy under HP in the Co-rich region (x≥0.5). At 0.5≤x≤0.75, the alloy shows a pressure-induced structural/magnetic phase transition from bcc-FM to hcp-non-FM phase just like pure iron but at higher pressures. The x=0.9 sample has an fcc structure in the pressure range investigated but presents an FM to non-FM transition at P=64 GPa, a significantly lower pressure compared with pure Co (predicted ≈120 GPa), showing that Fe impurities strongly affect the HP Co response.     

High pressure behavior of nano-crystalline CeO2 up to 35 GPa: a Raman investigation

The present paper reports the results of in situ Raman studies carried out on nano-crystalline CeO₂ up to a pressure of 35 GPa at room temperature. The material was characterized at ambient conditions using X-ray diffraction and Raman spectroscopy and was found to have a cubic structure. We observed the Raman peak at ambient at 465 cm^?1, which is characteristic of the cubic structure of the material. The sample was pressurized using a diamond anvil cell using ruby fluorescence as the pressure monitor, and the phase evolution was tracked by Raman spectroscopy. With an increase in the applied pressure, the cubic band was seen to steadily shift to higher wavenumbers. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa. CeO₂ was found to undergo a phase transition to an orthorhombic α -PbCl₂-type structure at this pressure. With the release of the applied pressure, the observed peaks steadily shift to lower wavenumbers. On decompression, the high pressure phase existed down to a total release of pressure.     

Simultaneous electrical and X-ray diffraction studies on neodymium metal to 152 GPa

Using designer diamond anvils and angle dispersive X-ray diffraction technique at a synchrotron source, we have performed simultaneous electrical and structural studies on neodymium metal to 152 GPa in a diamond anvil cell. Four-probe electrical resistance measurement shows a 38% decrease in the electrical resistivity, associated with the delocalization of the 4f-shell electrons, starting at 100 GPa up to a final pressure of 152 GPa. The continuous decrease in electrical resistivity is consistent with the observation of a gradual phase transition to α-U structure in this pressure range. The (1 1 1) diffraction peak of α-U structure first appears at 100 GPa and increases in intensity with increasing pressure to 152 GPa. This increase in intensity is attributed to an increasing volume fraction of α-U phase and an increase in structural y-parameter from 0.07 at 118 GPa to 0.095 at 152 GPa. In contrast to the abrupt pressure-induced f-electron transition seen in cerium and praseodymium, the continuous evolution of α-U structure and electrical resistivity in neodymium confirms the gradual nature of 4f delocalization process in this element.     

Optimization of Paris–Edinburgh press cell assemblies for in situ monochromatic X-ray diffraction and X-ray absorption

We describe some important improvements allowed by the development of new cell assemblies coupled to opposed conical sintered diamond anvils in the Paris–Edinburgh press. We provide X-ray absorption and diffraction experiments carried out at pressures up to 16.5 GPa. The maximum temperature reached was 1800 K for P<10 GPa and 1300 K for higher pressures. The sintered diamond anvils are X-ray transparent and give access to a much larger X-ray window than the tungsten carbide anvils, even at the highest pressure. Therefore, X-ray measurements are performed using in situ cross-calibration simultaneously. We also describe a new heating setup used to reach high temperatures, despite the low conductivity of the sintered diamond core by deviating the electrical current using copper strips. These improvements are illustrated by recent data collected using angle dispersive in situ X-ray diffraction on liquid Fe-18%wt S and using EXAFS at the barium K-edge on Ba₈Si₄₆ silicon clathrates and at the iodine K-edge on iodine-intercalated nanotubes.