Raman and X-Ray Investigation of Pyrope Garnet （Mg0.76Fe0.14Ca0.10）3Al2Si3O12 under High Pressure

The compressional behavlour of natural pyrope garnet is investigated by using angle-dlspersive synchrotron radiation x-ray diffraction and Raman spectroscopy in a diamond anvil cell at room temperature. The pressureinduced phase transition does not occur under given pressure. The equation of state of pyrope garnet is determined under pressure up to 25.3 GPa. The bulk modulus KTO is 199 GPa, with its first pressure derivative K′TO fixed to 4. The Raman spectra of pyrope garnet are studied. A new Raman peak nearly at 743 cm^-1 is observed in a bending vibration of the SiO4 tetrahedra frequency range at pressure of about 28 GPa. We suggest that the new Raman peak results from the lattice distortion of the SiO4 tetrahedra. All the Raman frequencies continuously increase with the increasing pressure. The average pressure derivative of the high frequency modes （650-1000 cm^-1） is larger than that of the low frequency （smaller than 650 cm^-1）. Based on these data, the mode Grǖneisen parameters for pyrope are obtained.     

Phase Transition in CCl4 under Pressure: a Raman Spectroscopic Study

High-pressure Raman studies at room temperature are performed on CCl₄ up to 13GPa. The Raman bands of the internal modes (v₂, v₄ and v₁) show entirely positive pressure dependence. The slopes dω/dP of the internal modes exhibit two sudden changes at 0.73GPa and 7.13GPa, respectively. A new lower frequency mode (225cm^-1) appears at 3.03GPa, and the splitting of v₂, ν₃ and v₄ occurs at about 7.13GPa. Moreover, Raman spectra of Fermiresonance show that the relative position of the v₁ + v₄ combination and the ν₃ fundamental firstly interchanges corresponding to that at ambient pressure, then the v₁ +v₄ combination disappears in the gradual process of compression. It is indicated that the pressure-induced phase transition from CCl₄ II to CCl₄ III occurs at 0.73GPa, and CCl₄ III undergoes a transition to CCl₄ IV below 3.03GPa. Further CCl₄ IV transforms in a new high-pressure phase at about 7.13GPa, and the symmetry of the new high-pressure phase is lower than that of CCl₄ IV. All the transitions are reversible during decompression.     

Electron Transport Property of CdTe under High Pressure and Moderate Temperature by In-Situ Resistivity Measurement in Diamond Anvil Cell

In situ resistivity measurement has been performed to investigate the electron transport property of powered CdTe under high pressure and moderate temperature in a designed diamond anvil cell. Several abnormal resistivity changes can be found at room temperature when the pressure increases from ambient to 33 GPa. The abnormal resistivity changes at about 3.8 GPa and 10 GPa are caused by the structural phase transitions to the rock-salt phase and to the Cmcm phase, respectively. The other abnormal resistivity changes at about 6.5 GPa, 15.5 GPa, 22.2 GPa and about 30 GPa never observed before are due to the electronic phase transitions of CdTe. The origin of the abnormal change occurred at about 6.5 GPa is discussed. The temperature dependence of the resistivity of CdTe shows its semiconducting behaviour at least before 11.3 GPa.     

Raman Scattering Spectroscopy of Phase Transition in n-Pentadecane under High Temperature and High Pressure

The Raman spectroscopy of n-pentadecane is investigated in a moissanite anvil cell at normal temperatures and a diamond anvil cell under pressure to about 3000MPa and at temperature from 298 to 573K. Result indicates that at room temperature the vibration modes, assigned to the symmetric and asymmetric stretching of CH3 and CH2 stretching, shift to higher frequency and display a pressure dependent quasi-linear curve. A liquid-solid phase transition appears at a pressure of 150 MPa. The high temperature solidus line of n-pentadecane follows a quadratic function of P = 0.02369T^2 - 9.117T ＋ 725.58, in agreement with previous conclusion derived from studies of other hydrocarbons. Upon phase transition, fitting the experimental data obtained in a temperature range of 283 553 K to the Clausius-Clapeyron equation allows one to define the thermodynamic parameters of n-pentadecane of dP/dT = 0.04738T - 9.117.     

First-Principles Calculations of Phase Transition and Stability of Si2CN4 under High Pressure

A pressure-induced phase transition and stability in Si2 CN4 polymorphs under high pressure are studied by firstprinciples calculations. The result shows that the phase transition pressure of α- and β-Si2 CN4 to the cubic spinal phase is 29.9 GPa and 27.5 GPa predicted by thermodynamic method respectively. Under ambient condition, all of the three Si2CN4 polymorphs are metastable with positive formation enthalpy. Unlike the stability of Si3N4 polymorphs, α-Si2 CN4 is more stable than the β phase.     

Improvement of Atomic-Layer-Deposited Al2O3/GaAs Interface Property by Sulfuration and NH3 Thermal Nitridation

Fermi level pinning at the interface between high-h gate dielectric and GaAs induced by unstable native oxides is a major obstacle for high performance GaAs-based metal-oxide-semiconductor （MOS） devices. We demonstrate the improved Al2O3/GaAs interracial characteristics by （NH4）2S immersion and NH3 thermal pretreatment prior to A1203 deposition. X-ray photoelectron spectroscopy （XPS） analysis confirms that sulfuration of GaAs surface by （NH4 ）2S solution can effectively reduce As-O bonds while Ga-O bonds and elemental As still exist at Al2O3 /GaAs interface. However, it is found that N incorporation during the further thermal nitridation on sulfurated GaAs can effectively suppress the native oxides and elemental As in the sequent deposition of Al2O3. Atomic force microscopy （AFM） shows that the further thermal nitridation on sulfurated GaAs surface can also improve the surface roughness.     

Isothermal equation of state for the skutterudites CoSb3 and LaFe3CoSb12

The thermoelectric materials CoSb₃ and LaFe₃CoSb₁₂ with skutterudite structure were subjected to high pressures using a diamond anvil high-pressure cell up to 20 GPa. Energy-dispersive X-ray diffraction was used to determine the dependence of the lattice parameter on pressure. No major change in the X-ray diffraction spectra was observed for both compounds, constituting evidence that both compounds are stable within this pressure range, despite their relatively open structures. Three distinct isothermal equations of state for solids under high pressure were fitted to the experimental data to determine the bulk modulus for both compounds. The filled skutterudite showed a greater compressibility than the unfilled one, this difference can be understood in terms of the larger lattice parameter of the former.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

Phase Transition of FeS in Terms of In-Situ Resistance Measurement

Electrical properties of stoichiometric iron sulfide （FeS） are investigated under high pressure with a designed diamond anvil cell. The process of phase transition is reflected by changing the electrical conductivity under high pressure, and the conductivity of FeS with the NiAs structure is found to be much smaller than other phases. Two new phase transitions without structural change are observed at 34.7 GPa and 61.3 GPa. The temperature dependence of the conductivity is found to be similar to that of a semiconductor when the pressure is higher than 35 GPa     

High-Pressure and High-Temperature Behaviour of Gallium Oxide

High-temperature and high-pressure behaviours of β-Ga2O3 powder are studied by energy-dispersive x-ray diffrac- tion in a diamond anvil cell （DAC）. It is found that the phase transition from the monoclinic β-Ga2O3 to the trigonal α-Ga2O3 occurs at around 19.2 GPa under cold compression. By heating the powder to 2000 K at 30 GPa, we confirm that α-Ga2O3 is the most stable structure at the high pressure. Furthermore, the structural transition from β-Ga2O3 to α-Ga2O3 is irreversible. After laser heating, the recrystallized Ga2O3 has a preferable （012） orientation. This interesting behaviour is also discussed.     

High pressure electrical transport measurements of Cs2MoS4 and KTbP2Se6 and X-ray diffraction study of Cs2MoS4

We report the results of electrical resistance measurements at high pressures on Cs₂MoS₄ and KTbP₂Se₆. The results of high pressure X-ray diffraction study of Cs₂MoS₄ are also presented. Interestingly, in the case of Cs₂MoS₄ the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct-indirect band crossing. In the case of KTbP₂Se₆,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs₂MoS₄.     

A Simple System to Measure Superconducting Transition Temperature at High Pressure

A simple hydride system is fabricated to measure the superconducting transition temperature 1c unaer hign pressure using a diamond anvil cell （DAC）. The system is designed with centrosymetric coils around the diamond that makes it easy to keep balance between the pick-up coil and the inductance coil, while the superconducting states can be modulated with a low-frequency small external magnetic field. Using the device we successfully obtain the Tc evolution as a function of applied pressure up to 10 GPa for YBa2 Cu3 O6＋δ superconductor single crystal.     

Ionic conductivity of NTE materials c- Zr1−xErxW2O8−x/2

It is found that the c- Zr_1−xEr_xW₂O_8−x/2 solid solutions which are well known to have isotropic NTE properties clearly exhibit oxygen-ionic conductance and their ionic conductivities are measured to be about 10⁻⁴ S cm⁻¹ at 673 K, which is comparable to that of ceria-based solid electrolytes.     

Thermal Expansion Anomaly of Tb2Fe14Cr3 Compound

We investigate the thermal expansion property of the Tb₂Fe₁₄Cr₃ compound by means of x-ray diffraction. The result shows that the Tb₂Fe₁₄Cr₃ compound has a hexagonal Th₂Ni₁₇-type structure. Negative thermalexpansion is found in the Tb₂Fe₁₄Cr₃ compound from 296 to 493K by x-ray dilatometry. The coefficient of the average thermal expansion is α=-2.82 × 10^-5K^-1. In the temperature range 493--692K, the coefficient of the average thermal expansion is α=1.59 × 10^-5K^-1. The physical mechanism of thermal expansion anomaly of the Tb₂Fe₁₄Cr₃ compound is discussed according to the temperature dependence of magnetization measured by a superconducting quantum interference device.     

Pressure-induced structural phase transition in rare-earth trihydrides. Part II. SmH3 and compressibility systematics

High-pressure studies of samarium trihydride have been performed in a diamond anvil cell at pressures up to 30 GPa at room temperature. As a result, a reversible structural phase transformation from the hexagonal to the cubic phase has been observed. The transition pressure and the lattice parameter follow the tendency of the other lanthanide trihydrides investigated earlier [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 477; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 481]. The compressibility systematics of lanthanide and yttrium trihydrides has been established.     

Axial ratio anomalies and electronic topological transitions in Cd0.80Hg0.20 at high pressures

High-pressure angle-dispersive X-ray diffraction measurements show that Cd_0.80Hg_0.20 alloy remains in the hcp structure up to 50 GPa. We observe subtle anomalies in the pressure variation of the lattice parameters and their ratio, and in normalized stress versus strain. Electronic-structure calculations, as well as experimental and theoretical results for Cd, suggest that these anomalies are related to the occurrence of electronic topological transitions. Our results support Lifshitz's prediction that electronic phase transitions can cause anomalies in structural and elastic properties of materials.     

Charge and orbital ordering in Na0.5CoO2

The ground state of Na_0.5CoO₂ has been calculated using the full potential local orbital method and local density approximation plus Hubbard U (); the results demonstrate that charge and orbital ordering evidently exist in the present system in association with the antiferromagnetic state. Notable structural features observed between 300 and 30 K have been carefully examined using in situ TEM investigations, a superstructure with a wave vector of Q1=a^∗/2, becoming commonly visible below , can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q2=a^∗/4 below the phase transition of , which suggests a more complicated orbital ordered state existing at lower temperatures.     

High-Pressure Phase Transition in CTAB-Micellar Solutions： A Raman Spectroscopic Study

We use a diamond anvil cell for the first time to investigate the Raman spectra of an aqueous micellar solution of hexadecyltrimethylammonium bromide （CTAB） at pressures up to 3.85 GPa. The pressure-induced phase transition between the micellar and coagel phases is found to occur at 0.64 GPa and 60℃. This phase transition has a pressure hysteresis, and thus exhibits the first-order phase transition properties. Further experimental results show that although the structure of the coagel phase is similar to that of the CTAB crystal, the interchain distance is slightly larger in the coagel phase than that in the CTAB crystal.     

Electronic Structures of PbMoOa Crystals with F-Type Colour Centres

Electronic structures of PbMoO4 crystals containing F-type colour centres with the lattice structure optimized are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational （DV-Xa） method. The calculated results show that F and F＋ centres have donor energy levels in the forbidden band. The optical transition energies are 2.166eV and 2.197eV, respectively, corresponding to the 580nm absorption bands in PbMoO4 crystal. The 580nm absorption band in PbMoO4 is originated from the F-type colour centres.