First-principles calculations of the structural, electronic, and optical properties of LiF up to 300 GPa

The structural, electronic, and optical properties of LiF are investigated at high pressures using the plane-wave pseudo-potential density functional method (DFT) within the generalized gradient approximation (GGA). From the analysis of Gibbs free energies, we find that no phase transition takes place for LiF in the presented pressure range from 0 to 300 GPa. The result is consistent with the theoretical prediction obtained from the ab initio calculations [N.A. Smirnov, Phys. Rev. B 83 (2011) 014109] that the rock-salt structure is thermodynamically stable up to 1000 GPa. Meanwhile, good agreement between the calculated equation of state parameters and the experimental results is obtained, and a direct energy gap of 8.65 eV is estimated in the DFT-GGA for LiF with rock-salt structure. In addition, the dielectric function and optical properties such as reflectivity, absorption coefficient, and refractive index dependence of the photon energy from 0 to 50 eV and wavelength from 0 to 200 nm at different pressures are also calculated and analyzed. It is found that the rock-salt LiF is transparent from the partially ultra-violet to the visible light area and hardly is the transparence affected by the pressure. Furthermore, the curve of optical spectrum will shift to high energy area with increasing pressure.     

Raman scattering in metals up to 50 GPa

Abstract

Measurements of k ≈ 0 optical phonons by Raman scattering are reported for Zn metal and for metallic high pressure phases of Si and Ge up to 50 GPa. Mode Griineisen parameters are determined and the experimental results are compared with theoretical results.     

Structural changes in perylene from UV Raman spectroscopy up to 1 GPa

Vibrational properties and structural changes under pressure of a highly luminescent molecular organic crystal have been investigated by ultraviolet resonant Raman spectroscopy with a 244‐nm excitation. Resonant Raman modes of α‐perylene crystal up to 1GPa were followed under hydrostatic pressure in an anvil cell with a sapphire window transparent to ultraviolet light. Nonlinear evolution of intra‐molecular modes is induced by pressure. Abrupt shifts of Raman wavenumbers suggest structural and planar modifications of the molecules in the crystal. We interpret these shifts as a first‐order phase transition to a lower volume of unit cell. The luminescence of perylene crystal is gradually modified as a consequence of these structural changes. The present experimental setup allows investigating with Raman spectroscopy very luminescent molecules involved in chemical reactions and molecular organic crystals under relatively high pressure (up to 1GPa). Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

1.0～4.4GPa下奥长石拉曼光谱特征的变化

常温、1.0～4.4GPa下,利用激光拉曼光谱研究了奥长石晶体结构随压力的变化。发现,压力为2.9GPa时,517cm-1附近出现新的谱峰,奥长石开始相变。3.4GPa时,源于奥长石结构中M—O伸缩振动的288cm-1拉曼谱峰频移发生突变,517cm-1附近谱峰消失,奥长石由三斜晶系完全相变为单斜晶系(P1-I1)。随压力增加,归属于奥长石四面体结构中Si—O—Si弯曲振动的458及516cm-1谱峰随压力增加有规律地向高频方向偏移,斜率分别是1.667cm-1/GPa和3.560cm-1/GPa,而源于Al—O—Al弯曲振动的480cm-1谱峰与压力没有明显的线性变化关系。卸压过程中,288cm-1拉曼谱峰频移保持不变,458,480及516cm-1谱峰向低频偏移。长石类矿物的相变压力与结构中八元环所含阳离子种类有关。     

Raman spectra and structural peculiarities of GaAs nanowires

The Raman scattering spectra of nanowire samples in the region of fundamental modes (50–320 cm^?1) are studied to analyze the structural peculiarities of GaAs nanowires (nanowhiskers) grown on a Si (111) substrate. Factor analysis of the experimental data array consisting of 55 spectra, each of which contains 300 points, is carried out. It is found that the number of linearly independent contributions in these data arrays amounts to only two spectra. The form of each linearly independent contribution is close to zincblende (ZB) and wurtzite (WZ) spectra. It is established that ZB and WZ phases exist in nanowhiskers. Comparison with the calculated frequencies of the normal vibrations of hexagonal GaAs polytypes makes it possible to assume that the spectra of crystal nanowhiskers are combinations of ZB and WZ spectra with good accuracy and agree with the scheme of phonon branch folding of the ZB Brillouin zone.     

First-principles study of structural and thermodynamic properties of osmium

We employ the first-principles plane wave pseudopotential density functional theory method to calculate the equilibrium lattice parameters of osmium and the thermodynamic properties of hcp structure osmium. The obtained lattice parameters are in good agreement with the experimental data investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell and the available theoretical data of others. Through the quasi-harmonic Debye model, the dependencies of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive volume V/V₀ on pressure P, the Debye temperature Θ_D and the heat capacity C_V on pressure P and temperature T, as well as the variation of the thermal expansion α with temperature and pressure are obtained successfully.     

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.     

Electrical properties of zinc oxide under uniform pressure up to 25 GPa

The dependences of the Hall coefficient R _H (P) and resistivity ρ(P) of bulk n-ZnO crystals on hydrostatic pressure up to 8 GPa and quasi-hydrostatic pressure up to 25 GPa at T = 300 K have been measured. With an increase in pressure up to the polymorphic transition P _pt ? 9 GPa, an exponential increase in R(P) and ρ(P) is observed, which is caused by the increase in the ionization energy of the shallow-energy donor center. At P > P _pt, the resistivity decreases by several orders of magnitude.     

常温及常压至1.2GPa条件下异辛烷的拉曼光谱研究

利用碳化硅压腔在室温(25℃)下,研究了异辛烷(2,2,4-三甲基戊烷)在常压至1.2GPa条件下的拉曼光谱特征。研究结果表明,异辛烷CH2和CH3的碳氢伸缩振动的拉曼位移随着压力的增大均呈线性向高频方向移动,其拉曼位移与压力的函数关系为:ν2 873=0.002 8P+2 873.3;ν2 905=0.004 8P+2 905.4;ν2 935=0.002 7P+2 935.0;ν2 960=0.012P+2 960.9。在1.0GPa附近,异辛烷的拉曼位移出现突变,与显微镜下观察发生的异辛烷液-固相变一致。结合异辛烷在常压下的熔点数据,获得了异辛烷的液-固两相相图,并根据克拉贝龙方程获得了液-固相转变过程中的摩尔体积变化量ΔVm=4.46×10-6 m3.mol-1和熵变ΔS=-30.32J.K-1.mol-1。     

Electrical properties and Tc of CaLaBaCu3Ox at pressures up to 9 GPa

Abstract

By means of a four-point resistivity method the critical temperature (T_c) of the tetragonal high temperature super-conductor CaLaBaCu₃O_x was studied for pressures up to 9 GPa. The pressure dependence of T_c is small and negative, dT_c/dp = - 0.77 K/GPa, and agrees with the general trend observed in previous data.     

Anomalies in the variation of elastic properties of cesium during phase transformations under a pressure up to 5 GPa

The pulsed ultrasonic method is used for studying polycrystalline cesium under a pressure up to 5 GPa. Elastic parameters and compression ratio of cesium are determined and anomalies in the elastic properties of cesium during phase transitions CsI-CsII-CsIII-CsIV are revealed. It is found that the CsII-CsIII electron-structure transformation is preceded by anomalous compressibility of the fcc phase of cesium and by softening of longitudinal and transverse acoustic modes of the cesium phonon spectrum.     

Raman spectra of aragonite at the pressure of 0.1-2 GPa and ambient temperature

Raman spectra of aragonite were studied at ambient temperature and pressure of 0.1 - 2 GPa in a moissanite anvil cell using Raman spectrum technique. The relations between the Raman shifts of aragonite and the system pressure are given as follows: v153 (cm(-1)) = 0.003 5p (MPa) +154.0, v206 = 0.006 0p + 206.3, v704 = 0.002 1p + 704.2, v1085 = 0.003 5p + 1 085.3. No phase transition occurred in aragonite within the range of experimental pressure. Similar to other carbonate minerals (magnesite, dolomite), the measured relative pressure-shift of the Raman line of the symmetric stretching vibration of aragonite is greater at 0.1-2 GPa than at ultrahigh pressure, which indicates that the C-O bond compressibility of the CO3 groups is related to the pressure, and it is more compressible at 0.1 - 2 GPa than at higher pressure.     

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.     

First-principles study of the structural properties and magnetism of NpNiSn

First-principles calculations based on density functional theory have been performed on the intermetallic compound NpNiSn. The electronic density of states, bonding properties and equilibrium volume have been studied using relativistic full-potential APW+lo calculations. The magnetocrystalline anisotropy energy has been estimated from total-energy calculations and the a-axis is predicted to be the easy axis of magnetization. The LSDA+U method has been employed to mimic the orbital polarization and to obtain the total magnetic moments.