相变及空位缺陷对高压下GeO_2光学性质的影响

本文采用第一性原理方法,在100 GPa的压力范围内,计算了GeO_2理想晶体和含锗、氧空位点缺陷晶体的光学性质.吸收谱数据表明,压力诱导的三个结构相变对GeO_2晶体的吸收谱均有影响:第一个相变将导致其吸收边蓝移,而第二和第三相变将使得其吸收边红移.锗和氧空位点缺陷的存在将导致GeO_2的吸收边红移,但氧空位点缺陷引起的红移更明显.尽管如此,分析发现,在100 GPa的压力范围内,压力、相变以及空位点缺陷等因素都不会导致GeO_2晶体在可见光区出现光吸收现象(是透明的).波长在532 nm处的折射率数据显示,在GeO_2的四个相区,其折射率均随压力增加而降低;而且,GeO_2的三个结构相变以及锗、氧空位点缺陷都会导致其折射率有所增大.本文预测,GeO_2有成为冲击光学窗口材料的可能.     

Mg2C高压性质的从头计算法研究

运用基于密度泛函理论的平面波赝势方法,结合广义梯度近似,系统地研究了Mg2C在高压下的结构相变、电子结构和光学性质。计算结果表明Mg2C在高压下将发生两次相变,一次是从反萤石到反氯化铅结构的一阶相变在30.09 GPa,另一次是从反氯化铅结构到Ni2In型结构的二阶相变在260 GPa。此外,对压力下电子结构和光学性质的分析表明,Mg2C的带隙宽度随着压力增加而增加,与Mg2Si在压力下表现出金属性有很大不同。     

Structural transitions in NaNH_2 via recrystallization under high pressure

Multiple phase transitions are detected in sodium amide(NaNH_2), an important hydrogen storage material, upon compression in diamond anvil cells(DAC) by using Raman spectroscopy and x-ray diffraction(XRD) measurements.Additional Bragg reflections appear on lower and higher angle sides of the original ones at ～1.07 GPa and 1.84 GPa,accompanied by obvious changes in Raman spectroscopy, respectively.It reveals that NaNH_2 undergoes the high-pressure phase sequence(α–β–γ) up to 20 GPa at room temperature.Spectral analysis indicates an orthorhombic structure with PBAN space group for the γ phase.We also experimentally observe high pressure induced recrystallization in alkaline amide compounds for the first time.     

Calculation of the probability of optical transitions in strong electric fields

An algorithm is proposed for calculating the spectrum of the cross section for photoionization of carriers on deep centers in electric fields on the basis of the form function of an optical transition.An experiment and calculations were performed for the complex V_Ga-S_As in GaAs. The proposed model is compared with theoretical works based on the single-coordinate approximation. It is concluded that the single-coordinate model is applicable for describing the field-dependence of the cross section for photoionization of an electron on a V_Ga-S_As center. Data on the influence of an external electric field on the change in the moments of the form function of the absorption band of the complex V_Ga-S_As in GaAs are obtained. It is concluded that an electric field influences the adiabatic potentials of the center investigated.     

Polarized optical studies of ordered InGaP2 under pressure

In this paper, we discuss polarized photomodulated reflectivity (PR) and photoluminescence (PL) studies of MOCVD grown InGaP₂ epilayers lattice-matched to a GaAs substrate. These structures were grown on a (001) face with a misorientation of two degrees along <110>. The PR spectra of the ordered sample show a strong polarization dependence. For the electric field E parallel to [1̄10] two features in the PR spectra are seen; for E∥[110], however, additional features are observed. A comparison with the spectra of disordered samples of the same alloy composition has enabled a determination of the band gap reduction due to ordering. The linewidths of the PR peaks are approximately 5-10 meV which has enabled us to study them in detail as a function of hydrostatic pressure at cryogenic temperatures. The pressure dependence is slightly sublinear with the first order term of 8-9 meV kbar^-1 for pressures well below the Γ-X crossover. Also observed is the indirect level crossing which occurs under pressure at about 40-kbar. An analysis of PR lineshapes at 1-bar is also presented at several commonly used experimental temperatures.     

Kinetics of phase transitions under high pressure

Abstract

Phase transitions under high pressure have attracted increasing attention i n connection with high pressure synthesis of new materials (e.g. superhard materials, ceramics, semiconductors, high temperature superconductors) and the exploration of geological processes like the formation of rocks and minerals. For t h e investigation of thermodynamic and electronic equilibrium properties a broad spectrum of methods for pressure generation and physical measurements have been developed ^1,2,3,4. Tha measurement of equilibrium properties, however, gives only poor insight into the detailed mechanisnts of phase changes. The accurate determination of phase equilibria and phase diagrams becomes inore and more difficult a t low temperatures due to increasing hysteresis effects end t h e extrapolation of experimental data to zerc, teiiipwature a nd the compar ison with quantumiiiechanical ah in.itio calculations are questionable.     

Nonmetal-metal transitions in liquid substances under high pressure

Abstract

In the melts of Te, Se, S, I₂ and Mg₃Bi₂ the nonmetal-metal transitions were found under pressure. The transitions are accompanied by a decrease of the volume. The transitions seem to terminate at high temperature by “critical regions”. For S and Se the kinetics of the transitions and the pressure influence on the solidification of the melts were investigated.

The existence of the transitions of this kind gives an explanation of anomalies of melting curves of some substances.     

Structural phase transitions in brookite-type TiO2 under high pressure

We have studied polycrystalline brookite TiO₂ using energy-dispersive X-ray diffraction at pressures up to 27.8 GPa and derived an ambient-pressure bulk modulus of 255 GPa using Birch-Murnaghan's equations of state with a fixed value of 4 as its first derivative. The transition from brookite-type to baddeleyite-type was observed to start at 15.8 GPa and finished at 22.8 GPa. Upon decompression, the α-PbO₂ structure appeared at 3.5 GPa and the baddeleyite-type structure remained down to 1.6 GPa, the lowest pressure in the present work.     

Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

相变及空位缺陷对SrF2在高压下光学性质的影响

本文采用第一性原理方法, 计算了SrF2的理想晶体和含锶、氟空位点缺陷晶体在100 GPa压力范围内的光学性质. 吸收谱数据表明, 压力因素引起的两个结构相变对SrF2的吸收谱均有影响: 第一个相变将导致其吸收边蓝移, 第二个相变将导致其吸收边红移. 空位点缺陷的存在将使得SrF2的吸收边红移, 其中氟空位点缺陷引起的红移行为更显著. 尽管如此, 这些红移并未使得SrF2晶体在可见光区出现光吸收的现象(是透明的). 波长在532 nm处的折射率数据指明, 在SrF2的三个结构相区, 其折射率均随压力的增加而增大, 且SrF2的高压结构相变也使得其折射率增大. 锶空位点缺陷将导致SrF2的折射率降低, 但氟空位点缺陷的存在对其基本没有影响. 分析表明, SrF2晶体有成为冲击窗口材料的可能.     

Second-order phase transitions under pressure in polycrystalline compounds and rocks

Bulletin of the Russian Academy of Sciences: Physics - The dependence of thermal conductivity in mono-and polycrystalline tellurium and rocks at hydrostatic pressures of up to 400 MPa in the...     

Structural phase transitions of tellurium nanoplates under pressure

In situ high-pressure angle dispersive x-ray diffraction experiments using synchrotron radiation on Te nanoplates were carried out with a diamond anvil cell at room temperature. The results show that Te-I with a trigonal structure transforms to triclinic Te-II at about 4.9 GPa, Te-II transforms to monoclinic Te-III at about 8.0 GPa, Te-III turns to rhombohedral Te-IV at about 23.8 GPa, and Te-IV changes to body centered cubic Te-V at 27.6 GPa. The bulk moduli B0 of Te nanoplates are higher than those of Te bulk materials.     

Nonlinear dependence of optical resonance widths at CO2 transitions on pressure

A nonlinear dependence of the Lamb-dip width at CO₂ transitions on pressure caused by scattering at small angles under collisions has experimentally been found. The analysis of the results allowed us to determine some characteristic parameters of elastic scattering under collisions, such as the total scattering cross-section and the characteristic elastic scattering angle, which proved to be equal to 480 Ų and 10^-2 rad, respectively.