High-pressure X-Ray diffraction and optical absorption studies of CsI

Static-compression data and absorption spectra for CsI have been collected to 61 GPa (610 kbar) at room temperature. The band gap closes with increasing pressure and CsI is expected to metallize at 105 (± 15) GPa. A second order phase transition to the CuAu I structure is observed at 39 (± 1) GPa. The elastic constants measured at low pressures do not predict that an elastic instability, and hence a structural distortion, would occur at elevated pressures. Similarly, an ionic pair-potential model which reproduces the properties of CsI at low pressures does not show the distortion to be stabilized at high pressures.     

High pressure photoinduced ring opening of benzene

The chemical transformation of crystalline benzene into an amorphous solid (a-C:H) was induced at high pressure by employing laser light of suitable wavelengths. The reaction was forced to occur at 16 GPa, well below the pressure value (23 GPa) where the reaction normally occurs. Different laser sources were used to tune the pumping wavelength into the red wing of the first excited singlet state S(1)((1)B(2u)) absorption edge. Here the benzene ring is distorted, presenting a greater flexibility which makes the molecule unstable at high pressure. The selective pumping of the S(1) level, in addition to structural considerations, was of paramount importance to clarify the mechanism of the reaction.     

High pressure μSR studies

The high pressureSR spectrometer [1] formerly located at CERN has been transferred to theE1-beamline at PSI and put back into operation with only minor modifications. The essential features of the high pressure apparatus are described below. The instrument covers a pressure range up to 0.7 GPa which can be extended to 1.4 GPa depending on the design of the high pressure cell. First measurements at PSI were successfully carried out with a single crystalline sample of Gd metal. New developments in high pressure cell design are presented. They are expected to further improve the signal/background ratio and to extend the pressure range to 1 GPa. One type of cell will allow temperatures above 380 K.This work was supported by the German Federal Minister for Research and Technology (Bundesminister für Forschung und Technologie [BMFT]) under Contract Nr. 03KA2-TUM-4 and 03SE3STU.     

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.     

Electrical conductivity and thermal EMF of CsI at high pressures

Abstract

The pressure dependence of thermal EMF and the resistivity-temperature dependence of CsI has been measured at pressures 20-50 GPa. In CsI non-monotonous change of resistivity, thermal EMF and activation energy of charge carriers has been observed at pressures above 40 GPa. The sign of thermal EMF corresponds to the electron conductivity. At pressures below 47 GPa the resistivity-temperature dependence is of the type characteristic of non-degenerate semiconductors, at pressure above 49 GPa it is characteristic of degenerate semiconductors (or metals). The observed properties are connected probably with the continuous distortion of B2 to an hcp-like phase.     

Electronic transition and the metallization effect in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The optical absorption spectra from bismuth ferrite (BiFeO₃) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ～1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe³⁺ ions near the transition.     

相变及空位缺陷对高压下BeO光学性质的影响

为了探究BeO晶体能否成为冲击波实验中的候选窗口材料,本文采用密度泛函理论(DFT)的第一性原理方法,计算了150 GPa的压力范围内BeO理想晶体和含氧空位点缺陷晶体的光学性质.吸收谱数据显示,BeO高压结构相变对其吸收谱的吸收边几乎没有影响.并且,在150 GPa压力范围内,BeO理想晶体在可见光区没有光吸收行为.氧空位点缺陷的存在将使得其吸收边出现明显的红移现象,但在可见光区仍然没有光吸收(是透明的).波长在532 nm处的折射率数据表明:在BeO的WZ和RS结构相区,其折射率会随着压力增加而缓慢降低,而高压结构相变和氧空位缺陷将使得其折射率显著增大.计算数据分析表明BeO有成为冲击窗口材料的可能,并且本文所获信息将对未来进一步的实验有重要参考价值.     

高压下BaLiF3晶体光学性质的第一性原理研究

本文采用第一性原理方法, 在190 GPa的压力范围内, 计算了BaLiF3理想晶体和含空位点缺陷晶体的光学性质. 吸收谱数据表明, 压力因素不会导致BaLiF3晶体在可见光区有光吸收的行为. 空位点缺陷的存在会使得BaLiF3的吸收边红移(其中氟空位点缺陷引起的红移最显著) , 但这些红移不会导致它在可见光区内出现光吸收的现象. 波长在532 nm处的折射率数据显示, BaLiF3的折射率将随压力升高而增大. 氟空位点缺陷将导致BaLiF3的折射率增大, 但钡空位点缺陷和锂空位点缺陷的存在对其基本没有影响. 本文预测, BaLiF3晶体有成为冲击光学窗口材料的可能.     

高压下KMgF_3晶体光学性质的第一性原理研究

本文采用第一性原理的方法,在100 GPa的压力范围内,计算了KMgF_3的理想晶体和含空位缺陷晶体的光学性质.吸收谱数据表明,在100 GPa范围内,压力因素不会导致KMgF_3晶体在可见光区有光吸收行为.钾、镁和氟空位缺陷的存在会使得KMgF_3晶体的吸收边红移(其中氟空位缺陷引起的红移最显著),但这些红移不会导致它在可见光区出现光吸收的现象.能量损失谱数据显示,压力因素不仅会使得KMgF_3晶体的能量损失谱有蓝移的行为,而且还会引起它的较强谱峰个数发生变化.在100 GPa处的缺陷晶体数据指明,氟空位缺陷会导致其能量损失谱的两个较强谱峰的峰值强度明显降低.分析表明,KMgF_3晶体有成为冲击窗口材料的可能,并且本文所获得的结果对未来的实验探究有参考作用.     

掺氧化钙及相变对高压下立方氧化锆电子结构和光吸收的影响

研究表明,立方氧化锆可作为冲击波实验中的窗口材料.为了使得该材料在常态下保持结构稳定,需添加稳定剂——氧化钙.然而,掺杂会导致其在29 GPa的冲击压力下从立方转变为斜方Ⅱ结构相.因此,该材料在冲击压缩下的电子结构和光学吸收性质以及作为光学窗口的适用压力范围是值得研究的重要问题.本文运用第一性原理的方法,分别计算了在100 GPa范围内两种结构氧化锆的电子结构和光学吸收性质.结果表明:(1)在立方结构相区,冲击压力将导致其吸收边蓝移,而在斜方Ⅱ结构相区,却使得其吸收边红移;(2)在立方结构相区,掺杂将引起能隙变窄(吸收边红移),但对于斜方II相区,却导致能隙变宽(吸收边蓝移);(3)冲击结构相变使得能隙变窄,吸收边红移.本文数据建议,掺氧化钙的立方氧化锆在95GPa的冲击压力范围内可作为光学窗口材料.     

高压下LiYF4晶体光学性质的第一性原理研究

本文采用第一性原理方法,在100 GPa的压力范围内, 计算了LiYF4理想晶体和含空位点缺陷晶体的光学性质.吸收谱数据表明,在100 GPa范围内,压力和相变因素的存在不会改变LiYF4晶体在250-1000 nm的波段内没有光吸收的事实. 氟、钇空位点缺陷的出现会使得LiYF4的吸收边蓝移,而锂空位点缺陷将导致它的吸收边微弱红移(但在250-1000 nm的波段内它仍不具有光吸收行为).波长在532 nm处的折射率数据显示, 在LiYF4的三个结构相区,其折射率均随压力的增加而增大. LiYF4从白钨矿结构到褐钇铌矿结构的相变会使得其折射率略微增加,但从褐钇铌矿结构到类黑钨矿结构的相变将导致其折射率显著降低. 同时,空位缺陷的存在将引起LiYF4的折射率明显增大. 分析指明,LiYF4有成为冲击窗口材料的可能. 本文所获得的信息对未来的实验研究有参考作用.     

Transport and optical properties of iron borate FeBO<Subscript>3</Subscript> under high pressures

The optical absorption spectra of iron borate FeBO₃ were measured in diamond anvil cells at high pressures up to P=82 GPa. The electronic transition with an abrupt jump in the absorption edge from ～3 to 0.8 eV was observed at P≈46 GPa. The resistance and its temperature dependence were directly measured for FeBO₃ at high pressures up to 140 GPa. It was established that the electronic transition at P≈46 GPa was accompanied by the insulator-semiconductor transition. In the high-pressure phase, the thermoactivation gap decreases smoothly at 46<P<140 GPa approximately from 0.55 to 0.2 eV following the linear law. The extrapolated value of the pressure at which the sample becomes fully metallic is equal to about 210 GPa.     

Lu_2O_3晶体在高压下的光学性质第一性原理研究

本文采用第一性原理方法,计算了Lu_2O_3(氧化镥)的理想晶体和含氧、镥空位点缺陷晶体在100 GPa压力范围内的光吸收谱和折射率性质.结果表明:在100 GPa范围内, Lu_2O_3理想晶体在可见光及红外光区不具有光吸收现象,空位点缺陷的存在将导致吸收边红移,其中氧空位点缺陷引起的红移行为更显著,但这些吸收边仍未进入可见光区的高波段. 532 nm处的折射率数据表明,在立方结构和单斜结构相区, Lu_2O_3晶体的折射率随压力的增加而增大,高压结构相变以及氧、镥空位的存在也会使得折射率增大.结合温度效应分析推测, Lu_2O_3晶体在近红外区有可能透明.     

First-principles calculation of the insulator-to-metal transition pressure in CsI

The room temperature compression isotherm and electronic band structure of CsI have been computed with the nonrelativistic self-consistent augmented-plane-wave method. The isotherm is in excellent agreement with available static and ultrasonic measurements, and hence allows an improved extrapolation of Asaumi and Kondo's measurements of the absorption edge to gap closure in the range 38–46 ų and 65–105 GPa when uncertainties in the experimental determination of the band gap are accounted for. The calculated band structure suggests that the band gap is direct and it is in close agreement with the corresponding experimentally determined band gap when the Slater exchange potential is used. These consistencies between the measurements and calculations suggest that the insulator-to-metal transition occurs in the 100±10 GPa range, somewhat higher than Asaumi and Kondo's estimate of 70 GPa, but in good agreement with the prediction from Herzfeld's theory.     

Electronic and structural transitions in NdFeO<Subscript>3</Subscript> orthoferrite under high pressures

The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO₃ orthoferrite single crystals is studied in diamond anvil cells. At P～37.5 GPa, an electronic transition at which the optical absorption edge jumps from ～2.2 to ～0.75 eV is observed. The equation of state V(P) is studied on the basis of the X-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at P～37 GPa with a jump of ～4% in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30–40 GPa is a transition of the insulator-to-semiconductor type.     

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.     

Suppression of Bragg reflection glitches of a single‐crystal diamond anvil cell by a polycapillary half‐lens in high‐pressure XAFS spectroscopy

In combination with a single‐crystal diamond anvil cell (DAC), a polycapillary half‐lens (PHL) re‐focusing optics has been used to perform high‐pressure extended X‐ray absorption fine‐structure measurements. It is found that a large divergent X‐ray beam induced by the PHL leads the Bragg glitches from single‐crystal diamond to be broadened significantly and the intensity of the glitches to be reduced strongly so that most of the DAC glitches are efficiently suppressed. The remaining glitches can be easily removed by rotating the DAC by a few degrees with respect to the X‐ray beam. Accurate X‐ray absorption fine‐structure (XAFS) spectra of polycrystalline Ge powder with a glitch‐free energy range from ?200 to 800 eV relative to the Ge absorption edge are obtained using this method at high pressures up to 23.7 GPa, demonstrating the capability of PHL optics in eliminating the DAC glitches for high‐pressure XAFS experiments. This approach brings new possibilities to perform XAFS measurements using a DAC up to ultrahigh pressures.     

Effect of pressure on optical and electrical properties of tungsten diselenide single crystals

Optical absorption spectra of tungsten diselenide (WSe₂) single crystals subjected to different values of pressure (0, 2, 4 and 6 GPa) were obtained in the spectral range 700–1450 nm with the help of a UV–VIS–NIR spectrophotometer. The spectra were thoroughly analyzed in the absorption edge region for obtaining direct as well as indirect band gaps in this material. The high temperature resistivity and thermoelectric power on WSe₂ single crystals at various pressures were also studied. The results and their implications are discussed in the article.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Pressue Dependence of the Ferroelectric-Paraelectric Transition Temperature in KNBO 3 . Comparision with the Ice VII-VIII Transition

New results on the phase diagram of KNbO 3 determined, from 10 to 650 K up to 30 GPa, with Raman scattering are given. On increasing pressure the Curie temperature T c was found to shift down regularly from 700 K, at atmospheric pressure, to 20 K at ～25 GPa. Up to 20 GPa, the pressure dependence of T c follows a classical regime described by the Ising model where the dipolar interaction follow a i 2 /v law, where i is the off-center displacement and v the unit cell volume. This behaviour found also for the ice VII-VIII is very likely encountered in a number of (anti)ferroelectric-paraelectric transformations involving positional disorder.