High Temperature-High Pressure Structural Studies of Cerium

Energy dispersive X-ray-diffraction with the large volume press MAX 80 at HASYLAB/DESY is used in the present study for the determination of the p - V - T -relations around the critical point of the f - n -phase transition in Cerium. Furthermore the f - l -phase boundary is studied experimentally to remove the last question marks from the phase diagram of Cerium in the accessible region of pressure and temperature.     

Observation of the Cinnabar Phase in ZnSe at High Pressure

In this paper we describe the results of an energy dispersive X-ray diffraction experiment carried out in the ZnSe 1 m x Te x alloy and pure ZnSe under high pressure. In the downstroke the cinnabar phase is observed between the rocksalt and the zincblende phases. The analysis of the whole series of compositions ( x =0, 0.05, 0.1 and 0.2) enables us to establish its lattice parameters in ZnSe ( a =3.785 + and c =8.844 + at 10.5 GPa). The X-ray diffraction pattern simulation suggests that the internal parameters u and v are close to 0.5, indicating that the cinnabar phase in ZnSe is similar to that observed in GaAs and ZnTe. The cinnabar's stability range decreases as the Te content is reduced.     

高压下金红石的拉曼光谱分析

常温下利用金刚石压腔装置(DAC)对金红石加压至40GPa,进行拉曼光谱的原位分析.发现压力约为13GPa时,金红石结构转变为斜锆石结构(ZrO2).21.1GPa相变完全.直至实验最高压力,没有进一步相变出现.在卸压中,斜锆石结构转变为α-PbO2结构.实验压力通过红宝石用拉曼光谱测压的计算方法确定,快速方便.     

Pressure Study on New Superconducting Germanium Clathrates

We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T_{C}\approx 0.24\,\hbox{K} out of the resulting bad metal ( \rho_{0}\approx 1.5\,\hbox{m}\Omega\;\hbox{cm} ). With increasing pressure, the structural phase transition is shifted to lower temperature but T C increases drastically. T C reaches a maximum value of 3.85 K at the critical pressure p_{C}\approx 2.8\,\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \approx 1.05\,\hbox{K}) than in the undoped case at ambient pressure but decreases slightly with increasing pressure.     

Electrophysical Properties of ZnAs 2 and CdAs 2 at Hydrostatic Pressure up to 9 GPa

Specific electroresistance and Hall coefficient on oriented ZnAs 2 and CdAs 2 single crystals in the region of room temperatures at hydrostatic pressure up to 9 GPa were measured. In p -ZnAs 2 specific electroresistance falls for one order of magnitude with the increase of pressure, and Hall coefficient falls for two orders in magnitude, and at P =7 GPa specific electroresistance and Hall coefficient come out to a saturation. Under mentioned conditions the phase transition in investigated p -ZnAs 2 samples was not observed, in all probability it occurs under the pressure P >10 GPa. Two groups of n -CdAs 2 samples oriented on [1 0 0] and [0 0 1] directions were investigated. The reversible structural phase transition was observed in investigated n -CdAs 2 samples at P =5.5 GPa from the dependencies of specific electroresistance 𝜌 ( P ) and Hall coefficient R H ( P ). On the basis of the values of concentrations and mobilities before and after phase transition a conclusion was made that semiconductor-semiconductor transition takes place in n -CdAs 2 . Maxima that earlier weren't observed, were detected on dependencies 𝜌 ( P ), R H ( P ) at P =1.8 GPa and at P =3 GPa.     

Non-Conventional Behaviour of Castor Oil Under High Pressure

Castor oil shows non-conventional physical and chemical behaviour when submitted to pressure. This paper gives a brief presentation of the specific physical properties of castor oil under pressure up to 1.0 GPa. The changes of permittivity, dielectric loss and the structure of the high-pressure induced phases of castor oil are also described.     

High-Pressure Behavior of Nano Titanium Dioxide

Nanocrystalline rutile Titanium dioxide has been studied by X-ray diffraction at ambient temperature up to 47.4 GPa. The material is found to transform to the monoclinic baddeleyite structure between 20 and 30 GPa, which is higher than the corresponding pressure range for bulk material. Upon decompression, the baddeleyite phase transforms to the f -PbO 2 phase at about 4-2 GPa. The experimental bulk moduli are 211(7) GPa for the rutile phase, 235(16) for the baddeleyite type and 212(25) GPa for the f -PbO 2 type phase. The results are compared with previous measurements of bulk rutile Titanium dioxide.     

Effect of pressure on magnetic phases of (Fe1-x Ni x )49Rh51

The effect of high pressure and magnetic field on the stability of magnetic phase transitions in (Fe_1?x Ni _x )₄₉Rh₅₁ has been studied. It has been shown that the increase in the Ni content suppresses the stability of the low-temperature antiferromagnetic phase and leads to its disappearance. Pressure on the other hand restores the stability to the antiferromagnetic order. These findings are explained using first-principles band-structure calculations.     

High Pressure and High Temperature Studies on Manganese Oxides

High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature. The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C. Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure. f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO 2 is identified to be a low-density polymorph f -MnO 2 . This unusual result of formation of low-density f -MnO 2 , having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu's (1976) laser-heated DAC experiment.     

X-Ray Diffraction Investigation of Oleic Acid under High Pressure

Pressure has a specific influence on the light scattering in those liquids in which all molecules have an unsaturated double bond C=C. Strong changes of intensity of transmitted and scattered light in oleic acid were observed during its transition to the high-pressure phase. In this paper, local order in liquid of oleic acid has been studied using the X-ray diffraction photographic Laue method. Experiments were run at room temperature under atmospheric pressure as well as under pressures up to 0.25 GPa. At pressures above 0.2 GPa a strong increase of ordering of molecules, similar to a smectic liquid crystal phase, has been observed. This confirms results obtained previously by studies of light scattering under high pressure.     

High Pressure X-Ray Absorption and Diffraction Study of InAs

We have performed X-ray absorption (XAS) and diffraction (XRD) measurements at high pressure on samples of powdered InAs, up to 50 and 80 GPa, respectively. In the lower pressure range, our data are consistent with the following structural sequence: Zincblende M NaCl M Cmcm . The first order transition from the semiconducting Zincblende phase to the metallic NaCl phase is clearly seen by the shift in the absorption onset at the As K-edge and the strong modifications of the extended X-ray absorption fine structure (EXAFS) due to the changes in the local structure from a 4-fold to a 6-fold coordinated environment. XAS shows the high pressure phase to be locally site-ordered. The diffraction data, analized by Rietveld fitting, gives a volume discontinuity of j V/V 0 ～0.18 for the first order transition. There is no apparent volume discontinuity associated to the NaCl M Cmcm transition.     

氨基酸DLH及其稀土镧配合物的高压红外和拉曼光谱研究

测定了DL-2-氨基-4-磺酸基-丁酸 [DLH, DL-Homocysteic acid, (NH+₃)-CH(COOH)-(CH₂)₂-SO-₃] 及其稀土La配合物[La(DLH)₂Cl₃·H₂O=LaL₂]在不同压力下的红外和拉曼光谱。DLH 在50 kbar左右压力以下存在两个压力诱导相转变区,它们分别在17和37 kbar左右,两者均为二级相转变,认为分子间氢键的存在是出现两个压力诱导相转变区的原因。在红外光谱中,SO-₃的对称伸缩振动的压力灵敏度(dν/dp)表现出与其他振动模式不同的变化趋势,它们在低压相区的平均压力灵敏度为0.30 cm-1·(kbar)-1、中压相区为0.32 cm-1·(kbar)-1、高压相区为0.41 cm-1·(kbar)-1,低压相区与高压相区的比值为0.72, 而其他振动模式刚好相反,低压相区与高压相区的比值为4.8。稀土La配合物LaL₂的生成,改变了分子间的氢键,在50 kbar左右压力以下只观察到1个压力诱导相转变区(27 kbar附近)。在红外光谱中,配合物LaL₂中SO-₃的反对称伸缩振动的压力灵敏度(dν/dp)也表现出与其他振动模式不同的变化趋势,它们在低压相区的平均压力灵敏度与高压相区的平均压力灵敏度的比值为0.43, 而其他振动模式的比值为2.5。     

Optical Properties of High Pressure Phases in ZnTe 1− x Se x

We report on the optical properties of high pressure semiconducting phases in ZnTe 1 m x Se x . In the Te rich side, the cinnabar phase is observed in the upstroke between typically 9.5 and 12.5 GPa with a pressure interval of existence that decreases with increasing the Se content. In most studied samples, the indirect absorption edge could be determined, with values of the bandgap increasing with the Se content and ranging from 1.2 to 1.7 eV. In the downstroke, the cinnabar phase is observed in the whole composition range but its bandgap can not be unambiguously determined in the Se-rich side, as it coexists with rocksalt or zincblende phases. The indirect semiconducting rocksalt phase is observed in the Se-rich side, with an indirect bandgap of the order of 0.7 eV. Within the experimental errors, the bandgaps of both the cinnabar and NaCl phases are pressure insensitive, in agreement with first-principles pseudopotential band structure calculations, that predict very low pressure coefficients for both indirect transitions.     

Photo-induced Conversion of Furylfulgide Single Crystal Under High Pressures

We have studied pressure effects on single crystal of photochromic furylfulgide for researching the possibility of photo-induced structural change. Pressure dependence of the absorption spectra was investigated up to 8.2 GPa at room temperature. Pressure-induced conversion to C-form molecules was observed at 8.2 GPa in single crystal composed only of E-form molecules, while at 5.7 GPa in single crystal containing small amount of C-form ones. Photochromic reaction was observed under high pressure as well as the ambient pressure. The dependence of absorption intensity due to C-form molecules suggests the possibly of cooperative structural change in furylfulgide single crystal at 5.5 GPa.     

Suppression of Jahn-Teller Distortion and Insulator-To-Metal Transition in LaMnO 3 at High Pressures

Structural, vibrational and electronic properties of LaMnO 3 under pressures up to 38 GPa have been studied by synchrotron X-ray powder diffraction, Raman spectroscopy, optical reflectivity, and transport measurements. The cooperative Jahn-Teller distortion of the MnO 6 octahedra of the perovskite-type structure is continuously suppressed with increasing pressure, a process which appears completed at ～20 GPa. The system remains insulating to 32 GPa, where an insulator-metal transition is observed. This transition is attributed to strengthened Mn--O--Mn interactions due to the increasing overlap of atomic orbitals.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.     

Theoretical Description of High-Pressure Phases of Semiconductors

First-principles ( ab initio ) electronic structure calculations allow detailed studies of the energetics of materials under high pressures. Here we illustrate some of the important themes using mainly examples from work on Si and Ge.     

Xanes and raman spectroscopy under pressure of bromobenzene

Abstract

The behaviour of bromobenzene (BBe) compressed in a diamond anvill cell up to 30 GPa was studied by XANES and Raman spectroscopy. The liquid-solid transition and a solid-solid transition were observed at 0.9 GPa and 9 GPa respectively. Above 24 GPa, an irreversible transformation occurs to a solid orange-yellow compound which can be recovered at zero pressure. The polymerization mechanism, in connection with the occurence of Br-bonded Sp² and Sp³ carbons in the solid compound, is discussed.     

B-C-N Compound Synthesized Under High Temperature and High Pressure

Crystalline hexagonal B(N 1 m x C x ) and cubic B-C-N compounds have been synthesized from a precursor produced from melamine and boric acid by application of high temperature and high pressure. The synthesized products were characterized by X-ray diffraction. The lattice parameters for the hexagonal crystal are a=2.506 Å, c=6.657 Å, and that for the cubic crystal is a=3.596 Å. The X-ray photoelectron spectra of the B-C-N compound indicate the presence of B-N, C-N, C-C, and B-C bonds, which suggests that boron, carbon, and nitrogen atoms all bond with one another and that the B-C-N crystal is a compound in which the three kinds of atoms are mixed atomically. The composition of the B-C-N compound is B 0.47 C 0.23 N 0.30 . A strong absorption band at 1000～1120 cm m 1 attributable to the cubic B-C-N phase is observed in the infrared spectrum. The photoluminescence spectrum of hexagonal B-C-N powder measured at room temperature features a broad peak centered at 374 nm, corresponding to the band-edge emission of h-B-C-N, and is similar to that of w-GaN.     

Pressure and Temperature Probe for Studies of Biological Materials

In our previous paper [1] a high pressure technique for monitoring pressure up to 700 MPa and temperature from m 40 °C to +100 °C in several pressure vessels simultaneously was reported. This technique, applied in Unipress High Pressure Multivessel Apparatus for studies of biological materials, revealed some limitations. In this paper we propose a new solution which allows to overcome them. In this solution two different pressure media are used, separated from each other: one suitable for biological studies and the other proper for electric sensors. A new integrated pressure/temperature probe is presented in which manganin pressure gauge is confined in a metal bellows separating the two pressure transmitting media. The bellows can be easily assembled or disassembled, allowing promptly to refill pressure medium or to replace the pressure gauge. Temperature is measured by constantan/copper thermocouple. The probe is linked to data acquisition system. Taking into account temperature dependence of the manganin pressure gauge, simultaneous measurements of the resistance of pressure gauge and thermocouple voltage allow to compute pressure at any temperature. The new probe is integrated with the bottom closure of the pressure vessel which also incorporates capillary inlet. Such a design leaves free access from the top of the vessel, allowing easy mounting the studied samples as well as other additional probes.