Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

Raman and optical absorption studies under pressure have been conducted on KTb(MoO₄)₂ up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO₄)₂ is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.     

高压下LiB3O5晶体-非晶体相变的研究

 采用X射线粉末衍射和同步辐射能散X射线衍射技术，对LiB₃O₅（LBO）晶体进行了压致非晶化相变的研究，压力达45.1 GPa。实验结果表明，LBO存在两个中间相变，压力分别为5 GPa和15.8 GPa。而当压力增加到25~31.6 GPa时，LBO出现非晶态，这个非晶化相变是不可逆的。     

β-BaB2O4晶体高压相变的X光衍射研究

 我们用金刚石压砧装置（DAC）高压X射线粉末衍射照相方法，研究了β-BaB₂O₄晶体的相变。压力从0.1 MPa逐步加至17.6 GPa，发现BBO由室温下的六方相经历了若干次相变后，当压力加至11.5 GPa时，转变为无定形相。在11.5 GPa压力前的相变是可逆的，一旦进入无定形相，则相变是不可逆的。     

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.     

Equation of state and structural characterization of Cu4I4{PPh2(CH2CH = CH2)}4 under pressure

Combined high pressure single crystal X-ray diffraction experiments and ab initio simulations based on the density functional theory have been performed on a copper(I) iodide cluster formulated [Cu₄I₄{PPh₂(CH₂CH?=?CH₂)}₄] under high pressure up to 5?GPa. An exhaustive study of compressibility has been done by means of determination of isothermal equations of state and structural changes with pressure at 298?K taking advantage of the single crystal is more precise than powder X-ray diffraction for this type of experiments. It allows us to report the evidence of the existence of an isostructural phase transition of second order at 2.3?GPa not detected so far.     

微区Raman光谱在TiO_2高压结构相变研究中的应用

本文以金红石单晶TiO2和锐钛矿多晶TiO2为研究对象,应用金刚石小压机和原位拉曼光谱测量技术,系统研究了室温高压下TiO2的结构相变。原位拉曼测量表明,金红石单晶TiO2在压力达到12.91GPa时开始发生由金红石结构向斜锆石结构(MI)的相变,当压力达到14.16 GPa时,相变完成;继续加压到21.65 GPa,没有发现进一步的相变;卸压时由斜锆石结构转变为PbO2结构,相变发生在大约7.11 GPa处。锐钛矿多晶TiO2在压力达到4.26 GPa时开始向PbO2结构转变,当压力达到8.34 GPa时相变完成;继续加压到12.94 GPa,样品开始发生由PbO2结构向斜锆石结构的相变,当压力达到18.74 GPa时相变完成;继续加压到21.39 GPa,没有发现进一步的相变;卸压时也由斜锆石结构转变为PbO2结构,起始相变压力点应高于8 GPa。     

Structural investigation of the zincblende-β tin transition for gasb by in situ X-ray absorption spectroscopy

Abstract

The transition to the β tin structure has been observed by X-ray absorption spectroscopy for GaSb at 7.9 GPa. In the low pressure phase, the bulk modulus has been determined. In the high pressure phase, the results for the Ga-Sb and Ga-Ga distances are in good agreement with previous x-ray diffraction data. The pseudo Debye-Waller factor associated to the Ga-Sb distance is abnormally high, indicating that disorder exists for this bond.     

Raman spectroscopy investigation on the pressure-induced structural and magnetic phase transition in two-dimensional antiferromagnet FePS3

The layered van der Waals antiferromagnetic FePS₃ has received considerable attention because long range magnetic ordering can remain with single atoms layer, which offers potential applications in future ultrathin devices. Here, we perform Raman spectroscopy to systematically explore the variations of lattice vibration and crystal structure under pressure up to 18.9 GPa. We observe two structural phase transitions at approximately 4 GPa and 13 GPa, respectively. Moreover, by monitoring spin-related Raman modes, we demonstrate a pressure-induced magnetic structure transition above 2 GPa. These modes disappear accompanying the second structural phase transition and insulator-to-metal transition (IMT), indicating the suppression of long-range magnetic ordering, in agreement with earlier neutron powder diffraction experiments.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Structural and optical properties of InSe under pressure

Abstract

We have investigated the high pressure behavior of InSe by x-ray powder diffraction and optical measurements. The rhombohedral γ-polytype of InSe (space group R3m) exhibits a strongly anisotropic compressibility characteristic of the layer-type structure. Mode Gruneisen parameters of intralayer modes have been determined by Raman scattering. At 10.3(5) GPa InSe undergoes a phase transition to the rocksalt structure, which remains stable up to at least 30 GPa. Optical reflectivity measurements show the cubic high pressure phase to have metallic character.     

Pressure Response of the Higher Fullerene C 84 Studied by Raman and X-ray Scattering

The pressure behavior of the intramolecular phonon modes of the fullerene C 84 and its structural stability have been studied for the first time by means of Raman spectroscopy and synchrotron X-ray diffraction at high pressure up to ～10 GPa. The volume of the cubic unit cell has been measured as a function of pressure. The experimental data fitted by the Murnaghan equation-of-state (EOS) gave K_{0}=19.5\pm 0.9\,\hbox{GPa} and K_{0}^{\prime}=16.4\pm 0.6 . The pressure coefficients and Grüneisen parameters of the intramolecular phonon modes of C 84 have been determined and compared with those of other fullerenes. The data obtained do not show any phase transition and the pressure behavior of the material is entirely reversible in the pressure region investigated.     

Phase transitions in KIO(3)

The high-pressure behavior of KIO(3) was studied up to 30?GPa using single crystal and powder x-ray diffraction, Raman spectroscopy, second harmonic generation (SHG) experiments and density functional theory (DFT)-based calculations. Triclinic KIO(3) shows two pressure-induced structural phase transitions at 7?GPa and at 14?GPa. Single crystal x-ray diffraction at 8.7(1)?GPa was employed to solve the structure of the first high-pressure phase (space group R3, a?=?5.89(1) ?, α?=?62.4(1)°). The bulk modulus, B, of this phase was obtained by fitting a second order Birch-Murnaghan equation of state (eos) to synchrotron x-ray powder diffraction data resulting in B(exp,second)?=?67(3)?GPa. The DFT model gave B(DFT,second)?=?70.9?GPa, and, for a third order Birch-Murnaghan eos, B(DFT,third)?=?67.9?GPa with a pressure derivative of [Formula: see text]. Both high-pressure transformations were detectable by Raman spectroscopy and the observation of second harmonic signals. The presence of strong SHG signals shows that all high-pressure phases are acentric. By using different pressure media, we showed that the transition pressures are very strongly influenced by shear stresses. Earlier work on low- and high-temperature transitions was complemented by low-temperature heat capacity measurements. We found no evidence for the presence of an orientational glass, in contrast to earlier dielectric studies, but consistent with earlier low-temperature diffraction studies.     

Laser-induced crystalline phase transition from rutile to anatase of niobium doped TiO2

The investigation of the laser-induced crystalline phase transition of niobium doped TiO₂ single crystal is discussed in this paper. The TiO₂ single crystal was studied by using Raman spectroscopy and electron backscatter diffraction before and after irradiation by 266?nm?ns laser at the threshold intensity of 56?MW/cm². Experimental results show an evidence of anatase phase formation from rutile single crystal. Moreover, the analysis of the experimentally obtained and calculated crystallographic texture indicates that the converted TiO₂ layer has a polycrystalline structure with the preferred orientations. According to the Raman spectroscopy the amount of anatase phase increases with the number of laser pulses indicating the dose-dependent conversion process.     

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.     

Phase transition of cadmium fluoride under high pressure

We investigated the high pressure phases of CdF₂ by a joint theoretical and experimental study. The structural and electronic properties of CdF₂ were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF₂ under high pressure.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

KNb0.77Al0.23O2.77的高压相变研究

 利用水热法合成了立方相KNb_0.77Al_0.23O_2.77。在水热晶化过程中得到了包括正交相钙钛矿和一新相在内的中间相产物。结构相变行为采用XRD、DSC、高压Raman和XPS进行了表征。结果表明，低于1 000 ℃的热处理不能导致KNb_0.77Al_0.23O_2.77发生结构相交。而在4.49 PGa高压下可能发生了到一新相的结构转变。高温高压处理可以导致另一新相的出现。最后对该立方相的相变机理进行了研究。     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Structural and magnetic phase transitions occurring in Pr0.7Sr0.3MnO3 manganite at high pressures

The crystal and magnetic structures and the vibrational spectra of Pr_0.7Sr_0.3MnO₃ manganite are studied within the pressure range up to 25 GPa by methods of X-ray diffraction and Raman spectroscopy. Neutron diffraction studies have been performed at pressures up to 4.5 GPa. The magnetic phase transition from the ferromagnetic phase (T _C = 273 K) to the A-type antiferromagnetic phase (T _N = 153 K) is found at P ≈ 2 GPa. This transition is characterized by a broad pressure range corresponding to the phase separation. The Raman spectra of Pr_0.7Sr_0.3MnO₃ measured under high pressures significantly differ from the corresponding spectra of the isostructural doped A_{1 ? x} A′ _x MnO₃ manganites, (where A is a rare-earth ion and A′ is an alkaline-earth ion) with the smaller average ionic radius 〈r _A〉 of A and A′ cations. Namely, the former spectra do not include clearly pronounced stretching phonon modes. At P ～ 7 GPa, there appears the structural phase transition from the orthorhombic phase with the Pnma space group to the orthorhombic high-pressure phase with the Imma symmetry. In the vicinity of the phase transition, anomalies in the pressure dependences of the lattice parameters, unit cell volume, and phonon frequencies corresponding to the characteristic lattice vibration modes are observed.     

室温下Fe62Ni27Mn11（wt%）合金的压致fcc-hcp相变

 本文采用Mao-Bell型金刚石对顶砧（DAC）及高压在位（in situ）粉末X光衍射照相方法研究了Fe₆₂Ni₂₇Mn₁₁（wt%）合金在0~43.2 GPa压力范围内的压致结构相变和等温压缩行为，实验结果表明，该合金在低压时为fcc结构，在19.4 GPa压力附近出现压致fcc→hcp结构相变，直到43.2 GPa一直保持fcc、hcp二相共存；相变过程中，二相的molar体积相同；高压hcp相得晶格参数比值c/a基本上不随压力而变，可以表示为c/a=1.630±0.006；在卸压过程中，hcp相可保持到5.8 GPa，当卸压到常压时，该合金完全恢复到fcc结构；用Murnaghan等温固体状态方程对其压缩数据进行最小二乘法拟合，得到B₀=(166±12) GPa，B₀'=5.2±0.5；本文还给出了该合金的压致fcc→hcp结构相变模型，并对存在很宽的二相共存区间问题进行了初步探讨。