Pressure-induced structural transition in n-pentane: a Raman study

Pressure-induced Raman spectroscopy studies on n-pentane have been carried out up to 17 GPa at ambient temperature. n-Pentane undergoes a liquid-solid transition around 3.0 GPa and a solid-solid transition around 12.3 GPa. The intensity ratio of the Raman modes related to all-trans conformation (1130 cm-1 and 2850 cm-1) to that of gauche conformation (1090 cm-1 and 2922 cm-1) suggests an increase in the gauche population conformers above 12.3 GPa. This is accompanied with broadening of Raman modes above 12.3 GPa. The high-pressure phase of n-pentane above 12.3 GPa is a disordered phase where the carbon chains are kinked. The pressure-induced order-disorder phase transition is different from the behavior of higher hydrocarbon like n-heptane.     

High-pressure- and low-temperature-induced changes in [(CH3)2NH(CH2)2NH3][SbCl5

The structure of N,N-dimethylethylenediammonium pentachloroantimonate(III), [(CH3)2NH(CH2)2NH3][SbCl5], NNDP, was investigated at 100 and 15 K at ambient pressure, as well as at pressures up to 4.00 GPa at room temperature in the diamond-anvil cell. The stable structure at low temperatures and low pressures consists of isolated [SbCl5]2- anions and [(CH3)2NH(CH2)2NH3]2+ cations. The inorganic anions have a distorted square pyramidal geometry. They are arranged in linear chains parallel to the c axis. In contrast to the low-temperature studies, where no phase transition was detected, pressure induces a P2(1)/c --> P2(1)/n phase transition between 0.55 and 1.00 GPa, accompanied by a doubling of the a unit-cell parameter. This solid-solid transition results from changes in the electron configuration of the Sb(III) atom and formation of the Sb-Cl bridging bonds between inorganic polyhedra to form, at approximately 1.0 GPa, isolated [Sb2Cl10]4- units consisting of [SbCl6]3- octahedra and [SbCl5]2- square pyramids connected by a common corner. The intermolecular distances continuously decrease with further increase in pressure, and at approximately 3.1 GPa, zigzag [{SbCl5}n]2n- chains containing corner-sharing [SbCl6]3- octahedra are formed. The unit-cell volume of NNDP decreases by 18.15% between room pressure and 4.00 GPa. The linear distortions of the [SbCl5]2- and [SbCl6]3- polyhedra decrease with increasing pressure and decreasing temperature and indicate a reduction in the stereochemical activity of the lone electron pair on the Sb(III) atom.     

Pressure-induced phase transitions on a liquid crystalline europium(III) complex

The effect of pressure on the phase behavior of the liquid crystalline complex [Eu(bta)(3)L(2)] (bta is benzoyltrifluoroacetonate, and L is the Schiff base 2-hydroxy-N-octadecyl-4-tetradecyloxybenzaldimine) was studied by X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy. The pressure was varied between ambient pressure and 8.0 GPa. [Eu(bta)(3)L(2)] exhibits a smectic A (SmA) phase at room temperature. The complex undergoes a transition from the SmA phase to a solid lamellar structure around 0.22 GPa and another transition from the solid lamellar phase to an amorphous state from 1.6 to 3.5 GPa. At low pressures, the smectic layer spacing increases, and the intermolecular distance decreases. Above 3.5 GPa, both the interlamellar and the intermolecular spacings hardly change, but the intensity of X-ray reflections exhibits a remarkable decrease and eventually vanishes. An interpretation of the changes in the molecular structure is given. It was found that less interdigitation of the alkyl chains situated in adjacent layers and/or a full extension of the alkyl chains occurred at low pressures and that the second phase transition was accompanied by a transfer of the hydrogen atom from the nitrogen atom of the imine group to the oxygen atom of the Schiff base ligand. The effect of applying pressure equals that of the lanthanide contraction on the phase behavior.     

The effect of Fe-doping on superconductivity of YBa2Cu30 y

In the present paper, an approximate band-structure treatment based on the EHMO approach is suggested and used to calculate the electronic structures of the Fe-doped superconductors YBa₂Cu_3–x Fe _x O _y . The present treatment gives, indeed, average band structures and average densities of states as the doping fraction increases. From investigations of the influence of the Fe-doping at the Cu-site on their properties, it is shown that as the Fe-doping fractionx in YBa₂Cu_3–x Fe _x O _y is raised from 0.0 to 0.5, (i) the broad anisotropic bands arising from the 1D Cu-O chains and the 2D Cu-O planes are displaced and depart from the Fermi levelE _f toward the high-energy zone by degrees, while the total electronic densities of states nearE _f are drastically decreased; (ii) the band arising from the Cu-O chains doped by Fe is gradually separated from the broad anisotropic bands arising from the 2D Cu-O planes; (iii) at the doping fractionx = 0.5, the Fe-doping results in an energy gap (about 0.2 eV) near E_f; (iv) the oxygen content is not a predominant factor for the superconducting properties of the Fe-doped Y-Ba-Cu-O system; (v) the total densities of states atE _f,N(E _f), and their decrease caused by the Fe-doping arise mainly from the 2D Cu-O planes, which implies the important role of the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system.     

New organically templated copper(I) sulfites: the role of sulfite anion as both soft and hard ligand

Two new organically templated layered copper(I) sulfites, namely, {H2pip}{Cu3(CN)3(SO3)} (1) and {H2pip}{NaCu2(SO3)2Br(H2O)}.2H2O (2) (pip = piperazine), have been synthesized by hydrothermal reactions of copper(I) cyanide or copper(I) bromide with NaHSO3 and piperazine. Both compounds exhibit a layered structure. The 2D layer of {Cu3(CN)3(SO3)}2- in 1 is composed of 1D chains of copper(I) cyanide interconnected by sulfite anions via both Cu-S and Cu-O bonds, whereas the 2D layer of {NaCu2(SO3)2Br}2- in 2 is formed by 1D chains of copper(I) bromide and 1D sodium(I) aqua chains that are interconnected by sulfite anions via Na-O, Cu-S, and Cu-O bonds. Chemical bonding in 1 and 2 has been also investigated by theoretical calculations based on DFT methods.     

Study on the effects of fluorine and oxygen deficiency on YBa_2Cu_3O_7 by ab initio method

The calculations of clusters modeling the fluorine-doping and oxygen deficiency of YBa2Cu3O2,have been performed by the method of all-electron ab initio Hartree-Fock with self-consistent crystal field Results show that in CuO planes electric charge significantly increases,the chemical valence of Cu decreases and the covalent bonding of Cu-O greatly weakens owing to oxygen deficiency,while the effect of F restores the local electronic structure of YBa2Cu3O7 The reported opinion that F occupied the oxygen vacancy in Cu-O chains seems disputable according to the calculated bonding characteristics.     

BH3 under pressure: leaving the molecular diborane motif

Molecular and crystalline structures of (BH(3))(n) have been theoretically studied in the pressure regime from 1 atm to 100 GPa. At lower pressures, crystals of the familiar molecular dimer are the structure of choice. At 1 atm, in addition to the well-characterized β diborane structure, we suggest a new polymorph of B(2)H(6), fitting the diffraction lines observed in the very first X-ray diffraction investigation of solid diborane, that of Mark and Pohland in 1925. We also find a number of metastable structures for oligomers of BH(3), including cyclic trimers, tetramers, and hexamers. While the higher oligomers as well as one-dimensional infinite chains (bent at the bridging hydrogens) are less stable than the dimer at ambient pressure, they are stabilized, for reasons of molecular compactness, by application of external pressure. Using periodic DFT calculations, we predict that near 4 GPa a molecular crystal constructed from discrete trimers replaces the β diborane structure as the most stable phase and remains as such until 36 GPa. At higher pressures, a crystal of polymeric, one-dimensional chains is preferred, until at least 100 GPa.     

Pressure-induced phase transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.     

Synthesis, structure, magnetic properties and structural distortion under high pressure of a new osmate, Sr2CuOsO6

A new osmate Sr₂CuOsO₆ was synthesized and its structure refined using powder synchrotron X-ray diffraction. The results of the Reitveld refinements indicate complete B-cation order in a double perovskite crystal structure. Furthermore, the analysis of the B-cation bond lengths indicates a symmetric coordination around Os, as opposed to a significant distortion of Cu-O bond lengths. The octahedral distortion around Cu(II) is characteristic of a Jahn-Teller distortion. Within the crystal structure of Sr₂CuOsO₆, the long Cu-O bonds are aligned in the same direction along the c-axis in the tetragonal unit cell. This parallel arrangement of long Cu-O bonds produces a lattice parameter ratio, c/(2^1/2a), that is greater than unity. The magnetic susceptibility of Sr₂CuOsO₆ was measured using a SQUID magnetometer and was observed to be consistent with an assignment of Cu(II)-Os(VI) formal oxidation states, thus confirming the bond valences calculated on the basis of the crystal structure. In perovskites, octahedral tilting and bond shortening are two competing compression mechanisms. Compression mechanisms of this double perovskite were characterized using high-pressure synchrotron X-ray diffraction. Application of hydrostatic pressure up to 6 GPa significantly decreased the lattice parameter ratio, demonstrating the primary compression mechanism is a shortening of the long Cu-O bond.     

Rubidium polyhydrides under pressure: emergence of the linear H3(-) species

The structures of compressed rubidium polyhydrides, RbH(n) with n>1, and their evolution under pressure are studied using density functional theory calculations. These phases, which start to stabilize at only P = 2 GPa, consist of Rb(+) cations and one or more of the following species: H(-) anions, H(2) molecules, and H(3)(-) molecules. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH(5) and RbH(3) , which metallize above 200?GPa. At the highest pressures studied, our evolutionary searches find an RbH(6) phase which contains polymeric (H(3)(-))(∞) chains that show signs of one-dimensional liquid-like behavior.     

高压下两种8-羟基喹啉络合物的发光行为和结构变化

测定了在高压条件下两种金属（钙和锌）的8 羟基喹啉络合物的晶体粉末样品的发光行为和原位X光衍射光谱.结果表明,压力对其发光性质产生极大的影响.随着压力的增加,8 羟基喹啉钙的发光强度在3 GPa以内时大大增加,随后发光强度快速下降,到7 GPa左右时几乎为零.而8 羟基喹啉锌的发光强度随压力的增加而逐渐降低,到7 GPa左右时约为常压的10％.高压下的原位X光衍射结果表明,8 羟基喹啉钙的晶体在3～4 GPa开始 发生非晶化相变,在7 GPa时该非晶化相变完成,样品的X光衍射完全消失.而8 羟基喹啉锌在压力的作用下（至16 GPa）没有发生明显的相变.     

Electrical properties and behaviors of cuprous oxide cubes under high pressure

An accurate in situ electrical resistivity measurement of cuprous oxide cubes has been conducted in a diamond anvil cell at room temperature with pressures up to 25 GPa. The abnormal electrical resistivity variation found at 0.7-2.2 GPa is attributed to the phase transformation from a cubic to a tetragonal structure. Three other discontinuous changes in the electrical resistivity are observed around 8.5, 10.3, and 21.6 GPa, corresponding to the phase transitions from tetragonal to pseudocubic to hexagonal to another hexagonal phase, respectively. The first-principles calculations illustrate that the electrical resistivity decrease of the tetragonal phase is not related to band-gap shrinkage but related to a higher quantity of electrons excited from strain-induced states increasing in band gap with increasing pressure. The results indicate that the Cu(2)O cubes begin to crush at about 15 GPa and completely transform into nanocrystalline at 25 GPa.     

The Local Atomic Structures of Liquid CO at 3.6 GPa and Polymerized CO at 0 to 30 GPa from High‐Pressure Pair Distribution Function Analysis

The local atomic structures of liquid and polymerized CO and its decomposition products were analyzed at pressures up to 30 GPa in diamond anvil cells by X‐ray diffraction, pair distribution function (PDF) analysis, single‐crystal diffraction, and Raman spectroscopy. The structural models were obtained by density functional calculations. Analysis of the PDF of a liquid CO‐rich phase revealed that the local structure has a pronounced short‐range order. The PDFs of polymerized amorphous CO at several pressures revealed the compression of the molecular structure; covalent bond lengths did not change significantly with pressure. Experimental PDFs could be reproduced with simulations from DFT‐optimized structural models. Likely structural features of polymerized CO are thus 4‐ to 6‐membered rings (lactones, cyclic ethers, and rings decorated with carbonyl groups) and long bent chains with carbonyl groups and bridging atoms. Laser heating polymerized CO at pressures of 7 to 9 GPa and 20 GPa resulted in the formation of CO₂.     

Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy

We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.     

Effect of pressure on thermal conductivity and pressure collapse of ice in a polymer-hydrogel and kinetic unfreezing at 1 GPa

We report a study of aqueous solutions of poly(vinylalcohol) and its hydrogel by thermal conductivity, κ, and specific heat measurements. In particular, we investigate (i) the changes in the solution and the hydrogel at 0.1 MPa observed in the 350-90 K range and of the frozen hydrogel at 130 K observed in the range from 0.1 MPa to 1.3 GPa, and (ii) the nature of the pressure collapse of ice in the frozen hydrogel and kinetic unfreezing on heating of its high density water at 1 GPa. The water component of the polymer solution on cooling either first phase separates and then freezes to hexagonal ice or freezes without phase separation and the dispersed polymer chains freeze-concentrate in nanoscopic and microscopic regions of the grain boundaries and grain junctions of the ice crystals in the frozen state of water in the hydrogel. The change in κ with temperature at 1 bar is reversible with some hysteresis, but not reversible with pressure after compression to 0.8 GPa at 130 K. At high pressures the crystallized state collapses showing features of κ and specific heat characteristic of formation of high density amorphous solid water. The pressure of structural collapse is 0.08 GPa higher than that of ice at 130 K. The slowly formed collapsed state shows kinetic unfreezing or glass-liquid transition temperature at 140 K for a time scale of 1 s. Comparison with the change in the properties observed for ice shows that κ decreases when the polymer is added.     

Benzene under high pressure: a story of molecular crystals transforming to saturated networks, with a possible intermediate metallic phase

In a theoretical study, benzene is compressed up to 300 GPa. The transformations found between molecular phases generally match the experimental findings in the moderate pressure regime (<20 GPa): phase I (Pbca) is found to be stable up to 4 GPa, while phase II (P4(3)2(1)2) is preferred in a narrow pressure range of 4-7 GPa. Phase III (P2(1)/c) is at lowest enthalpy at higher pressures. Above 50 GPa, phase V (P2(1) at 0 GPa; P2(1)/c at high pressure) comes into play, slightly more stable than phase III in the range of 50-80 GP, but unstable to rearrangement to a saturated, four-coordinate (at C), one-dimensional polymer. Actually, throughout the entire pressure range, crystals of graphane possess lower enthalpy than molecular benzene structures; a simple thermochemical argument is given for why this is so. In several of the benzene phases there nevertheless are substantial barriers to rearranging the molecules to a saturated polymer, especially at low temperatures. Even at room temperature these barriers should allow one to study the effect of pressure on the metastable molecular phases. Molecular phase III (P2(1)/c) is one such; it remains metastable to higher pressures up to ～200 GPa, at which point it too rearranges spontaneously to a saturated, tetracoordinate CH polymer. At 300 K the isomerization transition occurs at a lower pressure. Nevertheless, there may be a narrow region of pressure, between P = 180 and 200 GPa, where one could find a metallic, molecular benzene state. We explore several lower dimensional models for such a metallic benzene. We also probe the possible first steps in a localized, nucleated benzene polymerization by studying the dimerization of benzene molecules. Several new (C(6)H(6))(2) dimers are predicted.     

Crystal Structure of the High Pressure Phase(II) in CuGeO3

Crystal structures of the ambient pressure and temperature phase (phase I) and high pressure phase (phase II) in CuGeO₃ were studied by means of the high pressure single‐crystal X‐ray diffraction method in a diamond anvil cell using high power X‐ray generator and imaging plate detector. The pressure dependence of the atomic displacements in the phase I was investigated under the hydrostatic pressure of 0.1 MPa and 2.9 and 3.9 GPa. The lattice is particularly compressive in the b direction. In phase I the rippled layers are formed by the corner‐shared chains of GeO₄ tetrahedra and edge‐linked planar CuO₄. Major effects of pressure, directly related to the shortening of the b‐axis, consist of an enhanced folding of the rippled layers towards the b‐direction and of a shortening of the weak Cu–O bond. The crystal structure of phase II is monoclinic, a = 4.935(57), b = 6.754(14), c = 6.208(11) Å, β = 92.67(3)°, space group; P2₁/c. The transition from phase I to II involves a corrugated arrangement of the both cation with some oxygens around the c‐axis. Ge ion at the transition point of 6.4 GPa changes its coordination number from four‐fold to five‐fold, and Cu ion occupies a position of seven‐fold site. The structure of phase II is explained as a slab structure having unique edge‐ and corner‐sharing arrangements of GeO₅ and CuO₇ polyhedra. The average Ge–O and Cu–O distances in phase II is 1.92 and 2.17 Å, respectively, at 6.5 GPa.     

Pressure response of three-dimensional cyanide-bridged bimetallic magnets

Effects of pressure on the structures and magnetic properties of three types of 3-D cyanide-bridged bimetallic coordination polymer magnets, MnIICrIII ferrimagnet [Mn(en)]3[Cr(CN)6]2.4H2O (1; en = ethylenediamine), NiIICrIII ferromagnet [Ni(dipn)]3[Cr(CN)6]2.3H2O (2; dipn = N,N-di(3-aminopropyl)amine), and NiIIFeIII ferromagnet [Ni(dipn)]2[Ni(dipn)(H2O)][Fe(CN)6]2.11H2O (3), were systematically examined under hydrostatic pressure up to 19.8 GPa using a piston-cylinder-type pressure cell and a diamond anvil cell. The ferrimagnet 1 showed the reversible crystalline-to-amorphous-like phase change, and the magnetic phase transition temperature (TC) was reversibly changed from 69 K at 0 GPa to 126 K at 4.7 GPa. At higher pressure, the net magnetization was suppressed with increasing pressure, and the magnetic state at 19.8 GPa was assumed to be paramagnetic. The initial ferrimagnetic phase of 1 was not recovered after releasing the pressure from 19.8 GPa. The magnetic phase of 2 was reversibly converted between ferromagnetic and paramagnetic-like phase in the range 0 相似文献   

High-pressure-induced phase transitions in pentaerythritol: X-ray and Raman studies

The high-pressure response of pentaerythritol crystals has been examined to 10 GPa in diamond-anvil cells using angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The results reveal two first-order phase transitions: one at 4.8 GPa from phase I, tetragonal I(), to phase II, orthorhombic Pnn2C2v10, with a small approximately 0.5% volume change, and the other at 7.2 GPa to phase III with an unknown crystal structure. We found that phase I exhibits a large crystallographic anisotropy which rapidly decreases with increasing pressure: the ratio of linear compressibilities between two primary crystal axes decreases from betao= 8.1 at 1 atm to betaP = 2.6 at 4 GPa. We suggest that this apparent decrease in crystal anisotropy is due to the disruption of hydrogen bonding in the (001) plane of phase I and eventually leads to an orthorhombic distortion from a quadrilateral network structure in phase I to a quasi one-dimensional structure in phase II. The crystal structure of phase III exhibits a disordered character, and it is likely a conformational variant of phase II.     

Structural and EPR study of the dependence on deuteration of the Jahn-Teller distortion in ammonium hexaaquacopper(II) sulfate, (NH4)

The variation of the EPR spectra with degree of deuteration of the partially deuterated Tutton salt ammonium hexaaquacopper(II) sulfate, (NH4)2[Cu(H2O)6](SO4)2, has been measured at 293 K. The measurements indicate that the structure changes quite abruptly from that of the pure hydrogenous salt to that of the fully deuterated salt at approximately 50% deuteration. The structure of a crystal in which approximately 42% of the hydrogen atoms were replaced by deuterium was elucidated at 15 K by single-crystal time-of-flight neutron diffraction. The hexaaquacopper(II) complex exhibits an orthorhombically distorted, tetragonally elongated octahedral coordination geometry (Cu-O bond distances of 2.281(1), 2.007(1), and 1.975(1) A). The structure is very similar to that reported for the undeuterated salt at 9.6 K, and markedly different from that of the fully deuterated compound at 15 K, which has similar Cu-O bond lengths but with the directions of the long and intermediate bonds interchanged. There is no evidence for disorder or partial switching of the Cu-O bond directions. This is consistent with the temperature dependence of the EPR spectrum of the approximately 42% deuterated compound, which indicates a thermal equilibrium between the two structural forms close to room temperature similar to that reported for the undeuterated compound, but complete reversion to the low-temperature phase on cooling to 5 K. The possible influence of deuteration upon the hydrogen-bonding distances and the bearing of this upon the structural modifications of the compound are discussed.