水性聚氨酯相变储能材料的相变机理

以聚乙二醇(PEG)为软段、六亚甲基二异氰酸酯-1,4-丁二醇-二羟甲基丙酸(HDI-BDO-DMPA)为硬段,制备了一系列水性聚氨酯相变储能材料(WPUPCM)。在相变过程中,由于软段PEG由聚合前的固-液相变转化为聚合后的固-固相变,因此,所制备的WPUPCM表现出固-固相变特性。为了研究其固态相变行为的本质和形成机理,揭示该类聚合物的能量贮存和转换机理,应用偏光显微镜、扫描电镜、原子力显微镜、X射线衍射仪分析研究了PEG、WPUPCM在相变过程中的晶体结构变化特征,讨论了WPUPCM固-固相转变的机理。     

高压下D,L-扁桃酸结构相变的拉曼光谱研究

采用金刚石对顶砧高压装置,在室温下对D,L-扁桃酸(C8H8O3)进行了原位高压拉曼光谱研究,实验最高压力为2.2 GPa.结果表明,原来的一些拉曼峰在0.6 GPa左右突然消失或者劈裂,并同时出现了一些新的拉曼峰.通过进一步分析D,L-扁桃酸的拉曼频率随压力的变化,发现许多拉曼峰的移动在0.6 GPa时都出现了拐点.D,L-扁桃酸在0.6 GPa发生了由正交相(Pbca)到单斜相(P21/c)的压致结构相变.通过分析相变前后晶体结构及拉曼振动模式的变化,认为此压致结构相变是由高压下分子的密堆积效应和氢键结构的重新排列导致.完全卸压至常压后,卸压拉曼光谱与常压拉曼光谱一致,表明此压致相变可逆.     

高压条件下的凝聚态化学

本文介绍了高压条件对凝聚态物质电子结构和晶体结构的影响,其中包括高压对元素外层电子结构、能带结构和晶体缺陷的影响,高压导致的原子配位数的增加、元素非正常氧化态、结构相变和态变。同时从十个方面介绍了高压条件下凝聚态物质间的化学反应,最后对高压条件下凝聚态化学未来的发展做了展望。     

高压下铌酸锌钶铁矿结构相变的原位拉曼光谱和X射线衍射研究

通过原位高压拉曼光谱和X射线衍射对ZnNb₂O₆晶体在29 GPa以下的结构转变进行了研究.拉曼光谱显示, 多数拉曼峰强度减弱, 且随着压力增加向高波数方向移动.压力频移曲线分别在10, 16 和20 GPa处形成了拐点.原位X射线衍射谱在10.6 GPa以上有旧峰消失和新峰出现.结果分析表明, ZnNb₂O₆钶铁矿结构压缩过程中发生了一个可逆压致相变, 此相变从10 GPa左右开始, 到16 GPa左右完成, 继续增加压力到20 GPa以上则形成无序状态.     

高温高压条件下ZSM-5沸石分子筛相变的研究

本文在高温高压条件下研究了具有热力学亚稳定性的全硅ZSM-5沸石分子筛的相转变。结果表明,压力对沸石分子筛的结构具有显著的影响。在4.0 GPa的冷压条件下,首次发现了全硅ZSM-5沸石分子筛通过骨架原子的重新调整而直接转变为全硅ZSM-11沸石分子筛,而且这一相转变是不可逆的。采取高压淬火的技术手段,在2.5GPa和800℃条件下,实现了全硅ZSM-5沸石分子筛向β石英相的转变;而在4.0GPa,1300℃条件下,全硅ZSM-5沸石分子筛转变为柯石英。首次得到在室温条件下稳定存在的β石英和柯石英。     

正磷酸铝高压下相变的分子动力学模拟

在300K、-20GPa至40GPa范围内,对正磷酸铝系统进行了定温定压分子动力学模拟.随着压力不断增加,正磷酸铝中的磷氧四面体基本不变,只是各向同性稍有压缩 ;铝氧四面体变得越来越畸变,但铝对氧的配位数仍保持4.所计算的晶胞压缩率与现有实验数据符合良好,晶格常数a比c减少得更厉害.在 20GPa处发生了一级相变,对径向分布函数分析的结果表明,这主要是铝氧四面体中氧 氧位置发生滑移的结果.从高压下回到零压,模拟表明又回复到原来的晶体结构.在 -15GPa下,晶体结构崩溃.对所得结果进行了讨论.     

二碘化铕的相变结构及荧光性质

本文用ＤＴＡ、Ｘ－ｒａｙ粉末衍射和荧光光度计等方法研究了ＥｕＩ２的相变和结构及荧光性质，用退火和淬火的方法得到了ＥｕＩ２的三种不同结构，并在荧光光谱上有不同的反映，首次发现ＥｕＩ２的低温变体，ＥｕＩ２－Ⅲ，正交晶系，空间群为ｐｎｍａ，ａ＝１．２２８７ｎｍ，ｂ＝０．４８８７ｎｍ，ｃ＝０．８３８４ｎｍ，Ｚ＝４。     

Li0.025Na0.975NbO3低温铁电相变及晶体结构

Li_(0.025)Na_(0.975)NbO_3 undergoes a ferroelectric-ferroelectric phase transition at 180K (on cooling) or 260 K (on heating). The structure determinations of both phases have been carried out by single crystal diffraction analysis. The ferroelectric room-temperature phase is orthorhombic, with space group Pc2_1b; the cell parameters are a=0.5508(3), b=1.5521(6), c=0.5568(2) nm. The structure is same with Li_(0.02)Na_(0.98)NbO_3. The ferroelectric low-temperature phase is also orthorhomhie, a=0.5497(7), b=0.7913(18), c=0.5565(10) nm, V=0.2421 nm, Z=4. There is disordernal position distribution of O(2) atomes in the low-temperature phase. Its “apparent space group” is P2/n2/n2/n. The relation of the ferroelectric properties to the low-temperature phase structure is discussed.     

高压后EuI2的结构及光谱性质研究

研究了高压后ＥｕＩ２的结构及光谱性质，发现高压后其结构由单斜转化为正交，对称性和Ｅｕ＾２＋配位数均有增加，４ｆ＾６５ｄ能级间距加大，能级重心降低。还讨论了发射光谱的红移现象，指认了新出现的４４９ｎｍ处的发射峰，并报道了ＥｕＩ２新的正交结构（ＥｕＩ２－Ｖ）。     

Structural Phase Transitions and Magnetic Superexchange in MIAgIIF3 Perovskites at High Pressure**

Pressure-induced phase transitions of M^IAg^IIF₃ perovskites (M=K, Rb, Cs) have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M=K and Rb consists of a transition from orthorhombic to monoclinic and back to orthorhombic, associated with progressive bending of infinite chains of corner-sharing [AgF₆]⁴⁻ octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF₃; this is associated with substantial deformation of the Jahn–Teller-distorted first coordination sphere of Ag^II and association of the infinite [AgF₆]⁴⁻ chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF₃ hosts lattices.     

Structural Phase Transitions of a Molecular Metal Oxide

The structural phase of a metal oxide changes with temperature and pressure. During phase transitions, component ions move in multidimensional metal–oxygen networks. Such macroscopic structural events are robust to changes in particle size, even at scales of around 10 nm, and size effects limiting these transitions are particularly important in, for example, high-density memory applications of ferroelectrics. In this study, we examined structural transitions of the molecular metal oxide [Na@(SO₃)₂(n-BuPO₃)₄Mo^V₄Mo^VI₁₄O₄₉]⁵⁻ (Molecule 1 ) at approximately 2 nm by using single-crystal X-ray diffraction analysis. The Na⁺ encapsulated in the discrete metal-oxide anion exhibited a reversible order–disorder transition with distortion of the Mo–O molecular framework induced by temperature. Similar order–disorder transitions were also triggered by chemical pressure induced by removing crystalline solvent molecules in the single-crystal state or by substituting the countercation to change the molecular packing.     

Crystal Structures and Phase Sequences of Metallocenium Salts with Fluorinated Anions: Effects of Molecular Size and Symmetry on Phase Transitions to Ionic Plastic Crystals

Sandwich compounds often exhibit various phase transitions, including those to plastic phases. To elucidate the general features of the phase transitions in metallocenium salts, the thermal properties and crystal structures of [Fe(C₅Me₅)₂]X ([ 1 ]X), [Co(C₅Me₅)₂]X ([ 2 ]X), and [Fe(C₅Me₄H)₂]X ([ 3 ]X) have been investigated, where the counter anions (X) are Tf₂N (=(CF₃SO₂)₂N^?), OTf (=CF₃SO₃^?), PF₆, and BF₄. The Tf₂N salts commonly undergo phase transitions from an ordered phase at low temperatures to an anion‐disordered phase, followed by a plastic phase and finally melt at high temperatures. All these salts exhibit a phase transition to a plastic phase, and the transition temperature generally decreases with decreasing cation size and increasing anion size. The crystal structures of these salts comprise an alternating arrangement of cations and anions. About half of these salts exhibit phase transitions at low temperatures, which are mostly correlated with the order–disorder of the anion.     

Correlation Instability and Phase Transitions in Octahedral Fluoride Systems

An ab initio study of octahedral hexafluoride molecules and ions has shown that innersphere correlation interactions lower the initial O_h symmetry of these molecules and ions to C₁ and that the observed high symmetry is pseudosymmetry. The microscopic mechanism of correlation instability of octahedral molecules and ions is related to the electric dipole nature of dispersion interactions. The anomalously high values of entropy for polymorphic transformations in transition and chalcogenideelement hexafluorides fit the model of order–disorder phase transitions in a system of internally distorted octahedral groups.     

Transitiometric Analysis of Pressure Effects on Various Phase Transitions

A scanning transitiometer has been used in investigations of 1st and 2nd order phase transformations in polymers. It was demonstrated taking as an example fusion of polyethylene at 200 MPa with a temperature scan as inducing variable that by recording simultaneously the rate of heat exchange and the rate of volume variations it is possible to determine in a single experiment the pressure derivative of temperature of this 1st order phase transition. For phase transformations similar to the 2nd order transitions the transitiometric analysis permits simultaneous measurements of pairs of thermodynamic derivatives which permit determination of pressure effects according to the Ehrenfest equations. For the glass transition in polystyrene at high pressures the pressure effect was similar independently of the pair of thermodynamic derivatives used (heat capacity and thermal expansivity or compressibility and thermal expansibility).This revised version was published online in November 2005 with corrections to the Cover Date.     

Lithium Argyrodites with Phosphorus and Arsenic: Order and Disorder of Lithium Atoms,Crystal Chemistry,and Phase Transitions

Crystal chemical data of high‐ (HT) and low‐temperature (LT) modifications of lithium argyrodites with the compositions Li₇PCh₆ (Ch=S, Se), Li₆PCh₅X (X=Cl, Br, I), Li₆AsS₅Br, and Li₆AsCh₅I (Ch=S, Se) based on single‐crystal, powder X‐ray (113 K<T<503 K) and neutron measurements (5 K<T<293 K) are presented. In the HT modifications, the Li atoms are strongly disordered over a fraction of the available tetrahedral holes, whereas in the LT modifications they occupy ordered crystallographic positions with a pronounced site preference that is analysed on a crystal chemical basis. The Ch/X partial structures remain nearly unchanged upon the reversible phase transitions. Crystal chemical and crystallographic relations between HT and LT modifications based on the Frank–Kasper model of tetrahedral close packing are discussed. X‐ray single‐crystal data for HT‐Li₆PS₅I show the electron density of the disordered Li to be smeared out over an extended region preferably inside face‐sharing double tetrahedra. A series of temperature‐dependent powder neutron data for Li₆PS₅I gives clear evidence for an HT/LT phase transition at ≈175 K with an ordering of the Li atoms in different polyhedra with coordination numbers between three and four.     

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Zirconia

The nanocrystalline cubic, tetragonal, and submicron monoclinic phases of pure zirconia were prepared by thermal decomposition of carbonate and hydroxide precursors. The crystallization and isothermal phase transformations of the oxide were studied using high temperature X‐ray diffraction, X‐ray diffraction and Raman spectra of quenched samples. Cubic zirconia formed first, and then progressively transformed to tetragonal and monoclinic phases at temperatures as low as 320°C. The cubic, tetragonal, and monoclinic phases for ZrO₂ were found to be distinct functions of crystallite size, indicating the nanocrystalline nature of these phases. They were found to exist within critical size ranges of 50 to 140 Å, 100 to 220 Å and 190 to 420 Å (±5 Å), respectively. Thus, as the crystallites grow during annealing, they first transform from cubic to tetragonal and then from tetragonal to monoclinic at critical sizes. The classical Avrami equation for nucleation and growth was applied to the tetragonal to monoclinic phase transition.