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.     

高压下轻元素单质的结构相变

压力是物质独立于温度和化学组分的一个重要维度,能够有效改变物质内部原子间的相互作用,诱导形成具有新结构和新性质的高压新相,可以揭示许多常压下不曾有过的新现象和新规律,为制备出常压条件下无法得到的新材料提供了创新性源泉。元素单质在常压下大多具有简单的晶体结构,但在高压下却发生了非常复杂的结构相变,形成了多种新颖的高压相,产生了超导、超硬、金属-绝缘体相变等奇异现象,吸引了科学界的广泛关注。本文针对元素周期表中前12种轻元素单质的高压结构相变、高压相的新奇物理特性及相变产生的物理机制进行了探讨和论述,并对实验和理论高压结构相变的研究方法进行了展望。     

Photocontrolled Phase Transitions and Reflection Behaviors of Smectic Liquid Crystals by a Chiral Azobenzene

The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4‐octyl‐4′‐cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)‐(?)‐4‐n‐hexyl‐4′‐menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans‐to‐cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)‐(?)‐1,1′‐binaphthyl‐2,2′‐diol (BN), a reflection band located in the short‐wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)‐2,2′‐methylendioxy‐1,1′‐binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.     

Phase Transitions in Channel Clathrates of Thiourea with Hexachloroethane and Tetrachloroethane

An explanation is proposed for the nature of phase transitions in channel clathrates of thiourea with hexachloroethane and tetrachloroethane. Analysis of experimental temperature dependences of specific heat, entropy, and NQR spectra indicates the existence of first and secondorder phase transitions. In the thiourea + hexachloroethane compound, the secondorder phase transition (94.60 K, entropy gain Rln2) implies a transformation of two equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. In the thiourea + tetrachloroethane compound, the secondorder phase transition (224K, entropy change Rln 6) occurs with transformation of six equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. The next secondorder phase transition observed in this compound at 248 K leads to disappearance of the four possible orientations of guest molecules in the clathrate channel of hexagonal section and to a transition of guest molecules to chaotic orientation (entropy increment equals Rln 4).     

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.     

Structural Forces near Phase Transitions of Liquid Crystals

The structure of the fragile liquid‐crystalline phases has a strong impact on the forces between bodies immersed in a liquid crystal (LC). We have equipped an atomic force microscope with a precise temperature control and measured various liquid‐crystalline structural forces at temperatures close to the phase transitions. The observed forces agree well with predictions of Landau–de Gennes phenomenological theory of LCs, even at a nanoscale length. In addition to this, we have observed a molecular layer, adsorbed on the surfactant‐covered glass surface, and determined its thickness and elastic properties.     

Hydrogen-Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Two new crystalline rotors 1 and 2 assembled through N−H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4-diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through ¹H T₁ relaxometry experiments indicate very low rotational activation barriers (E_a) of 0.67 kcal mol⁻¹ for 1 and 0.26 kcal mol⁻¹ for 2 , indicating that DABCO can reach a THz frequency at room temperature in the latter. These E_a values are supported by solid-state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single-crystal X-ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here.     

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.     

Add a Pinch of Tetrel: The Transformation of a Centrosymmetric Metal into a Nonsymmorphic and Chiral Semiconductor

Centrosymmetric skutterudite RhP₃ was converted to a nonsymmorphic and chiral compound RhSi_0.3P_2.7 (space group P2₁2₁2₁) by means of partial replacement of Si for P. The structure, determined by a combination of X-ray crystallography and solid state ³¹P NMR, exhibits branched polyanionic P/Si chains that are unique among metal phosphides. A driving force to stabilize the locally noncentrosymmetric cis-RhSi₂P₄ and fac-RhSi₃P₃ fragments is π-electron back-donation between the Rh t_2g-type orbitals and the unoccupied antibonding Si/P orbitals, which is more effective for Si than for P. In situ studies and total energy calculations revealed the metastable nature of RhSi_0.3P_2.7. Electronic structure calculations predicted centrosymmetric cubic RhP₃ to be metallic which was confirmed by transport properties measurements. In contrast, the electronic structure for chiral orthorhombic RhSi_0.3P_2.7 contained a bandgap, and this compound was shown to be a narrow gap semiconductor.     

Reversible Phase Transitions in a Buckybowl Monolayer

Like penguins on ice , buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

     

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.     

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.     

In Situ Observation of Ferroelastic Domain and Phase Transition in a Three-Dimensional Molecular Crystal

Massive efforts have been devoted to designing molecular ferroic materials by molecular modification. For molecular ferroelastic, previous work is focused on the temperature-dependent ferroelastic domains, however, few are related to controlling the ferroelastic domain by the stress. Inspired by the “quasi-spherical theory” and fluorination effect, we designed a more flexible (MedabcoF)²⁺ (MedabcoF=1-fluoro-4-methyl-1,4-diazoniabicyclo[2.2.2]octane) cation by introducing a methyl group and a fluorine atom at the two symmetrical ends of the Dabco (1,4-diazoniabicyclo[2.2.2]octane) and synthesized a hybrid 3D perovskite (MedabcoF)Rb(BF₄)₃ ( 1 ) which displays three reversible phase transitions accompanying dual ferroelastic behavior. Besides, it not only exhibits ferroelastic domains switching by the thermal stimulation, and the sensitive reaction of in situ domains under the stress of it is also realized. This work not only achieves a force-controlled ferroelastic domain but develops a more profound comprehension of the relationship between the thermal motion behavior of guest cations and the intriguing properties of materials.     

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.     

氰戊菊酯微乳液相行为及其结构转变

氰戊菊酯微乳液相行为及其结构转变;相行为; 微乳液; 电导率; 氰戊菊酯     

Order-Disorder Structural Phase Transitions in Alkali Perchlorates

Order-disorder structural phase transitions in alkali perchlorates MClO₄ (M=Na, K, Rb, Cs) are investigated using molecular dynamics simulation. The potentials in the simulations are based on the Gordon-Kim modified electron gas formalism extended to molecular ions. The simulations yield first-order phase transitions in perchlorates from low temperature orthorhombic structures to high temperature cubic NaCl structures. The perchlorate ions are found to be orientational disordered in the high temperature phases.     

Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin (ls)-[Co^III(nEGEspy)₂(3,6-DTBSQ)(3,6-DTBCat)] (ls-[Co^III], 3,6-DTBSQ = 3,6-di-tert-butyl semiquinonato, 3,6-DTBCat = 3,6-di-tert-butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls-[Co^III] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high-spin (hs)-[Co^II(nEGEspy)₂(3,6-DTBSQ)₂] (hs-[Co^II]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls-[Co^III] exhibited partial VT (33 %) and only a 9.3 % VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.