Reversible phase transition of the 1:1 complex between 18-crown-6 and n-propylammonium triiodide

Solid-state structure of the crystalline 1:1 complex [C 3 H 10 N(18-crown-6)] + [I 3 ](1) between 18-crown-6 and n-propylammonium triiodide has been determined at 293 and 93 K,respectively,showing a change from monoclinic P2 1 /m to monoclinic P2 1 /a.Crystal structural analysis shows that in addition to van der Waals’ forces,conventional N-H···O hydrogen bonds are the key interactions.Measurements of unit cell parameters versus temperature show that the values of one of the three axes and the crystal volume change abruptly and remarkably at 220 K,indicating a first-order phase transition.The lack of the mirror plane in the low temperature structure is the most important differences between the two structural forms.Differential scanning calorimetry(DSC) measurement confirms that 1 undergoes a reversible phase transition at about 220 K with a thermal hysteresis of 3.5 K.The relatively large latent heat makes 1 a good candidate for phase change materials.The phase transition is accompanied by an anomaly of dielectric constant during heating and cooling process near the phase transition temperature.     

The structure and dielectric properties of ionic compounds with flexible ammonium moiety

The 3-morpholin-4-yl-propyl-ammine tetrafluoroborate(MPA+)(BF4)(1) and the 3-morpholin-4-ylpropyl-ammine perchlorate(MPA+)(Cl O4)(2) were synthesized and separated as colorless block crystal.The(MPA+)(BF4) undergoes a reversible phase transition at ca.347.3 K with a hysteresis of16.1 K.Dielectric measurement also can confirm the transition.The crystal structure determined at300 K(monoclinic,P21/c,a = 9.978(2) ,b = 7.5206(15) ′,c = 15.058(3) ,b = 96.99(3)8,v = 1121.6(4)  3,z = 4) and 373 K(triclinic,P-1,a = 10.021(7) ,b = 7.662(3) ,c = 7.920(4) ,a = 95.012(14)8,b = 101.67(4)8,g = 91.48(3)8,v = 592.7(6) 3,z = 2) reveal that this compound has two phase.The most distinct is the BF4 anion is disorder at 373 K,which is probably the driving force of the phase transition.DSC measurement,dielectric measurement and the crystal structure of the compound 2 did not change at 300 K and 100 K.     

Switchable Dielectric Phase Transition Induced by a Twisting Transformation in Diglycine Methanesulfonate

A new glycine‐based reversible phase transition compound diglycine methanesulfonate ( 1 ) has been successfully synthesized. Differential scanning calorimetry (DSC) measurements of 1 showed a pair of broad peaks around 134 K (T_c=phase transition temperature) with a slight thermal hysteresis during the heating/cooling cycle, thereby indicating that this compound undergoes a reversible second‐order phase transition. Dielectric measurements further confirmed the phase transition and revealed a switchable response to the ambient temperature change for the dielectric constants of 1 , namely, the dielectric constants have a distinctive step‐like anomaly switching between a high dielectric state in the room temperature phase (RTP) and a low state in the low temperature phase (LTP). Variable‐temperature single‐crystal X‐ray diffraction analyses show that 1 undergoes an atypical transition from the space group P2₁/m in the RTP to P2₁/c in the LTP. The origin of the switchable dielectric phase transition is ascribed to the movement of the moieties in 1 from the equilibrium position, and this stems from the twisting of the molecules in the compound. We believe that these findings will be useful in exploring switchable dielectric phase transition materials.     

High temperature crystal structures and superionic properties of SrCl2, SrBr2, BaCl2 and BaBr2

The structural properties of the binary alkaline-earth halides SrCl₂, SrBr₂, BaCl₂ and BaBr₂ have been investigated from ambient temperature up to close to their melting points, using the neutron powder diffraction technique. Fluorite-structured SrCl₂ undergoes a gradual transition to a superionic phase at 900–1100 K, characterised by an increasing concentration of anion Frenkel defects. At a temperature of 920(3) K, the tetragonal phase of SrBr₂ undergoes a first-order transition to a cubic fluorite phase. This high temperature phase shows the presence of extensive disorder within the anion sublattice, which differs from that found in superionic SrCl₂. BaCl₂ and BaBr₂ both adopt the cotunnite crystal structure under ambient conditions. BaCl₂ undergoes a first-order structural transition at 917(5) K to a disordered fluorite-structured phase. The relationship between the (disordered) crystal structures and the ionic conductivity behaviour is discussed and the influence of the size of the mobile anion on the superionic behaviour is explored.     

Synthesis and Crystal Structures of Tris-[3, 5-bis(trifluoromethyl)phenyl]arsine Oxide at 293 and 100 K and the Localization of the Trifluoromethyl Groups

Tris[3, 5-bis(trifluoromethyl)phenyl]arsine oxide ( 1 ) was synthesised by oxidation of tris[3, 5-bis(trifluoromethyl)phenyl]arsine with hydrogen peroxide in acetone. At 293 K, it crystallizes in the trigonal space group R3c (a = 20.2947(12) Å, c = 11.2484(13) Å, Z = 6, R₁ = 0.0254). The compound undergoes a phase transition upon cooling, and it crystallizes in the monoclinic space group Cc at 100 K (a = 13.8621(13) Å, b = 18.6537(17) Å, c = 11.2874(10) Å, Z = 4, R₁ = 0.0444). The crystal structures of both phases were determined. The fluorine atoms of the trifluoromethyl groups are strongly disordered at room temperature, which probably indicates a rotational motion in the plane of the fluorine atoms. This motion slows down while lowering the temperature, and the fluorine atoms are localized at 100 K. This point is illustrated by comparison of the experimental electron densities at the CF₃ groups. The packing pattern in both structures consists of parallel columns of ecliptically stacked molecules. The columns are hexagonally arranged.     

The uniqueness of the propyl compound in the series (CnH2n+1NH3)2MnCl4 with n = 1–10

The perovskite-like layer structures of (C_nH_2n+1NH₃)₂MnCl₄ with n = 1–10 have been investigated by X-ray methods to determine their lattice constants and the temperature of transition to the respective zero-tilt phases. It has been found that the propyl compound is distinctive in that it exhibits the highest temperature of transition to a zero-tilt phase, the longest MnCl bond, and the largest difference between the a and b lattice parameters. Furthermore, it undergoes a large number of phase transitions (five), some of which are accompanied by the formation of commensurable or incommensurable superstructures. The special position of this compound is attributed to geometrical peculiarities related to the terminal methyl groups. The propyl compound is the only member of the series in which coupled movements of terminal methyl groups of neighboring strata do not seem to be important below 400°K.     

Structural characterization, thermal, dielectric, vibrational properties and molecular motions in

[C₃N₂H₅]₆[Bi₄Br₁₈] has been synthesized and characterized by the X-ray (at 293 and 110 K), calorimetric, dilatometric and dielectric measurements. At room temperature it crystallizes in the monoclinic space group, C2/m. A crystal structure consists of disordered imidazolium cations and ordered discrete tetramers of [Bi₄Br₁₈]^6-. This compound reveals a rich polymorphism in a solid state. It undergoes three solid–solid phase transitions: from phase I to II at 426/423 K (heating–cooling), II→III at 227 K and III→IV at 219.5/219 K. A clear dielectric relaxation process is found in the room temperature phase II. Infrared studies of the polycrystalline [C₃N₂H₅]₆[Bi₄Br₁₈] showed that the ν(N–H), δ(ring) and δ(C–H) modes of the imidazolium cations appeared to be very sensitive to the IV→III phase transition. ¹H NMR measurements confirmed a key role of the imidazolium cations in the phase transitions mechanisms at low temperatures.     

Rotation barriers around the carbene-metal bond of transition metal complexes of cycloheptatrienylidenes

Rotation around the carbonmetal bond of a cycloheptatriene and three cycloheptatrienylidene complexes of iron have been examined using temperature-dependent NMR studies. Neither η¹-cycloheptatrienylidene-η⁵-cyclopentadienyldicarbonyliron hexafluorophosphate (3) nor 1,2-benzo-5-(η⁵-cyclopentadienylcarbonyltri-n-butylphosphineiron)cycloheptatriene (10) showed any temperature dependence to ?105°C. From this it is concluded that the former probably prefers the electronically less favored (but sterically preferred) conformation 3b while the rotation barrier for the latter is too small to be observed in this temperature range. Both η¹-cycloheptatrienylidene-η⁵-cyclopentadienylcarbonyltri-n-butylphosphineiron hexafluorophosphate (11) and η¹-4,5-benzocycloheptatrienylidene-η⁵-cyclopentadienylcarbonyltri-n-butylphosphineiron hexafluorophosphate (12) showed rotation barriers that could be measured in the temperature range employed (RT to ?105°C). The barrier in the benzannelated complex 12 is greater than in 11. This is consistent with Hoffmann's [2] prediction that an electronic component should contribute to conformational preferences and rotational barriers in carbene complexes of unsymmetrically substituted transition metals.     

Thermochromism in nickel(II) complexes: thermal, solid state H NMR and single crystal X-ray analysis of bis-(N,N-dimethyl-1,2-ethanediamine) nickel(II) perchlorate

The complex bis-(N,N-dimethyl-1,2-ethanediamine) nickel(II) perchlorate undergoes a first-order thermochromic phase transition at ca. 476 K, changing its color from orange to red. The room temperature X-ray crystal structure determination showed that the nickel ion possesses a square-planar geometry with two five membered chelate rings, in the δλ conformation, forming the NiN₄ chromophore. The broad-line ¹H NMR indicates the onset of a dynamic disorder of diamine chelate rings at the phase transition temperature region, while T₁ measurement of ¹H affords the activation energy of the puckering metal chelate rings to be 26 kJ mol⁻¹. The electronic spectrum revealed that a weakening of ligand field around the nickel is associated with the phase transition.     

C60固体中的相变

本文用（ｅｘｐ－６－１）势对固体Ｃ６０中分子间的相互作用进行计算，给出常温以下固体Ｃ６０中分子存在的状态，计算表明，常温下Ｃ６０高速旋转，随着温度降低，旋转速度逐渐减小，并且旋转轴向也逐渐趋于单一，当降到－２６０Ｋ时发生一级相变，此时ＦＣＣ晶胞中（０，０，０），１／２，１／２，０），（１，１／２，１／２）及（１／２，０，１／２）四套格点上的分子各只绕［１１１］三重轴旋转，Ｃ６０晶体从ＦＣＣ结构变为     

Iso-structural phase transition in tetramethylammonium nickel（Ⅱ） nitrite [（CH3）4N][Ni（NO2）3]

The title compound, tetramethylammonium nickel nitrite [（CH3）4N][Ni（NO2）3], has a hexagonal perovskite-type structure with formula ABX3. It undergoes two reversible phase transitions occurring at about 409.1 and 428.4 K, associated with dielectric transitions. DSC measurement and dielectric measurement confirm the transition. The variable-temperature X-ray structural determinations and the powder X-ray diffraction （PXRD） experiments reveal that this compound has the same space group P3ml （No. 164） at 293 K, 413 K and 438 K. The phase transitions are caused by the rotation of the [（CH3）4N]＋ cation.     

Phase Transition Originating from Order–Disorder Transformations of Carboxy Oxygen Atoms Coupled with Dynamic Proton Motions in [PhCH2NH(CH3)2]2C2O4⋅H2C2O4

A new molecular phase transition material, [PhCH₂NH(CH₃)₂]₂C₂O₄?H₂C₂O₄, which undergoes a reversible phase transition at 151.6 K, has been successfully synthesized. Differential scanning calorimetry (DSC), specific heat capacity, and dielectric measurements confirm its reversible phase transition with a large thermal hysteresis of 15.1 K, demonstrating that the phase transition is typical first order. Variable‐temperature single‐crystal X‐ray diffraction analyses reveal that the order–disorder transformations of carboxy oxygen atoms induce the structural phase transition. A slight reorientation of the oxalic acid unit is discovered to accompany the ordering of carboxy oxygen atoms at low temperature. The DSC measurement result of the deuterated analog is different to that of 1 , indicating that proton dynamic motions in hydrogen bonds also contribute to the phase transition.     

Structural control of dithiazolyl radicals: Case studies on 3′- and 4′-cyano-benzo-1,3,2-dithiazolyl, NCC6H3S2N

Dithiazolyl radicals with π-stacking motifs have attracted particular interest because of their ability to exhibit spin-switching between diamagnetic distorted π-stacks and paramagnetic regular π-stacked structures through a solid state phase transition. Previous studies indicate that inclusion of electronegative heteroatoms into the backbone favours lamellar structures. This methodology has been extended to the synthesis and characterisation of the title compound, 4′-cyanobenzo-1,3,2-dithiazolyl (4-NCBDTA). Its electronic structure is probed through DFT calculations, cyclic voltammetry and EPR spectroscopy and its crystal structure determined by X-ray powder diffraction at room temperature. Variable temperature SQUID magnetometry reveals that 4-NCBDTA undergoes two phase transitions, each exhibiting bistability; a high temperature phase transition occurs at room temperature (T_C↓ = 291 K, T_C↑ = 304 K, ΔT = 13 K); whilst the low temperature phase transition occurs below liquid nitrogen temperatures (T_C↓ = 37 K, T_C↑ = 28 K;ΔT = 9 K).     

Mössbauer spectroscopy of 57Fe in NH4MnCl3, NH4FeCl3 and NH4CoCl3

The Mössbauer spectra of ⁵⁷Fe doped NH₄MnCl₃ reveal that this compound undergoes a crystallographic phase transition at 258°K and becomes magnetic at 105°K. No crystallographic transition appeared either in NH₄FeCl₃ or in ⁵⁷Fe:NH₄CoCl₃; the latter orders magnetically around 27°K.     

Temperature-induced phase transitions in BaTbO3

The crystal structures of BaTbO₃ have been investigated over a wide temperature range between 40 and 773 K using high-resolution time-of-flight neutron powder diffraction. Two-phase transitions were observed. Below about 280 K, BaTbO₃ adopts an orthorhombic perovskite structure (space group Ibmm), which is characterized by rotation of TbO₆ octahedra about the pseudocubic two-fold axis. Above 280 K, BaTbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm), in which the tilting of the octahedra is around the pseudocubic four-fold axis. As the temperature is further increased, BaTbO₃ adopts the primitive cubic aristotype at about 623 K. This later phase transformation is characterized by a gradual decrease of the rotation angle, indicating a continuous phase transition, which is described by a critical exponent β=0.35.     

X‐Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure‐Induced Phase Transitions in a Mixed‐Valence Trinuclear Iron Complex

The mixed‐valence complex Fe₃O(cyanoacetate)₆(H₂O)₃ ( 1 ) has been studied by single‐crystal X‐ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure‐induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence‐trapped after the second phase transition. This sluggish pressure‐induced valence‐trapping is in contrast to the very abrupt valence‐trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure.     

Evidence of a phase transition of RbHCO3 from high-resolution solid-state 13C and 87Rb NMR by comparison with KHCO3

A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar.     

Neutron inelastic scattering spectra and phase transition of 1,2-dichloroethane crystal

Neutron inelastic scattering spectra of 1,2-dichloroethane crystal have been measured at three temperatures above and below 177°K where the crystal undergoes a broad phase transition. Two and three peaks have been observed in the high and low temperature phases, respectively. The frequency distribution g(v) has been calculated in the first Brillouin zone for the low temperature phase and is compared with the observed spectra. The results show that the phase transition at 177°K is associated with rotational motion of the molecule around the axis passing through two chlorine atoms.     

Phase Transition of Rubidium Lead Tetrathiophosphate RbPbPS4

The single crystal structure of RbPbPS₄ was determined at 293 K. The compound crystallizes in the orthorhombic space group Pnma (No. 62) with a = 17.486(1) Å, b = 6.7127(5) Å, c = 6.4191(5) Å, V = 753.5(1) ų, Z = 4. Differential scanning calorimetry shows a reversible structural phase transition at 182 K on cooling and at 184 K on heating. The phase transition is attributed to the displacement of the lead atoms which are located on the mirror planes at room temperature. In the low temperature modification, the Pb²⁺ ions are moved away from the mirror plane thus changing the coordination number from seven at low temperature to six at room temperature. The low temperature phase of RbPbPS₄ is non‐centrosymmetric with space group P2₁2₁2₁, which is a maximal translationengleiche subgroup index 2 (t2) of Pnma. The analysis demonstrates that the phase transition is of second order, or at least nearly so.     

Synthesis,structural phase transition,and characterization of potassium hydrogen bis-dichloroacetate

Potassium hydrogen bis-dichloroacetate (1) was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurement reveals that this compound undergoes a reversible phase transition at about 259 K with a heat hysteresis of 23.5 K. Dielectric anomaly observed at 260 K in the heating process further confirms the phase transition. The room temperature X-ray single-crystal structure determination indicates that 1 crystallizes in the monoclinic crystal system with a centrosymmetric space group P2₁/c, and cell parameters are a =?6.240(1), b =?23.177(4), c =?7.335(1) Å, β =?106.938(1)°, V =?1014.8(3) ų, and Z =?4. In the low temperature phase, 1 also crystallizes in monolinic with space group P2₁/c, and cell parameters are a =?6.180(1), b =?22.988(2), c =?7.200(1) Å, β =?108.098(1)°, V =?972.4(1) ų, and Z =?4. The structural phase transition is dominating caused by the torsion of bond angles.