Stress‐Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single‐crystal X‐ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn³⁺ compound [Mn(3,5‐diBr‐sal₂(323))]BPh₄, 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc‐P1 transition     

Hard mode Raman spectroscopy and its application to ferroelastic and ferroelectric phase transitions

The application of Raman spectroscopy for the investigation of phase transitions focused traditionally on the observation of soft modes in displacive systems. The present furthergoing study on displacive and order-disorder systems is based on the observation of systematic changes of the scattering profiles of hard modes during the phase transition. It reveals the temperature evolution of the order parameters, the role of order-parameter fluctuations and phonon density of states effects in model systems like improper ferroelastic Pb₃(P_1-x As _x O₄)₂, lead diluted ferroelastic (Pb_1-x Ba _x )₃(PO₄)₂, pure ferroelastic As₂O₅ and the relaxor ferroelectric Pb(Sc_0.5Ta_0.5)O₃. Allied with supplementary experimental techniques hard-mode Raman spectroscopy (HMRS) is an ideal method for the investigation of order-parameter coupling effects and the characterization of structural phase transitions.     

Monoclinic‐to‐orthorhombic phase transition of the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal with temperature‐dependent dynamic molecular disorder

As a function of temperature, the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal, C₆H₁₂N₄·2C₈H₈O₂, undergoes a reversible structural phase transition. The orthorhombic high‐temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300 K. At 164 K, a t₂ phase transition to the monoclinic subgroup P2₁/c space group occurs; the resulting twinned low‐temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/00/00). Systematic absence violations represent a sensitive criterium for the decision about the correct space‐group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low‐temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t₂ structural phase transition. The asymmetric unit of the low‐temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2‐methylbenzoic acid; it is smaller by a factor of 2 in the high‐temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three‐molecule aggregate; the low‐temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high‐temperature phase. In both phases, one of these three‐molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 K via 0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature‐dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.     

Modelling of hysteresis loops taken during the stress- and temperature-induced martensitic transformations

In this article, it has been shown that the process of reconfiguration of the crystal defects system noticeably contributes to the width of the stress–strain and strain–temperature hysteresis loops taken during the stress- and temperature-induced martensitic transformations of the shape memory alloys. It has been demonstrated that the contribution of the defects system to the hysteresis width strongly depends on the alloy temperature and the transformation cycle duration. It has been shown that the hysteresis effect can be observed not only in the course of the first-order phase transition of martensitic type, but also in the course of the gradual deformation of crystal lattice. The obtained results are applicable to the ferroelastic phase transitions in the different crystalline solids.     

Microscopic distortion and order parameter in langbeinite K2Cd2(SO4)3

The structural data on the orthorhombic phase of K₂Cd₂(SO₄)₃ at four different temperatures of Abraham et al. (1978) [J. Chem. Phys., 68, 1926] is reanalyzed in terms of frozen symmetry modes. The eigenvector of the primary distortion present in the orthorhombic phase is derived. The temperature variation of the amplitude of the order parameter is determined and compared with those of the amplitude of the frozen secondary distortion and the amplitude of the orthorhombic strain. In contrast with some previous literature, it is shown that, within experimental accuracy, all these magnitudes exhibit the correlation expected from a conventional Landau model.     

Deflected light structure in NaNH4SeO4·2H2O crystals

Deflection of light studies in function of temperature in NaNH₄SeO₄·H₂O crystal is presented. At 180?K, this compound undergoes a para-ferroelectric/ferroelastic phase transition of the second order. It changes the symmetry from the orthorhombic symmetry class 222 (space group P2₁2₁2₁) to the monoclinic symmetry class 2 (space group P2₁). A distinct deflection pattern, resulting from the ferroelastic domain structure, occurs in the low temperature phase. The intensity of deflected beams varies considerably with temperature. Detailed studies revealed the structure of deflected spots during cooling–heating cycle. These spots change intensity, become more dispersed and finally split with decreasing temperature. Moreover, a variation of deflection angles was observed. A possible explanation of these phenomena is given in a framework of crystals optics.     

Temperature‐dependent low wavenumber Raman scattering studies in LiCsSO4 crystal

The Raman lattice modes in the LiCsSO₄ crystal were measured in the temperature range of 17–303 K. Two right‐angle scattering geometries were used: z(xx)y and z(xz)y, to observe A_g and B_g (or B_2g) modes, respectively. Critical phenomena were observed in the spectra at temperatures of about 200, 180 and 100 K. They correspond to structural phase transitions in the crystal from D_2h to C_2h at 200 K and to C_s symmetry at 100 K. The nature of the transition at 180 K is supposed to be of a lock‐in type, but it needs further investigation. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigations of elastic properties at very low frequencies: Method and behaviour of (TMA)2ZnCl4

Abstract

A new apparatus to perform longitudinal elastic strain measurements over a frequency range from 10^?4 Hz to 10¹ Hz is presented. First results of measurements on TMAZC with the prototype of this device are given. These results will be discussed in connection with the new methods.     

Phase transitions in [NH(CH3)3]2MeCl4 (Me = Cd,Zn,Cu) crystals

Dielectric properties of the new [NH(CH₃)₃]₂ZnCl₄ and [NH(CH₃)₃]₂CdCl₄ crystals from the [(CH₃) _n NH_4-n ]₂MeCl₄ group have been investigated in a wide temperature range (4.2–320 K). A series of phase transitions has been discovered at T₃ = 325 K,T₄ = 251 K,T₅ = 193 K, for [NH(CH₃)₃]₂CdCl₄ and at T₃ = 309 K, T₄ = 282 K, T₅ = 269 K for [NH(CH₃)₃]₂ZnCl₄. A ferroelectric phase has been discovered in the temperature interval T₄—T₅ from the temperature and frequency dependence of the dielectric permittivity ε(T, v). According to optical investigations the existence of ferroelastic phases in the temperature interval T₁ = 349 K–T₂ = 391 K and below T₅ for [NH(CH₃)₃]₂CdCl₄ and both above T₃ and below T₅ for [NH(CH₃)₃]₂ZnCl₄ has been ascertained.     

First-principles study of the Li2TiGeO5 ferroelastic phase transition

The electronic structures of ferroelastic lithium titanium germanate are investigated by first-principles method. The structure changes caused by the phase transition are discussed. It is shown that the orthorhombic structure is more stable than the tetragonal structure. The remarkable ferroelastic property largely originates from the Ge–O hybridization, which is enhanced by the Ti–O hybridization. The effective density and potential shows the changes of atoms bonding accompanying the ferroelastic phase transition.