The ferroelastic transition of betaine borate

The ferroelastic transition of betaine borate at about 142 K and the nonlinear temperature dependence of the corresponding critical elastic modulus are related to the softening of optical phonons. The transition has to be considered a ferroelastic one of the optic type, i.e. pseudoproper one, therefore. It is shown by Raman spectroscopic results that the transition is induced by a strong bilinear coupling between a homogeneous strain and at least two optic lattice modes which are of the same symmetry as the strain. Optic and dielectric, investigations support the second-order character of and the classical behaviour at this transition, which have been reported in the literature. Differences and similarities to the ferroelastic transition of betaine fumarate are discussed.     

Giant thermodynamical optical effect near ferroelastic phase transition point in Cs3Bi2I9 layered crystal

For the first time we have found a new giant thermodynamical optical effect near the ferroelastic phase transition point in Cs₃Bi₂I₉ layered crystal. The effect is appeared as periodical oscillations in time of the reflection coefficient. This phenomenon is caused by the small temperature deviations in thermodynamical system the appearance of which in the reflection spectra is strongly amplified in the ferroelastic phase transition point. The optical oscillations are explained on the base of a model that takes into account the temperature dependence of the refractive index through the order parameter (spontaneous strain) of the crystal.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Pattern formation in Ferroelastic Transitions

We study pattern formation in ferroelastic materials using the Ginzburg–Landau approach. Since ferroelastic transitions are driven by strain, the nonlinear elastic free energy is expressed as an expansion in the appropriate (i.e., order parameter) strain variables. However, the displacement fields are the real independent variables, whereas the components of the strain tensor are related to each other through elastic compatibility relations. These constraints manifest as an anisotropic long-range interaction which drastically influences the underlying microstructure. The evolution of the microstructure is demonstrated for (i) a hexagonal-to-orthorhombic transition using a strain-based approach with explicit long-range interactions; and (ii) a cubic-to-tetragonal transition by solving the force-balance equations for the displacement fields.     

Mechanism of the ferroelastic transition of BiVO4

BiVO₄ has a pure ferroelastic transition at T_c = 528°K and atmospheric pressure. We elucidated the mechanism of this transition by studying the q≈0 soft optical phonon, with the symmetry of the ferroelastic strain, under large hydrostatic prèssures at room temperature. A free-energy analysis, including the optical-acoustical phonon couplings, shows that the transition is driven by the q≈0 soft optical phonon.     

Geometrical coupling between inhomogeneous strain fields: Application to fluctuations in ferroelastics

The compatibility conditions of elasticity theory are applied to the calculation of strain fluctuations in ferroelastic materials. A ferroelastic transition with an acoustic phonon mode instability and an order-disorder transition with striction are considered in the framework of the Landau-Ginzburg functional. Anisotropy of strain correlation functions and their critical behaviour in the symmetric phase near the transition point are analysed and compared with the results of the Ornstein-Zernike theory of fluctuations. Characteristic shape of the correlation functions (“butterflies”) are predicted.     

Shape-memory effect in PMN-PT(65/35) ceramic

We report the observation of fairly large and fully recoverable shape-memory strain in thin bars of lead magnesium niobate lead titanate ceramic for a composition (65/35) near its morphotropic phase-boundary. The recoverable shape strain produced by bending and thermal-cycling experiments is ∼0.3%, similar in magnitude to that reported for the shape-memory effect (SME) observed in some other dielectric ceramics. An explanation of the observed SME and the recoverable shape-strain is given in terms of the ferroelastic phase transitions, and the availability of a large number of competing phases and domain states near the morphotropic phase boundary in this material. The stress-induced shifting of the temperatures at which the ferroelastic phase transitions occur also plays an important role in ensuring good crystallographic reversibility around the thermal-cycling experiments.     

Polarized Raman study of the ferroelastic transition in lanthanum pentaphosphate

Polarized Raman spectra in LaP₅O₁₄ have been studied in the low frequency range (0–85 cm^-1) between 78 and 575 K across the 398 K ferroelastic transition temperature. By contrast to previous results, two softening modes with different symmetries are observed, only one of them having the same B_2g symmetry as the transition's order parameter. An anomalous behaviour is pointed out for the linewidths of these modes in the ferroelastic phase. It is also shown that higher order coupling terms between the ferroelastic spontaneous shear and the B_2g soft optic mode in addition to linear terms are necessary to explain the experimental results.     

Soft optical phonon in ferroelastic BiVO4

We report the observation of an underdamped q～O soft optical phonon in the Raman spectra of the paraelastic and ferroelastic phases of BiVO₄. This mode has the same symmetry as the ferroelastic strain. The temperature dependence of the soft optical phonon energy indicates that the ferroelastic transition is continuous and that the order parameter has a Landau-type behavior over a wide temperature range.     

Effect of electric field on the dielectric permittivity of betaine phosphite crystals in the paraelectric phase

Temperature dependences of the dielectric permittivity of betaine phosphite crystals are studied both without and under application of an electric bias. It is shown that, in view of the fact that the high-temperature improper ferroelastic (antiferrodistorsive) phase transition at T _c1=355 K is nearly tricritical, the nonlinear temperature dependence of inverse dielectric permittivity in the paraelectric phase and the effect of the field on the dielectric permittivity can be described within a phenomenological model containing two coupled (polar and nonpolar) order parameters with a negative coupling coefficient. An analysis of the model revealed that, in the case where two phase transitions, a nonpolar and a ferroelectric one, can occur in the crystal, all of its dielectric properties, including the polarization response in a field, can be described by one dimensionless parameter a. For the crystal under study, we have a=?2.5. This value of the parameter corresponds to a second-order ferroelectric transition far from the tricritical point, at which a=?1. It is shown that the polarization response in the paraelectric phase in an electric field calculated within this model differs radically from that in the ferroelectric phase-transition model for which the Curie-Weiss law holds in the paraelectric phase.     

Acoustic study of the ferroelastic phase transition in LiCsSO4 crystal

The paper reports on an acoustic study of the temperature dependences of the ultrasonic-wave velocity and attenuation in a LiCsSO₄ crystal within the 190–295 K temperature region, which includes the interval of the pseudoproper second-order ferroelastic phase transition (202 K). The velocity of the transverse xy acoustic mode is found to decrease by more than six times at the phase transition. The possibility of performing ultrasonic studies both in the region of the ferroelastic phase transition temperature and below it is demonstrated. The results are treated in terms of Landau’s theory. Waves not associated with the soft mode are shown to exhibit anomalies which are supposedly due to an intermediate phase, whose existence was suggested in a number of publications.     

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.     

A single-crystal neutron scattering study of lattice melting in ferroelastic [Formula: see text

We present the results of an extensive single-crystal neutron scattering study of the ferroelastic phase transition in [Formula: see text]. This material has previously been demonstrated to undergo a continuous loss of long-range order at its ferroelastic transition, which is the phenomenon known as lattice melting. We show that our data are consistent with a special form of lattice melting where the long-range order appears to be destroyed in a two-dimensional sense, but is preserved in the third dimension.     

Raman scattering investigation of ferroelastic Sb5O7I crystals

Sb₅O₇I undergoes a displacive phase transition at 481 K where the symmetry is changed fromC _6h ² toC _2h ⁵ . In the low temperature monoclinic phase the crystal is ferroelastic. The polarized Raman spectra of Sb₅O₇I have been measured at various temperatures below and above the phase transition. The frequencies and symmetries of most of the theoretically expected Raman active phonons in the ferroelastic phase have been determined. The observation of a soft mode in the ferroelastic phase which disappears above the phase transition together with the fact that the unit cell of the ferroelastic phase is twice as large as that of the paraelastic structure permits the conclusion that the phase transition results from a phonon instability at the Brillouin zone boundaryM-point of the hexagonal phase. The temperature dependent splittings and intensity changes of several Raman lines are discussed with respect to the ferroelastic property of the crystal and the phase transition.     

Ferroelastic phase transition in LiCsSO4 embedded into molecular sieves

Acoustic studies of a nanocomposite consisted of MCM-41 molecular sieves with nanoparticles of ferroelastic LiCsSO₄ within pores were carried out. The critical softening of the transverse ultrasound velocity was observed which evidenced the ferroelastic phase transition in confined particles. The transition was moved to low temperatures compared to that in bulk LiCsSO₄. It is shown that acoustic methods are very suitable to reveal the ferroelastic phase transitions under nanoconfinement.     

Investigations on the dynamics of ferro- and antiferroelectric betaine compounds

Measurements of the dielectric permittivity around the ferroelectric transition of betaine arsenate and around the antiferroelectric one of deuterated betaine arsenate reveal relaxation-type dispersions in the upper MHz-region. The data analysis points to two mechanisms which contribute to the low temperature phase transitions of these crystals. In case of betaine phosphate Raman-spectra demonstrate that besides the antiferroelectric transition at 86 K a second structural transition exists at 81 K. Both transitions correlate with dielectric anomalies and the unit cell doubles at 81 K.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Local geometry of the isofrequency phonon surface and the spectral structure of generalized lamb waves on a bounded ferroelastic substance

The singularities in the spectrum of bulk acoustic phonons polarized in the sagittal plane are investigated on a plate made of a uniaxial ferroelastic material undergoing a proper ferroelastic transition from the paraelectric to the ferroelectric phase. The singularities are induced by anomalies in the reflection of this type of normal elastic vibrations at the crystal boundary.     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

Internal strain and the ferroelectric transition in ferroelastic NH4HSO4 crystals as studied by EPR of VO2+ impurity probes

The ferroelectric phase transition and its relation to the spontaneous strain in ferroelastic NH₄HSO₄ crystals were investigated using VO²⁺ ions as an EPR probe. The impurity ions were found to be interstitially trapped at sites surrounded by crystallographically inequivalent NH₄⁺ and SO₄^2? ions. The polar VO²⁺ axes exhibited temperature-dependent displacements in two distinct directions with different energies. The differential properties of VO²⁺ ions in NH₄HSO₄ crystals were used to verify the presence of internal stress in the ferroelastic phases, and the corresponding strain was studied in the range between ?120 and + 100°C. The results indicate that the ferroelectric phase transition occurs as a consequence of lattice instability caused by the internal strain. At the second-order structural transition a dipolar lattice emerges in the crystal and the spontaneous polarization appears as a result of internal entropy transfer to the strained lattice.     

Ferroelastic domain structure of (CH3)4N CdCl3 (TMCC) crystal

The ferroelastic domain structure and the phase boundaries of TMCC have been studied in the temperature range 114-90 K by direct observation under polarised light. By applying an external, compressive and unidirectional mechanical stress the ferroelastic character of the domain structure has been confirmed. The orientation of the domain walls and phase boundaries are analysed. To characterise quantitatively the observed domain wall distribution the classical symmetry approach, based on the criterion of spontaneous strain compatibility, has to be extended to allow small rotations of the domain walls with respect to their ideal orientation. The observed switching process among the different domains can be understood as a mechanism that minimises the elastic energy. Received 21 July 2000