Ferroelastic Crystals Research Articles

Thermoelastic effect induced by ferroelastic domain switching

Temperature changes induced by externally applied mechanical stress were observed in the Pb3(PO4)2 ferroelastic crystal. These characteristic features can be explained by the thermoelastic effect as in other thermoelastic martensitic materials. In this study, the mechanical stress dependence of the thermoelastic effect was studied using a modified thermomechanical analysis system. The thermoelastic effect strongly depended on the mechanical stress and was enhanced by ferroelastic domain switching, responsible for the pseudoelastic behavior of ferroelastic materials. The ∂T∕∂σsin, the change of sample temperature with varying stress enhanced by pseudoelasticity, was ∼9.5×10−2°C∕MPa at 159 °C, ferroelastic temperature region under 1.2 MPa sinusoidal stress. This value is eight to nine times higher than the values obtained under bias stress. This result can be explained by a thermoelastic equation which includes the effective thermal expansion coefficient in the ferroelastic phase.

Memory and Perfection in Ferroelastic Inclusion Compounds

In a series of ferroelastic urea inclusion compounds (UICs), in which domain reorientation occurs upon application of an external anisotropic force, introduction of a relaxive impurity that disrupts a specific hydrogen-bonding network transforms a plastic (irreversible) domain-switching process into one that exhibits a striking memory effect and 'rubber-like behavior', a form of pseudoelasticity. As expected for a highly cooperative process, the ferroelastic response to the impurity concentration exhibits a critical threshold. Through synchrotron white-beam X-ray topography (SWBXT) of crystals under stress, videomicroscopy of spontaneous repair during crystal growth, acoustomechanical relaxation of daughter domains, kinetic measurements of spontaneous domain reversion, and solid-state {sup 2}H NMR of labeled guests, this work shows how relaxive impurities lower the barrier to domain switching and how differences in perfection between mother and daughter domains provide the driving force for the memory effects. Although the interfacial effects implicated here are different from the volume effects that operate in certain shape memory materials, the twinning and defect phenomena responsible for the rubber-like behavior and memory effects should be generally applicable to domain switching in ferroelastic and ferroelectric crystals and to other solid-state processes.

Fast proton conducting hydrogen sulphates and selenates: Impedance spectroscopy, Raman scattering and optical microscope study

In the paper the phenomenon of proton conductivity in hydrogen bonded acidic sulphate and selenate salts of alkali metals is discussed. The superionic proton conducting state in the crystals is achieved as a result of structural phase transition at T S. The specific features of H-bond make it easily adaptable to its environment, hence the fast proton transport is assisted by dynamics of neighbouring species. In the crystals there appear additional, symmetry equivalent positions for protons above T S, so the occupancy of H-bonds c < 1. This results in formation of a dynamically disordered H-bond network, which enables significant proton conductivity by Grotthuss mechanism. In most of M 3H(XO 4) 2-type crystals T S coincides with ferroelastic–paraelastic phase transition temperature T C. All the ferroelastic compounds exhibit an intermediate phase close to T S, where anomalies of physical properties are observed. The experimental data indicate, that intensive ferroelastic twinning assists transformation of H-bond network from ordered to disordered state. Therefore smooth and reversible superionic phase transition is observed in the ferroelastic crystals. On the other hand, non-ferroelastic Rb 3H(SO 4) 2 crystal, whose structure cannot be transformed gradually, undergoes an irreversible reconstructive transformation into a polycrystalline state at T S.

Behaviour of surface phonons in LiCsSO4 crystal at phase transition

AbstractThe paper reports results of Brillouin spectroscopy study performed on surface phonons in different crystallographic surfaces of crystals, in a wide temperature range but with particular emphasis on the temperatures covering the phase transition. The main aim of the study is to establish in which way the ferroelastic phase transition related to a strong soft bulk mode is reflected in the near‐surface region of the ferroelastic crystals. Results of the Brillouin scattering of the surface phonons propagating in the ferroelastic crystal LiCsSO4 in the temperature range covering its phase transition are presented. Surface acoustic phonons in different planes and at various depths from the surface have been studied. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electro-mechanical modeling of a ferroelastic relaxor single crystal

This paper examines the switching process occuring in ferroelectric and ferroelastic relaxor single crystals under electro-mechanical loadings. Relaxors undergoing a cubic to tetragonal phase transition are considered. The single crystal energy has three origins: elastic, electric and the incompatibilities of the spontaneous strain and electric displacement fieds between domains. The stress and the electric field fluctuate and present jumps at the domain walls. As a consequence, they induce electro-elastic interaction energy. Thus, it involves dissipation that the present work aims to capture through an electro-micromechanical approach.

Domain wall orientations in ferroelastics and ferroelectrics

This paper concerns the problem of orientations of domain walls in ferroelastic and/or ferroelectric single crystals. They are specified on the basis of elastic compatibility of the neighbouring domains. Results are presented in form of tables, which make it possible to find spontaneous tensors (i.e. strain and polarization) for each domain state in any ferroelastic and/or ferroelectric species. Domain wall orientations are listed in tables for all combinations of domain states. For each domain wall orientation, their charge or neutrality is indicated. Results are summarized in comprehensive table including numbers of possible domain states, domain wall types (W ∞, W f , S) and their charge or neutrality. Domain pairs with non-permissible domain walls (R case) are also specified.

Constitutive law for ferroelectric and ferroelastic single crystals: a micromechanical approach

This paper examines the switching process occuring in ferroelectric and ferroelastic single crystals under electro-mechanical loadings. Ferroelectrics undergoing a cubic to tetragonal phase transition are considered. The single crystal energy has three origins: elastic, electric and the incompatibilities of the spontaneous strain and electric displacement fieds between domains. The stress and electric fields fluctuate and present jumps at the domain walls. As a consequence, they induce electro-elastic interaction energy. Thus, it involves dissipation that the present work aims to capture through a micromechanical approach.

Landau theory and phase diagram ofKMn1−xCaxF3 ferroelastic crystal near the tricritical point: calorimetric and order parameterstudy

The cubic to tetragonal phase transition in the solid solutionK(Mn,Ca)F3 has been investigated by conduction calorimetry and x-ray diffraction. Thebehaviour of the excess specific heat, latent heat and spontaneous strainhas been explained in terms of a 2–4–6 Landau potential. The coefficientsA andC, prefactorsof Q2 and Q6 in the free energy expansion, are practically constant with composition, butB (theprefactor of Q4) and TC are functions of composition. The tricritical point occurs when the sign ofB changes fromnegative (in pure KMnF3) to positive (in Ca rich samples). However, the variation of the parametersB andTC with dopant concentrationx is non-linear. Thedependence of TC with composition is explained in terms of an internal stress due to the dopingion of Ca and is compared with the effect of external uniaxial stress on pureKMnF3.

Non-Linear Waves in Ferroelastics

Theoretical investigation of nonlinear periodical waves in ferroelastic crystals is made. The conditions of appreciable effect of media nonlinearity on wave characteristics are found. Analytical expressions for dependences of wave velocity on the wave length and oscillation amplitude are derived. It is shown that in the crystals soliton-like excitations can exist and characteristics of the solitons are found.

Argand Diagram and Oscillation Description of Electron State in Ferroelastic Crystals

The new method of Argand diagram was used to study the optical properties and energy spectra of Gd2(MoO4)3. The results were used to calculate the number, energy position, force and relaxation time for oscillators which constructs the energy spectra of Gd2(MoO4)3. The number of oscillators and their resonance energies were determined by using Argand diagram method.

Light Scattering in Ferroelastic Lead Phosphate-Type Crystals Due to Domain Structure

Ferroelastic lead phosphate-type (LP) crystals undergo a sequence of phase transition and [1]. The ferroic domain pattern depends on the spontaneous strain which can be diluted by the chemical composition. Light scattering from ferroelastic materials and its computer model was presented by T-matrix method at the structural phase transition. We discussed numerical procedure of T-matrix method and improved new computer program to calculate the scattering properties for domain structure of LP-type ferroelastic materials. Then, we report results of these calculations for polydispersive, randomly oriented spheroid with a refractive index, different semi-axis ratios and different size parameters. The calculation results are very close to our previous study of light scattering by spherical, cylindrical structures and convex particles.

1H and39K Nuclear Magnetic Resonance Relaxation Study in a KHSO4Single Crystal

¹H and³⁹K Nuclear Magnetic Resonance Relaxation Study in a KHSO₄Single Crystal

Incommensurate phases in the improper ferroelastic MgGeF6·6H2O:Mn2+ studied by means of EPR

The results of an EPR study of the inhomogeneous phases existing in thetemperature interval TC = 311.0 ± 0.3 K < T < Ti1 = 403 ± 0.3K in improper ferroelastic crystals of MgGeF6 · 6H2O:Mn2+are presented. On the basis of the analysis of the temperature and angle dependences of theexperimental parameters and numerical calculations, the conclusion has been drawn thatat Ti1the crystals considered undergo a transition to a structurally modulated phaseand the order parameter of this transition may be the angle of the Mg[H2O]62+octahedra rotation around the crystal C3-axis.From Ti1to TCthe modes of the modulated phase follow according to a completelyclassical scenario for incommensurate crystals: the origin ofthe incommensurate structure with plane-wave modulation atTi1,the appearance of structural phase solitons below Ti2 = 380 ± 0.3K and decrease of the soliton density to values of ≈ 0.85 as the temperaturedecreases to T ≈ 360K. Below that temperature the soliton density is almost unchanged down toTCand, to successfully describe the experimental spectra, the effect ofvariation of the spin–lattice relaxation rate over the spectrum has alsobeen taken into account within the temperature interval considered. AtTC thecrystals investigated undergo a first-order improper ferroelastic phase transition into amonoclinic phase with unit-cell doubling. A comparison with results on MgSiF6 · 6H2Ofrom a previous investigation and a theoretical analysis of the structure evolutionin these crystals are also presented.

Optical second order nonlinearity of transparent Ba2TiGe2O8 crystallized glasses

Optical second-order nonlinearity of ferroelastic Ba2TiGe2O8 crystal in transparent crystallized glasses was estimated by means of the Maker fringe technique. Transparent crystallized glasses were fabricated by crystallization of a glass with the composition of 30 BaO.15 TiO2.55 GeO2. The crystalline layer consisting of highly oriented Ba2TiGe2O8 crystals was formed at the surface of about 9 μm thickness, and a clear second-harmonic generation was observed. The optical second-order nonlinearity of d33 for Ba2TiGe2O8 crystal was found to be ∼10 pm/V, being comparable to d22 and d31 of LiNbO3, and demonstrating that Ba2TiGe2O8 crystal is an excellent candidate for nonlinear optical crystals.

Spontaneous twisting of the ferroelastic crystals below their Curie points

Using a torsion pendulum, the features of spontaneous twisting of several ferroelastic crystals at a temperature variation near the phase transition points have been studied as a function of a crystallographic orientation, a thermal and mechanical history, and an external static shear stress. It was assumed that a torque arises at the Curie point because of the specific domain structure organization resulted in the predominant formation of that certain domain types.

Balanced and unbalanced domain walls in Pb3(PO4)2

The equations of domain walls in the vicinity of a phase transition are obtained from the condition of the deformation balance at phase boundaries and domain walls for a ferroelastic lead orthophosphate crystal. All domain walls can be divided into three groups, in accordance with the conditions of their formation-balanced, conventionally balanced, and unbalanced domain walls. The structure of staggered domain walls is suggested.

EPR spectra of Cr3+-doped LuNbO4 crystals

Ferroelastic LuNbO4 single crystals containing 0.3 at. % Cr3+ ions have been grown by the floating zone technique, and their EPR spectra have been studied at a frequency of 9.8 GHz at room temperature. The lines on the spectra are due to the transitions caused by three paramagnetic centers formed as a result of the replacement of one isovalent position of a Lu3+ ion and two nonisovalent positions of Nb5+ions by Cr3+-ions. As a result of twinning, the line number is doubled and four principal directions arise along which the same spectra are obtained. The spectra of these centers were described by a spin Hamiltonian with S = 3/2, the D and E parameters ranging from 0.024 to 0.17 cm−1, and the g-factors g ∥ = 1.75–2.20 and g ⊥ = 1.90–2.13.

Surface Phonons in Ferroelastic Crystals

Results of the surface phonon velocity V R measurements in three ferroelastic crystals {Gd 2 (MoO 4 ) 3 , LiCsSO 4 and Rb 4 LiH 3 (SO 4 ) 4 } over the temperature range covering their transitions are presented. No significant elastic softening was observed in the course of our studies. In a few cases an anomalous increase of V R at T C was observed. The observed behavior may be attributed to the properties of the surface layer, which in the case Brillouin scattering experiment, is penetrated to about 300 nm from the surface.

X-ray study of sublattice structures in ferrielastics

The orthorhombic to monoclinic phase transition in CsLiSO4 and CsLiCrO4 has been studied with X-ray diffraction to clarify the mechanism of the phase transition. Temperature dependence of lattice constants and integrated intensity of Bragg reflection (h0l) has been measured. It is concluded that CsLiSO4 is a ferroelastic crystal with single order parameter. On the other hand, CsLiCrO4 is a ferrielastic crystal with two kinds of sublattice order parameters originating from Cs sublattice and CrO4 sublattice.

EPR investigation of phase transitions and incommensurate phases in improper ferroelastic MgGeF6·6H2O: Mn2+

The results of EPR investigation of improper ferroelastic crystals MgGeF6·6H2O: Mn2+ allow to conclude that at Ti1=403 K these crystals undergo transition to structurally incommensurate phase. The order parameter of this transition is probably the angle of Mg[H2O]6 2+ octahedra rotation around crystal C3 axis, which determines the presence of only even terms in expanding the EPR spectra fine structure parameter on the powers of order parameter. Below 360K the appearance of structural phase soliton lattice and effect of variation of the spin-lattice relaxation rate over the spectrum have been taken into account.