Ferroelectric-ferroelastic Transition Research Articles

Ferroelectric–Ferroelastic Transitions in (Na0.5Bi0.5)TiO3-BaTiO3 Single Crystals

The comprehensive understanding of (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lattice structure is highly desired to develop lead-free ferroelectric materials. However, most of the previous studies focused on the improvement of piezoelectric properties at room temperature, and many structural puzzles are left unclear. In this work, the lattice structure of a ferroelastic phase and the ferroelectric-ferroelastic transitions in both rhombohedral NBT and tetragonal NBT-8%BT single crystals are investigated in detail. Our results illustrate the complex process of the ferroelectric-ferroelastic transition of NBT. The variation of Ti-O modes and oxygen octahedra modes clearly indicates the gradual change of lattice symmetry from R3c to P4bm during a wide temperature range between 170 and 350 °C. A ferroelectric-ferroelastic transition is also confirmed in tetragonal NBT-8BT for the first time, and the lattice symmetry of P4bm is found to be maintained during the ferroelastic stage. This work reveals the lattice evolutions of the ferroelectric-ferroelastic transition of NBT-BT crystals and provides new insights for understanding the ferroelasticity and the evolution of phonon modes in a lead-free relaxor.

Study of ferroelastic domains in HTMA crystals at the ferroelectric–ferroelastic transition

The ferroelectric ferroelastic phase transitions of tetramethylammonium tetrachlorozincate (HTMA) were observed by standard X-ray topography and by synchrotron topography. Observation of the phase boundary is possible in the latter case. The different kinds of domain walls observed in the ferroelastic phase cast doubts on the commonly assumed symmetry group of this phase.

“On the influence of different parameters on the phase transition temperatures and ferroelectric domain structure in H(UO2)(AsO4). 4H2O”

Abstract Thermal bysteresis of the ferroelectric-ferroelastic transition has been observed in virgin and heat-treated samples by means of differential scanning calorimetry, X-ray diffraction of oriented samples and electron microscopy observation of replicas of cleavage surfaces. These replicas provided for the first time direct evidence of the ferroelectric domain structure. In the ferroelastic-paraelastic transition, transition temperature changes, under different atmospheric surroundings, have been observed.

Tb2(Mo O4)3 spontaneous shear strain measurements and free energy expression

The spontaneous shear strain uxy versus the temperature is measured by the y ray diffractometry technique in the (300 K-430 K) range. The first-order type of the ferroelectric-ferroelastic transition is clearly demonstrated (discontinuity of some 100 for uxy at the transition temperature Ttr) : our data are in accordance with phenomenological descriptions corresponding to the free energy expressions with four adjustable parameters.

Tanane: A real molecular ferroelectric crystal

Abstract “Tanane” exhibits a ferroelectric ferroelastic transition at 135°C. The symmetry change is the same as in KDP (142d → Fdd2) but it will be shown that the transition is more ferroelastic than ferroelectric. We have performed an X-ray structure determination of the high temperature phase and also measurement of the order parameter and the response function by optical method. Critical exponents (β and γ) have been measured showing non “mean-field” values. In the low temperature phase the switching time is very long and we will report several features of the domain wall behavior.

Temperature and Polarization Dependence of the Optical Absorption Edge of Gadolinium Molybdate

The optical absorption spectrum of gadolinium molybdate (GMO) was measured as a function of photon energy and polarization between 40 and 205 \ifmmode^\circ\else\textdegree\fi{}C for absorption coefficients between 10 and 300 ${\mathrm{cm}}^{\ensuremath{-}1}$. GMO undergoes a simultaneous ferroelectric-ferroelastic transition at 159 \ifmmode^\circ\else\textdegree\fi{}C; no discontinuity in the absorption spectrum was observed at this temperature. No differences were observed between spectra for light polarized along the $a$ or $b$ crystal axes, but the spectrum shifted to higher energy for light polarized along the $c$ axis and shifted uniformly with temperature. Two absorption bands, each consisting of at least three separate lines, were superimposed on the optical absorption edge. The absorption coefficients were found to obey a modified form of Urbach's rule which, within experimental error, can be described equally well by either of two equations. The implications of these two equations are discussed.

Polyethylene Sandwich Technique for Far Infrared Powder Samples

Abstract Recent research on the far infrared absorption of the free radical “tanane” (nitroxyde tetramethyl-2, 2, 6, 6,-piperidine oxyle, C9H18NO) in connection with its ferroelectric-ferroelastic transition at 14° C1, 2, 3 has shown that conventional sample preparation techniques are unsuitable. A new method which offers certain advantages has been developed and is described here since it has potential applicability to other powder samples and can be readily adapted for measurements at higher frequencies.

A comprehensive X-ray study of the ferroelectric–ferroelastic and paraelectric–paraelastic phases of Gd2(MoO4)3

The crystal structure of Gdz(MoO4)a has been investigated both at room temperature and at elevated temperatures below and above the ferroelectric-ferroelastic transition temperature (~ 160°C). Space group, unit-cell dimensions, and formula units per unit cell Pba2 (C8v), a= 103881 +00003, b= 104194+0.0004, c= 107007+0.0006/~,, Z = 4 at 25°C and PS421m (D~), a=7.393 +0.002, c= 10.670 + 0.004/~, Z= 2 at 183 °C. Full-matrix least-squares refinements have been carried out with anisotropic thermal parameters with two single-crystal diffractometer data sets measured at 25 and 183 °C, yielding conventional R values of 0.032 (3000 reflections) and 0-028 (880). The high-temperature structure comes close to the average structure of the two ferroelectric-ferroelastic orientations. The differences in interatomic distances between the two modifications are all less than 0.05 .~. Ferroelectric-ferroelastic switching can be accomplished through two macroscopically equivalent switching mechanisms. Changes in interatomic distances of nearest neighbors upon switching are all less than 0.05 ,~,. The main difference between the two ferroelectric-ferroelastic orientations results through movements of several oxygen atoms by as much as 0.7.~. The spontaneous polarization was calculated assuming point charges Gd 3+ and [MOO4]'from positional parameters: Ps = 0.175/tC.cm -2, in good agreement with experiment. Since most of the dipole moments cancel out within the unit cell, Gd2(MoO4)3 can be described as a canted antiferroelectric. The temperature dependence of intensities and peak widths of superstructure reflections has been monitored through the transition temperature. Below the transition temperature, temperature-dependent physical properties can be accounted for by gradual changes in positional parameters towards the high-temperature structure. The mechanism of the phase transition is discussed in terms of the 'positional order-disorder' and the 'soft mode' model. The results obtained in the refinement of the room-temperature structure are compared with results of an independent study of this structure by Keve, Abrahams & Bernstein.