J. Przeslawski

Li 2 TiGeO 5 --A Novel Ferroelastic Crystal

Newly discovered phase transition at 233.5 K in the Li 2 TiGeO 5 crystal was studied with various physical methods. Results of dielectric, dilatometric, calorimetric, linear birefringence measurements, ferroelastic domain structure observations, the Brillouin scattering studies demonstrated a ferroelastic character of the second order transition, of 4/mmmFmmm Aizu type.

Linear birefringence studies in [(CH3)2NH2]5Cd3Cl11 crystal

Abstract Linear birefringence measurements have been performed for the [(CH 3 ) 2 NH 2 ] 5 Cd 3 Cl 11 (DMACC) crystal. A diffused anomaly of the lineat birefringence in a temperature range 250–320 K and a very distinct anomaly typical for a second order phase transition was found at 178.5 K. A sequence of phase transitions: of a second order at a T 2 = 178.5 K, and of a first order at a T 3 = 127 K, characteristic for crystals with incommensurate phases was confirmed. Critical index values and a hysteretic behaviour of the linear birefringence also indicated a possibility of the incommensurate phase existence in the DMACC crystal.

Ferroelastic twins and linear birefringence in DMACA crystal

Abstract Comprehensive studies of the ferroelastic domain structure in the DMACA crystal have been performed. It was concluded that the point group of the hypothetical paraelastic phase is 3 m. The precise linear birefringence measurements have been carried out at the ferroelastic- ferroelectric phase transition region. A proportionality between the linear birefringence changes and Ps2 was verified in the region of about 10 K below Tc.

Calorimetric studies of phase transitions in imidazolium perchlorate crystal

The first precise measurements of specific heat changes have been performed for a C3N2H5ClO4 crystal using an ac calorimeter. Phase transitions at about 373 and 219 K were revealed. The continuous second-order phase transition at 373 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of the same order of magnitude as those obtained for hard ferroelectric materials. Thermal parameters (such as the excess enthalpy, ΔH, and the excess entropy, ΔS) of the continuous transition were estimated and discussed. During the first cooling, a distinct (~2 K) supercooling effect was observed at the discontinuous transition at about 219 K.

Linear birefringence changes in DMACA and TMACA crystals in the phase transitions regions

Abstract The ferroelastic-ferroelectric phase transition and the ferroelastic domain structure have been studied for DMACA crystal. The paraelastic point group 3m is proposed. The para-electric-intermediate-ferroelectric phase transitions have been investigated for TMACA crystal using the linear birefringence measurements. The critical index value and the presence of the memory effect in the intermediate phase support our suggestion that the phase can be regarded as an incommensurate.

Specific Heat Peculiarities in (NH4)2SbF5

Precise measurements of the specific heat in (NH4)2SbF5 crystal have been performed using an ac-calorimeter. Anomalies previously connected with phase transitions at 293 K, 169 K, and 139 K are detected and discussed. Two second order transitions at 293 K and 169 K are confirmed. Thermal parameters (as the excess enthalpy Δ H, and the excess entropy ΔS) are estimated and possible mechanisms are presented. The anomaly at 139 K is attributed to an overcritical trace of isostructural phase transition. Simplified model calculations of the excess of specific heat are compared with the Δ cp measured experimentally.

Observations of two ferroelectric response times in TGSe at T ≤ Tc

Abstract Time dependent dielectric constant measurements in ferroelectric triglycine selenate reveal an existence of two characteristic response times with distinct temperature dependences near TC. Possible reasons connected with this behaviour are considered.

Anomalous behaviour of linear birefringence and strain in taap crystal

Abstract Linear birefringence and lattice parameters were measured for the ferroelectric telluric acid ammonium phosphate in the temperature range 200–340 K. It is shown that anomalous temperature behaviour of the linear birefringence increment δ(Δn1) induced by the ferroelectric phase transition can be attributed to the temperature change of the spontaneous strain e13.

AFM Observations of Domain Structures in Thin Films of BiFeO3

We have observed surfaces of multiferroic thin films of the BiFeO3 using the atomic force microscopy. Heterostructures BiFeO3/YBa2Cu3O7 on LSAT substrates were studied by various AFM techniques to reveal ferroic properties. LFM and tapping mode revealed domains of average widths: 36 nm and 58 nm for samples of 25 nm and 200 nm thick, respectively. Domain structure observations and its transformation have been performed by piezoresponse technique. We have also recorded features indicating presence of the magnetic domain structure by magnetic force method. Domain widths were estimated and compared with a classical model.

General equation of state for the pressure and temperature-induced transition in the ferroelectric triglycine selenate

Abstract An experimental investigation of the phase transition induced by temperature and pressure in the uniaxial ferroelectric triglycine selenate, which exhibits a quasi-tricritical point at T c ≈21.7°C (at ambient pressure), has been performed. The combined pressure and temperature dependences of polarization versus electric field are shown to fit well by effective field theory using the general scaling combination [1−( T / T c + p / p c )] which involves temperature ( T ) and pressure ( p ), T c being the transition temperature at zero pressure and p c =−81.81 kbar the transition pressure extrapolated to zero absolute temperature.