R. Poprawski

Some properties of NaO0.5Bi0.5TiO3

Abstract The specific heat of Na0.5Bi0.5TiO3 crystal in the temperature range 300–650 K and the changes of spontaneous polarization of Na0.5Bi0.5TiO3 ceramic in the temperature range 4.2–90 K were measured. The calorimetric measurements showed that the anomaly of specific heat exists in the temperature range where the phase transition rate from the rhomhedral phase to the tetragonal phase is utmost. From low-temperature pyroelectric effect a phonon wave number equal to 128 cm−1 was found to have the main share in the temperature changes of spontaneous polarization.

Calorimetric Investigations of Phase Transitions in KNO3 Embedded Into Porous Glasses

Results of the Differential Scanning Calorimetry (DSC) studies of porous glasses filled by KNO3 ferroelectric in wide temperature range are presented. Anomalous heat capacity of the potassium nitrate confined in nanoscale porous glasses with two different radius of pore 23 and 160nm show that the phase I-III transition temperature was slightly affected by particle size, whereas the phase III-II transition temperature significantly decreased. During the heating, the phase II-I transition temperature increased up to 12.5°C comparing to the bulk material. The lowering of the potassium nitrate particle size significantly reduced the phase I–liquid transition temperature.

Spontaneous birefringence and pyroelectricity in KT1Opo4 crystals

Abstract Results on temperature dependence of spontaneous birefringence and pyroelectric properties for KT10P04 crystals are presented for a wide temperature range. KTi0P04 crystals have been shown to be ferroelectrics below 1165 K, the ferroelectric phase transition being of the second order. Based on the pyroelectric studies at low temperatures, the Einstein temperature for these crystals were estimated along with the wave number of phonons contributing to the pyroelectric effect.

Size effects in KDP-porous glass ferroelectric nanocomposites

An influence of the size effect on the phase transition temperature in potassium dihydrogen phosphate (KDP)-porous glass nanocomposites was studied in this work. Results of the dielectric measurements in a wide temperature range for KDP crystals embedded in porous glasses are presented. The phase transition temperature shift toward higher temperatures was observed for KDP embedded in porous glasses with the mean value of the pore size of 312 and 160 nm, whereas for smaller pore sizes (71 and 23 nm) the phase transition temperature decreased. The phase transition temperature dependence of the mean values of the pores dimensions of KDP-porous glass nanocomposites is the experimental evidence showing the non-monotonical character of the size effect on the phase transition temperature in ferroelectrics particles. It was found that the dielectric permittivity of KDP embedded into the porous glasses with the mean values of pore sizes of 160 nm and 312 nm practically did not depend on frequency, whereas for KDP embedded into the pores with dimensions of 71 nm and 23 nm a dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases.

Pressure effect on the ferroelectric phase transition in nanosized NH4HSO4

The dielectric permittivity and the specific heat results of the ammonium hydrogen sulphate NH4HSO4 embedded into porous glasses with different average pore sizes in a wide temperature and frequency ranges are presented. The influence of the pore size on the ferroelectric phase transition temperature is considered. The results suggest a possibility of the size-effect influence on the phase transition type. The results show a good agreement with a theory [W. L. Zhong et al., Phys. Rev. B 50, 698 (1994)] indicating a non-monotonic behavior of the low-temperature phase transition with a confined particle.

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.

Calorimetric investigations of overcritical behaviour in (TEA)2CuCl4 crystals

Abstract The results of experimental investigation of molar heat capacity of [N(C2H5)4]2CuCl4 - ferroelasto-ferroelectric crystals in a wide temperature range are presented. The typical anomaly for the first order phase transition of order-disorder type at 258 K and additional anomaly with maximum at 195 K has been observed. We suggest that the low temperature anomalies of physical properties of [N(C2H5)4]2CuCl4 crystals can be attributed to over critical traces of isomorphous phase transitions (similar to that observed in liquid-gas system). Results of numerical analysis of phenomenological model of phase transition in [N(C2H5)4]2CuCl4 crystals are presented and compared with the experimentally determined temperature dependence of the specific heat and entropy changes.

Investigation of phase transitions in NH4HSeO4 crystals by pyroelectric method

Abstract The pyroelectric properties of NH4HSeO4 crystals were measured in the temperature range of 100–300 K. A new phase transition was found at 109 K. It was noted that 80% deuteration of the NH4HSeO4 crystals caused these crystals to become non-ferroelectric.

Low-Temperature Thermal and Dielectric Properties of Na0.5Bi0.5TiO3

Thermal conductivity (at 5 to 300 K), dielectric (at 5 to 250 K) and sound velocity (at 5 to 150 K) measurements have been performed for the first time on ceramic samples of Na 0.5 Bi 0.5 TiO 3 . The data revealed a nearly temperature-independent plateau in thermal conductivity λ(T) at about 7 to 30 K and 0.2 W/mK. This plateau corresponds to a broad maximum in the heat capacity C expt T -3 plot for this material. Strong temperature and frequency dependences of electrical permittivity were observed. The results of the measurements presented here indicate a glass-like behaviour of Na 0.5 Bi 0.5 TiO 3 at low temperatures. Strongly frequency-dependent phonon scattering and/or interactions of phonons and domain walls with excitations, which arise from fluctuations between the rhombohedral matrix and regions of another phase, are responsible for this behaviour. Additional measurements performed for Na 0.5 Bi 0.5 TiO 3 single crystals (at 80 to 440 K) reveal the existence of very weak paramagnetic properties in this temperature range. The sound velocity, elastic stiffness moduli, phonon mean free path and dielectric losses have a small anomaly near 55 K indicating the presence of a new phase transition inferred in the present paper. The deduced elastic Debye temperature at 5 K is Θ elast D =190 K (180 and 185 K was recently obtained from calorimetric and pyroelectric measurements, respectively).

Phase transition in NH4H2PO4: porous glass composites

In this article, the results of the dielectric measurements after an impregnation of porous glasses (PGs) with ammonium dihydrogen phosphate NH4H2PO4 (ADP) are presented. As a result of dielectric and calorimetric studies, it was found that the phase transition in ADP crystals embedded in PG became diffused. In addition, the phase transition temperature decreased with a reduction of the pore size. We observed a shift of the phase transition temperature which is a superposition of the intrinsic size effect and the pressure effect created by the difference in thermal expansion coefficients of ADP nanoparticles and glass matrix. Accordingly, for the crystals with hydrogen bonds dTp /dp < 0 (for ADP dTp /dpp =0 = −3.4 × 10−8 K Pa−1), so in ADP-porous glass composite a small negative effective size effect occurs.