A. Ciżman

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.

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.

Birefringence imaging of phase transition in ferroelastic Li2TiGeO5

The ferroelastic phase transition in Li2TiGeO5 at 233.5 K was studied using an imaging version of the rotating-polariser method in the temperature range 110–290 K. The temperature dependences of the spontaneous birefringence and the extinction angle in the ferroelastic phase were determined. The evolution of the ferroelastic domain structure was observed. The critical behaviour of the linear birefringence studied at the tetragonal-orthorhombic phase transition was observed. The numerical value of the exponent β obtained from the experiment was 0.673 ± 0.004.

Ferroelectricity in bis(ethylammonium) pentachlorobismuthate(III): synthesis, structure, polar and spectroscopic properties

A brief description of the thermal, structural and dielectric properties of bis(ethylammonium) pentachlorobismuthate(III) ferroelectric with Ps that equals to 1.4 μC cm−2 at 180 K is presented. This paper focuses in particular on the molecular mechanism of a phase transition that is related mainly to the deformation of the anionic sublattice confirmed by temperature-variable powdered UV-Vis spectroscopy.

Pressure dependence of dielectric properties of the LiNaGe4O9 ferroelectric

Dielectric properties of lithium–sodium–tetragermanate (LNG) LiNaGe4O9 crystal were investigated under hydrostatic pressure. The phase diagram and phase transition temperature as a function of pressure was constructed. Up to 200 MPa the dependence of TC on pressure is linear with negative slope of 8.5 K/GPa. The influence of pressure on the Curie–Weiss law was observed. It was also shown that with increasing pressure the maximum value of the electric permittivity and Curie–Weiss constant are decreasing. The obtained results revealed complex mechanism of phase transition in the ferroelectric LNG.

Calorimetric investigation of (TEA)2MnCl4 crystals

Temperature dependencies of the specific heat in a temperature range of 14–260 K are presented for the crystals (TEA)2MnCl4. Two anomalies typical for the first order phase transition at T 1 = 224 K and T 2 = 231 K with entropy jumps of 15 J mol−1 K−1 and 12 J mol−1 K−1 were observed. The temperature dependence of the lattice heat was approximated by a linear combination of Debye and Einstein functions. Basing on the results an effective wave number of phonons with an essential contribution to the lattice oscillation energy was determined. Since phase transition sequence observed in (TEA)2MnCl4 and (TEA)2CuCl4 is similar and similar is the chemical composition one can suppose that the low temperature phase transition observed in (TEA)2MnCl4 crystals is an isostructural transition.

Size effect and dielectric properties of NH4H2PO4 — porous glass composites

NH4H2PO4 nano-composite antiferroelectric materials in porous glass have been studied by means of dielectric and dilatometric investigations. Dielectric spectroscopy measurements in a wide frequency range are reported here for the first time, for both the antiferro- and paraelectric phases of ammonium dihydrogen phosphate (ADP) embedded in a porous matrix. Low frequency relaxation processes above the phase transition temperature were shown to occur. An investigation of the thermal expansion revealed a negative volume jump at the phase transition point. It was found that the phase transition temperature in ADP crystals embedded in porous glass decreased with the decrease of the mean pore size. The experimentally observed shift of the phase transition temperature is caused by a combination of size and pressure effects.

Dilatometric Investigations of Phase Transitions in TEA2MnCl4 Crystals

Results of dilatometric measurements of [N(C 2 H 5 ) 4 ] 2 MnCl 4 single crystal along the main crystallographic axes in the temperature range of 170 ÷ 240 K are presented. The positive thermal deformation in b direction and the negative one in c direction were found. Anomalies in the thermal expansion and the thermal hysteresis confirm the first-order character of both phase transitions. The changes in the relative crystal volume at the phase transition temperatures (Δ V/V 228 K = 0.032 and Δ V/V 224 K = 0.022) were determined on heating. Based on the dilatometric and specific heat data the coefficients of the pressure dependent shift of the transition temperature were estimated for both phase transition temperatures. These are ΔT 1/Δp ≅ 0.9 K/MPa and ΔT 2/Δ p ≅ 0.3K/MPa. The roentgenographic data collected for the high-temperature phase as well as low-temperature phases could not be interpreted unambiguously.

Evidence of the ferroelastic phase transition in Na2TiGeO5 ceramics

Newly discovered phase transition at 281 K in Na2TiGeO5 ceramics was studied with thermal analysis techniques. The results of thermal conductivity and dilatometric studies unequivocally demonstrated a ferroelastic character of the second-order phase transition.