B. Kosturek

Structure and phase transition in the (C5H10NH2)SbCl6?(C5H10NH2)Cl crystal

Differential scanning calorimetry, dilatometric, dielectric dispersion and linear birefringence measurements have been used to study the phase transition at 202 K in the (C5 H10 NH2 )SbCl6 (C5 H10 NH2 )Cl crystal. The x-ray studies show that at room temperature it crystallizes in the orthorhombic symmetry (space group Pccn ), whereas below 202 K the structure changes its symmetry to monoclinic (P 21 /c ). The mechanism of the phase transition is connected both with the disordering of the piperidinium cations and the essential change in the NH...Cl hydrogen bond system. The title crystal exhibits the ferroelastic domain structure in the monoclinic phase.

Structure and phase transitions in the ferroelastic [C(NH2)3]3Bi2Br9 crystal

Differential scanning calorimetry, dilatometric, dielectric and linear birefringence measurements have been used to study the ferroelastic [C(NH2)3]3Bi2Br9 crystal. The x-ray studies showed that it crystallizes at room temperature in the monoclinic symmetry, space group P21/m. The crystal undergoes a complex sequence of phase transitions: at 311 K, 333.5 K, 350 K, 415 K and 425 K. All phase transitions were found to be of first order type. The ferroelastic domain structure is maintained from room temperature up to 425 K. The temperature measurements of the linear birefringence and optical observations suggest the tetragonal symmetry of the parent paraelastic phase above 425 K.

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.

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.

Structure and phase transition in pyridazine perchlorate

Single crystals of pyridazine perchlorate [(C4H4N2)HClO4] have been synthesized and characterized by x-ray diffraction, dielectric measurements and optical studies. At room temperature the crystal is monoclinic, space group P 21/n. The temperature dependences of the electric permittivity and birefringence exhibit anomalous behaviour, characteristic of the first-order phase transitions at 343 and 339 K on heating and cooling, respectively. Optical investigations under a polarizing microscope show that the crystal symmetry changes from the biaxial room temperature phase II to the optically uniaxial high temperature phase I. The domain pattern arising after cooling from phase I reflects observed symmetry changes at phase transition. The fully disordered structure of phase I can be described in the obverse hexagonal space group .

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.

Structural Phase Transition in Pyridazine Fluoroborate Crystal

Single crystals of pyridazine fluoroborate [(C4H4N2)HBF4] have been grown and characterized by X-ray diffraction, DSC, dielectric measurements and optical studies. At room temperature the crystal is monoclinic, space group P21/n. DSC measurement revealed the reversible phase transition at 333 and 327 K on heating and cooling, respectively. The temperature dependences of the electric permittivity exhibit anomalies characteristic for the first-order phase transitions at practically the same temperatures as those observed in DSC. Optical observations by means of polarizing microscope show that the crystal symmetry changes from the biaxial room temperature phase II to the optically uniaxial high temperature phase I. The orientation states domain arising after cooling from phase I reflects observed symmetry changes between monoclinic and hexagonal system at the phase transition. The crystal seems to be similar to pyridazine perchlorate [(C4H4N2)HClO4].

Optical investigations of phase transitions in N(CH3)4FeCl4

Abstract Optical microscope observations and linear birefringence measurements were performed to obtain more information on the phase transitions and phases properties of the N(CH3)4FeCl4 single crystal. At least four phase transitions of first order were detected (at 385 K, 303 K, 294 K and 265 K on cooling), some of them of very low kinetics and large thermal hysteresis. The phase above 386 K was found as an isotropic one

Linear birefringence studies on phase transitions in trichloroacetamide crystal

Abstract Temperature dependence of the linear birefringence was studied for the molecular crystal-trichloroacetamide (TCAA). Two phase transitions were found at about 354.5 K and 358.5 K. The linear birefringence increments in the intermediate phase were analysed by the log-log approximation procedure and the gb-exponents were determined. A kind of the global hysteresis and the memory effect were also noticed in the intermediate phase suggesting the incommensurate nature of this phase.

Pyroelectric, pressureelectric and optical properties of potassium pentaborate tetrahydrate crystals

Abstract Results of studies of pyroelectric properties, the influence of hydrostatic pressure on spontaneous polarization, electrooptic effect and spontaneous birefringence of potassium pentaborate tetrahydrate crystals are presented. It is shown that potassium pentaborate crystals are linear pyroelectric with no phase transitions in temperature of 4.2–430 K. Obtained temperature dependence of pyroelectric coefficient has been described with the help of Einstein and Debye functions.