Maria Połomska

FIR studies of α and β polymorphs of LiNH4SO4 single crystals

Abstract LiNH 4 SO 4 crystals occur in two polymorphous modifications. The α polymorph (space group P ca2 1 ) undergoes an irreversible transformation to the β modification (space group P c2 1 n) exhibiting a ferroelastic—paraelastic phase transition at about 284 K and a ferroelectric—paraelectric transition at T c ≈ 460 K. Using FIR spectroscopy we studied the differences in the extenal vibrations of the two polymorphs, and the α → β polymorphous transformation.

Raman Scattering Studies of Lead Free (1-x)K0.5Na0.5NbO3-xSrTiO3 Relaxors

We present the first results of FT NIR Raman studies of (K 0.5 Na 0.5 NbO 3 ) 1 − x (SrTiO 3 ) x (x = 0.25, 0.20, 0.15) relaxors made in wide temperature range. Although the crystal structure of the investigated relaxors exhibits a pseudocubic symmetry Raman spectra are observed. The Raman spectra of (K 0.5 Na 0.5 NbO 3 ) 1 − x (SrTiO 3 ) solid solution resemble those observed for orthorhombic K 0.5 Na 0.5 NbO 3 . The results are discussed in the frame of molecular dynamics of NbO 6 octahedral network. It was found that, the changes in dynamics of NbO 6 molecular groups appear at T ≈ 196 K for x = 0.25, T ≈ 254 K for x = 0.20 and T ≈ 336 K for x = 0.15.

Electric conductivity of some ferroelectric crystals

Abstract Steady state conductivity vs. temperature was measured for Pb5(Ge,Si)3O11 mixed crystals and, moreover, for TGS and LAS pure and deuterated crystals. Change in activation energy at Tc was observed with activation energy in the ferroelectric phase higher than in the paraelectric phase. In case of TGS the second change in activation energy at low temperatures was found with low temperature activation energy close to those of the paraelectric phase.

Domain structure evolution and phase transition in incommensurate ferroelastic KSc(MoO4)2

Abstract Investigations of domain structures in the three ferroelastic phases of KSc(MoO4)2 have been performed. It was found that all the phases exhibit 2/m point symmetry and that the sign of spontaneous optical anisotropy in phase III is opposite to the one in II and IV phases.

Superionic phase transition in Rb4LiH3(SO4)4, Rb4LiH3(SeO4)4 and K4LiH3(SO4)4 single crystals

Abstract DTA studies of a new family of ferroelastic crystals of Me4LiH3(XO4)4 stoichiometry revealed at Ts another phase transition in the paraelastic phase, accompanied by an anomalous increase in the capacitance. Complex impedance method was used to study temperature variation of electric conductivity and its anisotropy of Rb4LiH3(SO4)4, Rb4LiH3(SeO4)4 and K4LiH3(SO4)4 single crystals. The bulk conductivity of the crystals varies from 10−6 to 10−4 (ωm)−1 in the temperature range from ∼ 400 K to temperatures below melting. The conductivity σc is ∼ one order of magnitude lower than σa ∽ σb. At Ts the activation energy Wa ∽ Wb decreases from 0.94 – 1.35 eV below Ts to 0.27 –0.28 eV at T > Ts. Comparison of the data with the crystallographic structure allowed us to classify the transition as superionic.

Dielectric Relaxation in K0.5Na0.5NbO3-PVDF Composites

K1−x Na x NbO3 is considered as an alternative system for lead-based functional ceramics widely used in transducers, sensors and actuators. In some applications however, the composites of electroactive ceramic and polymer, with low acoustic impedance and dielectric permittivity that can be tailored to various requirement, are more useful. We studied dielectric relaxation processes in polyvinylidene fluoride (PVDF) loaded with K0.5Na0.5NbO3 powder in wide temperature and frequency range. The powder was prepared by solid state synthesis and characterized by HRTEM and NIR Raman spectroscopy. The dielectric response of (K0.5Na0.5NbO3)0.32(PVDF)0.68 composite, of (0–3) connectivity, was found to be determined by the relaxation processes of the polymer matrix with an additional broad low-frequency anomaly at ∼ 305 K that we relate to a response of water absorbed by the shell of KNN grains.

Vibrational and Electronic Properties of Perylenediimide Linked to Fullerene and Tetrathiafulvalene

Solid-state electronic (UV-Vis) and vibrational (IR) spectra of the tetrathiafulvalene-perylenediimide (PDI-TTF) 1 and perylenediimide-fullerene (PDI-C60) 2 dyads as well as PDI 3 , TTF 8 and fullerene 9 derivatives used as references molecules were investigated in the frequency range between 400 and 45,500 cm−1. Electronic and vibrational features of the dyads were compared and analysed. Molecular geometry of the compounds and their IR and Raman theoretical spectra were calculated. It was stated, that small redistribution of the charges occurs in both moieties of these dyads. This effect is observed as shifts of both electronic and vibrational bands of the dyads in comparison with the respective bands of free components.

Ferroelastic and ferroelectric domain structure observations in LiNH4 SO4 and LiND4 SO4 with scanning electron microscope

Abstract Ferroelastic and ferroelectric domain structure in lithium ammonium sulphate and its deuterated analogue are observed with the scanning electron microscope. Samples are cutted perpendicularly to the three main crystallographic directions. The pictures of all these faces in ferroelectric and ferroelastic phases are presented.

Structural relaxation in superprotonic tetraammonium dihydrogen triselenate single crystals

Abstract (NH 4 ) 4 H 2 (SeO 4 ) 3 single crystals undergo a transition to superionic phase at T s =378 K and the phase is characterized by high structural disorder. When the crystal is cooled from highly disordered superprotonic phase to T ≪ T s the order, characteristic of low-temperature phase (trimers formed by three SeO 4 tetrahedra linked with two short hydrogen bonds, lying along [021] direction), has to be recovered. The recovery process, termed by us as structural relaxation, was studied by means of impedance and Raman spectroscopy. Results of conductivity recovery measurements allow to distinguish three rates of conductivity decrease. Time variation of SeO 4 skeletal modes after cooling the crystal from the superprotonic state to temperatures T ≪ T s points to the following processes of the recovery: i) relaxation to the low-temperature molecular structure (formation of trimers is proceeded by formation and disappearance of dimers as a transitional form) and ii) relaxation to the low-temperature crystallographic structure (trimers lying along [021] direction). We observe the process of dimer formation and disappearance within ∼30 min at 303 K, whereas the formation of trimers lasts ∼1 h at the same temperature.

NIR Raman studies of superionic phase transition in (NH4)4H2(SeO4)3

Abstract Near-infrared Fourier transform Raman spectroscopy has been used to study the molecular group dynamics in (NH 4 ) 4 H 2 (SeO 4 ) 3 single crystal in the vicinity of its superionic phase transition ( T s ⋍ 378 K ). A change in the dynamics of SeO 4 and SeO 4 H entities begins about 15 K below T s and ends at T s . The results of the Raman studies of (NH 4 ) 4 H 2 (SeO 4 ) 3 gave evidence that the orientational disorder of SeO 4 tetrahedra is responsible for the fast protonic conductivity. A very large temperature hysteresis (∼ 30 K) for the superionic phase transition in (NH 4 ) 4 H 2 (SeO 4 ) 3 was found.