O. Czupiński

Structure, phase transitions and molecular motions in 4-aminopyridinium perchlorate

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO4 has been determined at 100 K by means of x-ray diffraction as monoclinic, with space group P 21, with Z = 8. The crystal undergoes two structural phase transitions: one of first-order type, reversible, at 241/243 K (on cooling/heating respectively) and one of weakly first-order type, irreversible, at 277 K (on heating). The crystal dynamics is discussed on the basis of the temperature dependence of the 1 H nuclear magnetic resonance second moment (M2) and spin–lattice relaxation time T1. Both phase transitions are interpreted in terms of the changes in the motional state of (4-apyH)+ cations and ClO4− anions. The dielectric dispersion studies disclose a relaxation process over the high-temperature phase (above 241 K) in the audio-frequency region. The dielectric results are described by a Cole–Cole equation. The title crystal reveals pyroelectric properties below 241 K. The ferroelastic domain structure of (4-apyH)ClO4 is observed over the whole temperature range studied.

Polar and antiferroelectric behaviour of a hybrid crystal – piperazinium perchlorate

Monoprotonated piperazinium perchlorate, [NH2(CH2)4NH][ClO4], appeared to be a novel room temperature polar material (P1). Its acentric symmetry was confirmed by single-crystal X-ray diffraction, second harmonic generation (SHG) and pyroelectric measurements. Differential scanning calorimetry (DSC) measurements revealed a complex sequence of phase transitions above room temperature: I ↔ II at 433/422 K (heating–cooling), II ↔ III at 417/411 K, III ↔ IV at 403/395 K and IV ↔ V at 397 K (the lowest temperature phase transition recorded only upon heating). The characteristic feature of the structure of [NH2(CH2)4NH][ClO4] is the presence of two parallel cationic chains which are connected with each other by strong N–H⋯N hydrogen bonds. In phase V, these strongly polar non-equivalent chains contribute to spontaneous polarization. 1H NMR measurements disclosed the reorientational motions of the piperazinium ([NH2(CH2)NH]+) cations as well as the proton motion in the N–H⋯N hydrogen bonds along the piperazine chain. Over phase I, the overall motions of the ClO4− anions and reorientational motion of cations are postulated. The dielectric response, e′(T), accompanying the PT I ↔ II indicates possible antiferroelectricity in phase II.

Unusual structural phase transition in [N(C2H5)4][N(CH3)4][ZnBr4]

ABSTRACT The new hybrid organic–inorganic crystal [N(C2H5)4][N(CH3)4][ZnBr4] was grown and its physical properties and structural phase transition are presented. On the basis of thermal analysis (DSC (differential scanning calorimetry), DTA (differential thermal analysis), DTG), X-ray structural, dilatometric and dielectric studies as well as optical observation, the reversible first-order phase transition at 490/488 K on heating and cooling run, respectively, has been found. An appearance of domain structure of ferroelastic type gives evidence for an untypical lowering of crystal symmetry during the phase transition. At room temperature, the satisfying crystal structure solution was found in the tetragonal system, in the P21m space group.

Structural phase transition in a perovskite-type NH3(CH2)3NH3CuCl4 crystal – X-ray and optical studies

ABSTRACT X-ray powder studies and optical studies (polarized microscopic observation and linear birefringence studies) of the crystal NH3(CH2)3NH3CuCl4 are presented. The X-ray powder studies revealed a change of symmetry from orthorhombic room-temperature phase to monoclinic phase above 434 K. A reversible phase transition of the first order at 434 K on heating and 432 K on cooling was observed in birefringence studies. Optical polarized microscopic observation revealed monodomain and multidomain states in the room-temperature orthorhombic phase with domain walls in (110) and (1-10) planes. The hypothetical prototypic phase is expected to be tetragonal. The change of symmetry from orthorhombic to monoclinic and expected domain structure was found above 434 K in the (010) plane.