DYNAMICAL AND STRUCTURAL STUDIES OF FERROELECTRIC SALTS. A.V. Belushkin, Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russia
In many ferroelectrics, structural transformations at phase transitions are accompanied by significant changes in lattice dynamics. Often the microscopic mechanism of phase transitions can be understood only on the basis of a simultaneous analysis of both structural and dynamical data. Recent results obtained by neutron diffraction, as well as inelastic and quasielastic neutron scattering will be reviewed.
1. KH2PO4 (KDP) is an extensively studied crystal which
undergoes a ferroelectric phase transition at 122K (I
2d(Fdd2).
Pressure reduces the transition temperature and above 17 kbar, KDP remains
paraelectric down to 0K. The inelastic neutron scattering spectra measured from
KDP at 20K at ambient pressure and at 20 kbar reveal dramatic changes in the
lattice dynamics. These changes will be discussed together with the published
structural data obtained at different pressures.
2. When KDP is mixed with NH4H2PO4 (ADP), which undergoes an antiferroelectric phase transition at 148K, the ferroelectric transition temperature rapidly decreases. In the concentration range 0.2(x(0.8, the K1-x(NH)4H2PO4 mixed crystals and their deuterated analogues show neither a ferro- nor antiferroelectric transition. At low temperatures, they reveal properties characteristic of a proton dipolar glass. Using neutron diffraction on a deuterated mixed powder (x(0.6), structural changes were studied in the temperature range from 290K to 5K. At 5K, profile refinement was performed and revealed very unusual structural features. Structural anomalies observed in the temperature dependence will be discussed, together with the quasielastic neutron scattering data.
3. Structural and dynamical studies of (CH3)2NH2Al(SO4)2(6HO which undergoes a ferroelectric phase transition at 152K, will also be discussed.