Neutron scattering study of ferroelectric Sn2P2S6 under pressure

Ferroelectric phase transition in the semiconductor Sn2P2S6 single crystal has been studied by means of neutron scattering in the pressure-temperature range adjacent to the anticipated tricritical Lifshitz point (p=0.18GPa, T=296K). The observations reveal a direct ferroelectric-paraelectric phase transition in the whole investigated pressure range (0.18 - 0.6GPa). These results are in a clear disagreement with phase diagrams assumed in numerous earlier works, according to which a hypothetical intermediate incommensurate phase extends over several or even tens of degrees in the 0.5GPa pressure range. Temperature dependence of the anisotropic quasielastic diffuse scattering suggests that polarization fluctuations present above TC are strongly reduced in the ordered phase. Still, the temperature dependence of the (200) Bragg reflection intensity at p=0.18GPa can be remarkably well modeled assuming the order-parameter amplitude growth according to the power law with logarithmic corrections predicted for a uniaxial ferroelectric transition at the tricritical Lifshitz point

Repolarization of ferroelectric superlattices BaZrO3/BaTiO3

The study was supported by Russian Science Foundation, project No. № 17-72-20105

Phase incommensurable dans le composé mixte (NH4) 2(BeF4)0,82(SO4)0,18

Phase transitions in the solid solution (NH4)2(BeF 4)0.82(SO4)0.18 have been studied by means of X-ray scattering. The transition sequence is particularly complicated. The incommensurate phase appears between — 106 °C and — 120.5 °C, but in the — 106 to — 113 °C temperature range, the incommensurate superstructure is double. Moreover the so-called « lock in » phase is already present at the normal-incommensurate transition : it appears from — 102.5 °C. These results are briefly discussed. The possibility of inhomogeneities in the crystal is considered.Une étude par diffraction des rayons X des transitions de phase dans le composé mixte (NH4)2(BeF4)0,82(SO 4)0,18 est présentée. La séquence de transitions est particulièrement complexe. La phase incommensurable qui va de — 106 °C à - 120,5 °C, présente une surstructure incommensurable double de — 106 °C à — 113 °C. De plus la phase dite « de blocage » est déjà présente à la transition normale-incommensurable : elle apparaît dès - 102,5 °C. Ces résultats sont discutés brièvement. L'éventualité d'une structure biphasique inhomogène est envisagée

Incorporation of lanthanide ions in lead titanate

Lanthanide ion doping of lead titanate was investigated using structural and differential scanning calorimetry measurements. Dense ceramics samples were prepared with starting compositions, Pb 1-3x/2Ln x TiO 3, assuming A-site substitution of Ln = La, Nd, Sm, Eu, Gd and Dy, at 2 and 8 at.%. Doping with La 3+ reduced the tetragonality and cell volume. A systematic recovery in these values was observed for the smaller Ln 3+ ions, suggesting the onset of partial substitution of the Ln ions at the B-site. The ferroelectric-paraelectric transition temperatures also recovered, consistent with an increased probability of B-site occupation with decreasing ion size. Evidence for site occupancy obtained from the presence cation vacancies is also discussed

Switching Properties of Ferroelectric Perovskite Superlattices

14th European Conference on the Applications of Polar Dielectrics (ECAPD), Moscow, RUSSIA, JUN 25-28, 2018International audienceStructural, dielectric and switching properties of epitaxial ferroelectric superlattices (SL) of barium zirconate BaZrO3/barium titanate BaTiO3 are investigated. In SL there is a ferroelectric phase transition at a temperature of 393 degrees C close to the second type. The study of switching of SL in rectangular pulse fields showed that the dependence of the maximum switching current on the applied field is characterized by ``activation'' and ``linear'' regimes separated by a threshold field, which decreases when approaching the Curie point