Octahedral deformations and cationic displacements in the ferroelectric PbHf0.8Ti0.2O3: a neutron powder diffraction study from 10 to 770 K

Abstract

Neutron powder diffraction data, collected over the temperature range 10–770 K, have been analysed in order to make a detailed characterization of the sequence of phase transitions occurring in the Hf-rich ferroelectric PbHf0.8Ti0.2O3, titanium hafnium lead oxide. Over the whole temperature range this compound undergoes two phase transitions, which involve cationic displacements and octahedral deformations (tilt and/or distortion) leading to strongly distorted perovskite-type structures. The first transition appears around 415 K between two ferroelectric rhombohedral phases: a low-temperature nonzero-tilt phase F RL (space group R3c) and an intermediate zero-tilt phase F RH (space group R3m). The second one, detected around 520 K, is associated with a ferroelectric to-paraelectric transition between the F RH phase and the P C cubic phase (space group Pm3¯m). From high-resolution neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France, λ = 1.2251 Å), the crystallographic structure of the three successive phases has been accurately determined at the following temperatures: T = 10 K (F RL): space group R3c, Z = 6, a hex = 5.7827 (1), c hex = 14.2702 (4) Å, V hex = 413.26 (2) Å3; T = 150 K (F RL): space group R3c, Z = 6, a hex = 5.7871 (1), c hex = 14.2735 (4) Å, V hex = 413.98 (3) Å3; T = 290 K (F RL): space group R3c, Z = 6, a hex = 5.7943 (1), c hex = 14.2742 (5) Å, V hex = 415.04 (3) Å3; T = 440 K (F RH): space group R3c, Z = 6, a hex = 5.8025 (1), c hex = 14.2648 (4) Å, V hex = 415.94 (3) Å3; T = 520 K (P C ): space group Pm3¯m, Z = 1, a cub = 4.1072 (2) Å, V cub = 69.29 (1) Å3. In addition, a neutron powder thermodiffractometry experiment, performed between 290 and 770 K (diffractometer D1B-ILL, Grenoble, France, λ = 2.533 Å), has been used to study in situ the temperature-induced phase transitions. From sequential Rietveld refinements, the temperature dependence of the cation displacements and the rotation and/or distortion of oxygen octahedra was derived.