Acta Crystallographica Section B

Structural Science

Volume 56, Part 1 (February 2000)

research papers

Acta Cryst. (2000). B56, 27-38 [ doi:10.1107/S0108768199011453 ]

Octahedral deformations and cationic displacements in the ferroelectric PbHf_0.8Ti_0.2O₃: a neutron powder diffraction study from 10 to 770 K

C. Muller, J.-L. Baudour, C. Bedoya, F. Bouree, J.-L. Soubeyroux and M. Roubin

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 PbHf_0.8Ti_0.2O₃, 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 Pm3m). From high-resolution neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France, [lambda] = 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 Pm3m, 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, [lambda] = 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.

Keywords: Octahedral deformation; Cationic displacement; Ferroelectrics; Neutron powder diffraction.