Acta Crystallographica Section B

Structural Science

Volume 56, Part 1 (February 2000)

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, = 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, = 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.

Online 1 February 2000

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