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

Neutron powder diffraction data, collected over the temperature range 10-77 0 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 th is compound undergoes two phase transitions, which involve cationic displac ements and octahedral deformations (tilt and/or distortion) leading to stro ngly distorted perovskite-type structures. The first transition appears aro und 415 K between two ferroelectric rhombohedral phases: a low-temperature nonzero-tilt phase F-RL (space group R3c) and an intermediate zero-tilt pha se F-RH (space group R3m). The second one, detected around 520 K, is associ ated with a ferroelectric to-paraelectric transition between the F-RH phase and the P-C cubic phase (space group Pm (3) over bar m). From high-resolut ion neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France , lambda = 1.2251 A), the crystallographic structure of the three successiv e phases has been accurately determined at the following temperatures: T = 10 K (FRL): space group R3c, Z = 6, a(hex) = 5 7827(1), c(hex) = 14.2702(4) Angstrom, V-hex 413.26 (2) Angstrom(3); T = 150 K (FRL): space group R3c, Z = 6, a(hex) = 5.7871 (1), c(hex) = 14.2735(4) Angstrom, V-hex = 413.98 (3 ) Angstrom(3); T = 290 K (F-RL): space group R3c, Z = 6, a(hex) = 5.7943 (1 ), c(hex) = 14.2742 (5) Angstrom, V-hex = 415.04 (3) Angstrom(3); T = 440 K (F-RH): space group R3c, Z = 6, a(hex) = 5.8025 (1), c(hex) = 14.2648 (4) Angstrom, V-hex = 415.94 (3) Angstrom(3); T = 520 K (P-C): space group Pm ( 3) over bar m, Z = 1, a(cub) = 4.1072(2) Angstrom, V-cub = 69.29(1) Angstro m(3). In addition, a neutron powder thermodiffractometry experiment, perfor med between 290 and 770 K (diffractometer D1B-ILL, Grenoble, France, lambda = 2.533 A), has been used to study in situ the temperature-induced phase t ransitions. From sequential Rietveld refinements, the temperature dependenc e of the cation displacements and the rotation and/or distortion of oxygen octahedra was derived.