Ferroelectric-paraelectric phase transition in PbHf0.2Ti0.8O3 studied by neutron powder diffraction

Neutron powder diffraction data, collected over the temperature range 10-810 K, have been analysed in order to study the ferroelectric-paraelectric transition in the ferroelectric compound PbHf0.2Ti0.8O3. This transition appears at 670 K between the low-temperature tetragonal phase and the high-temperature cubic phase. From high-resolution neutron powder diffraction data (3T2-LLB), the tetragonal structure of the ferroelectric phase has been refined at 10, 300 and 400 K using a Rietveld-type method: space group P4mm, Z =1; at T=10 K, at=3.9299(4) A, ct 4.1239(5) A and Vt=63.689 A3; at T=300 K, at=3.9405(4) A, ct=4.1038(5) A and Vt=63.723 A3; and at T=400 K, at = 3.9468(4) A, ct 4.0901(5) A and Vt=63.713 A3. In addition, a neutron powder thermodiffractometry experiment (D1B-ILL) has been performed to study in situ the temperature induced phase transition. From sequential Rietveld refinements, the thermal dependence of the cationic displacements has been analysed and a spontaneous polarization has been derived. From a generalized effective field theory, the first-order character of the phase transition has been established. Finally, the structural results obtained on the ferroelectric PbHf0.2Ti0.8O3 are discussed in reference to PbTiO3 and PbHf0.4TiO0.6O3 compounds. (author)