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

Neutron powder diffraction data, collected over the temperature range 10-81 0 K, have been analysed in order to study the ferroelectric-paraelectric tr ansition in the ferroelectric compound PbHf0.2Ti0.8O3. This transition appe ars at 670 K between the low-temperature tetragonal phase and the high-temp erature cubic phase. From high-resolution neutron powder diffraction data ( 3T2-LLB), the tetragonal structure of the ferroelectric phase has been refi ned at 10, 300 and 400 K using a Rietveld-type method: space group P4mm, Z = 1; at T = 10 K, a(t) = 3.9299((4)) Angstrom, c(t) = 4.1239((5)) Angstrom and V-t = 63.689 Angstrom (3); at T = 300 K, a(t) = 3.9405((4)) Angstrom, c (t) = 4.1038((5)) Angstrom and V-t = 63.723 Angstrom (3); and at T = 400 K, a(t) = 3.9468((4)) Angstrom, c(t) = 4.0901((5)) A and V-t = 63.713 Angstro m (3). In addition, a neutron powder thermodiffractometry experiment (D1B-U -L) has been performed to study in situ the temperature induced phase trans ition. 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 c haracter of the phase transition has been established. Finally, the structu ral results obtained on the ferroelectric PbHf0.2Ti0.8O3 are discussed in r eference to PbTiO3 and PbHf0.4TiO0.6O3 compounds.