Temperature-dependent neutron powder diffraction evidence for splitting of the cationic sites in ferroelectric PbHf0.4Ti0.6O3

Temperature-dependent neutron powder diffraction experiments (diffractometer 3T2-LLB, Saclay, France, lambda = 1.227 Angstrom) have been performed on the perovskite-like lead hafnate titanate PbHf0.4Ti0.6O3. This compound belongs to the solid solution denoted pHT, which derives from the well known ferroelectric PZT series. It exhibits a ferroelectric-to-paraelectric phase transition around 620 K, between the low-temperature tetragonal phase and the high-temperature cubic phase. The tetragonal structure of the ferroelectric phase has been refined at 10 and 300 K using a Rietveld-type method: space group P4mm with Z = 1; a(t) = 3.999 (1), c(t) = = 4.120 (1) Angstrom, c/a = 1.030, V = 65.89 Angstrom(3) at 10 K; a(t) = 4.012 (1) and c(t) = 4.100 (1) Angstrom, c/a = 1.022, V= 65.99 Angstrom(3) at 300 K. The cubic structure of the paraelectric phase has also been refined at 720 K: space group Pm (3) over bar m, Z = 1, a(c) = 4.046 (1) Angstrom, V = 66.23 Angstrom(3) Cation displacements and oxygen-octahedra elongations have been observed as a function of temperature. Evidence for peculiar behaviour associated with the relative shifts of the Hf and Ti atoms (thought until now to be on the same crystallographic site) was found through an anomaly of the mean-square atomic displacements of the Hf/Ti pseudonucleus. The PDF Nos for PbHf0.4Ti0.6O3 are 48-49-9 and 48-49-10.