Rietveld neutron powder profile analysis of the compound Na(0.5)Bi(0.5)TiO(3) (NBT) is reported over the temperature range 5-873 K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813 K), to one of tetragonal (673-773 K) and then rhombohedral structures (5-528 K) has been established. Coexisting tetragonal/cubic (773-813 K) and rhombohedral/tetragonal (with an upper temperature limit of 145 K between 528 and 673 K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with a(H) = 5.4887 (2), c(H) = 13.5048 (8) A, V = 352.33 (3) A(3), Z = 6 and D(x) = 5.99 Mg m(-3), exhibits an antiphase, a(-)a(-)a(-) oxygen tilt system, omega = 8.24 (4) degrees, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with a(T) = 5.5179 (2), c(T) = 3.9073 (2) A, V = 118.96 (1) A(3), Z = 2 and D(x) = 5.91 Mg m(-3), possesses an unusual combination of in-phase, a(0)a(0)c(+) oxygen octahedra tilts, omega = 3.06 (2) degrees, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported.

The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.

No abstract