Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na_0.5Bi_0.5TiO₃

Abstract

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.

Dramatic Influence of A-Site Nonstoichiometry on the Electrical Conductivity and Conduction Mechanisms in the Perovskite Oxide Na0.5Bi0.5TiO3

“…[18][19][20][21][22][23][24][25] Our work shows there are two types of NBT based on the level of nonstoichiometry (resistive stoichiometric NBT and conducting Bi-deficient NBT). It's not clear whether the NBT samples used in the structural studies reported in the literature are resistive or conducting and whether or not these two electrically distinct materials have the same crystal structure.…”

“…For 18 O tracer diffusion measurements, the standard procedure for introducing an 18 O penetration profile into a solid from a large volume of gas was employed. 32,33 Dense samples (relative density > 95%) were annealed in highly enriched 18 …”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] NBT has a complex crystal structure. [18][19][20][21][22][23][24][25] Early neutron powder diffraction studies proposed NBT to exhibit a rhombohedral (space group R3c) structure at room temperature (RT). 18 Recent high resolution synchrotron powder x-ray diffraction data suggest a monoclinic structure with space group Cc is a better fit.…”

“…[18][19][20][21][22][23][24][25] Early neutron powder diffraction studies proposed NBT to exhibit a rhombohedral (space group R3c) structure at room temperature (RT). 18 Recent high resolution synchrotron powder x-ray diffraction data suggest a monoclinic structure with space group Cc is a better fit. 19 Furthermore, the average structure determined by neutron powder diffraction and synchrotron powder x-ray diffraction is different from the local structure, as revealed by X-ray and neutron total scattering experiments and pair distribution function (PDF) analysis.…”

“…In a previous study, 30 we used a combination of Impedance Spectroscopy, 18 O tracer diffusion studies and transport number measurements for oxide-ion conductivity to reveal the effect of Bi nonstoichiometry on the electrical properties of Na 0.50 Bi 0.50+x TiO 3+1.50x/2 where x = -0.01 (Na 0.50 Bi 0.49 TiO 2.985 ; NBi 0.49 T), 0 (Na 0.50 Bi 0.50 TiO 3 ; NBT) and 0.01 (Na 0.50 Bi 0.51 TiO 3.015 ; NBi 0.51 T). The defect chemistry of NBT is rather different from other well-known titanates such as (Ba,Sr,Ca)TiO 3 in that the electrical conductivity is dominated by oxide-ion conduction in NBT and NB 0.49 T rather than electronic conduction.…”