Synchrotron x-ray diffraction study of Ba4.5Nd9Ti18O54 microwave dielectric ceramics at 10-295 K

The structure of ceramic Ba4.5Nd9Ti18O54 was investigated by synchrotron x-ray powder diffraction from 10 to 295 K. Reitveld refinement and Le Bail profile analysis were applied to the data. Based on an orthorhombic structure, unit cell parameters of a = 22.3479(3) Angstrom, b = 7.6955(l) Angstrom, and c = 12.2021(2) Angstrom were obtained at room temperature and a = 22.3367(5) Angstrom, b = 7.6738(l) Angstrom, and c = 12.1842(3) Angstrom at 10 K. No evidence was found for any major structural change from 10 to 295 K. Within the tungsten bronze framework the two pentagonal channels were fully occupied by Ba; the remaining Ba atoms shared the rhombic channels with Nd. Thermal expansion of the unit cell was found to be anisotropic. The largest expansion occurs along the b cell edge, and the least along the a cell edge. It is proposed that the anisotropy is due to enhanced bending of the TiO6 polyhedra chains along the b direction.