Synchrotron X-ray Diffraction Study of Ba_4.5Nd₉Ti₁₈O₅₄ Microwave Dielectric Ceramics at 10–295 K

The structure of ceramic Ba_4.5Nd₉Ti₁₈O₅₄ 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) Å, b = 7.6955(1) Å, and c = 12.2021(2) Å were obtained at room temperature and a = 22.3367(5) Å, b = 7.6738(1) Å, and c = 12.1842(3) Å 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.