Structural Change Caused by Substitution of Nd for Sm in (Nd, Sm)AlO₃: Application of Synchrotron High-Resolution Powder X-ray Diffraction

Structural change caused by substitution of Nd for Sm in perovskite (Nd, Sm)AlO₃ was analysed by application of high-resolution powder X-ray diffraction using synchrotron radiation. The parallel, well monochromated and bright incident X-rays improved the full width at half-maximum (FWHM) to 0.027° in a wide 2θ range for the standard Si powder. Applying this high-resolution optical system, the lattice parameters of the solid solution (Nd_x, Sm_1-x)AlO₃ were precisely analysed for the phases from x = 0.0 to 1.0 with an interval of 0.2. The lattice parameters of a series of RAlO₃ vary system-atically with the average ionic radii of R³⁺, accompanying a structural change from orthorhombic to a trigonal system at around R³⁺ = 1.11 Å corresponding to average ionic radii of 0.7Nd³⁺ + 0.3Sm³⁺. In orthorhombic phases, deformation of the crystal lattice from its ideal cubic lattice is minimized at around x = 0.0-0.2 in (Nd_x, Sm_1-x)AlO₃ and increased with increasing average ionic radii of Nd³⁺ and Sm³⁺. The structure changes from orthorhombic to trigonal at around x = 0.7.