Changes in the crystal structure of layered perovskite compounds <i>A</i><sub>3</sub><i>B</i><sub>2</sub>O<sub>9</sub> (<i>A</i> = Ba, Sr; <i>B</i> = W, Re)

Layered perovskite compounds exhibit various electrical properties due to their crystal structure.This review focused on the synthesis and characterization of B-cation-deficient hexagonal perovskite A 3 B 2 O 9 (A = Ba, Sr; B = W, Re), which exhibits two-dimensional perovskite layers and perovskite layer stacking along the [111] direction in the simple perovskite notation.Ba 3 W 2 O 9 , Sr 3 W 2 O 9 , and Sr 3 Re 2 O 9 were obtained through high-pressure synthesis.Crystal structure analysis was conducted using single crystal X-ray diffraction and transmission electron microscopy.Ba 3 W 2 O 9 was found to be isostructural with Ba 3 Re 2 O 9 and exhibited no octahedral rotation and tilt.By contrast, Sr 3 W 2 O 9 and Sr 3 Re 2 O 9 were hettotype structures of Ba 3 W 2 O 9 at 298 K and demonstrated phase transitions related to octahedral rotation and distortion.The substitution of Sr for Ba at the A-site induced compressive chemical pressure and crystal structure changes.The crystallographic differences between Sr 3 W 2 O 9 and Sr 3 Re 2 O 9 were octahedral tilts related to the 5d electron configuration.In A 3 B 2 O 9 (A = Ba, Sr; B = W, Re), the substitution in the Aand B-site cations changed the octahedral rotation and distortion.