Double perovskites containing hexavalent molybdenum and tungsten: synthesis, structural investigation and proposal of a fitness factor to discriminate the crystal symmetry

Abstract

Double perovskites A ^II ₂ B ^II B ^VI O ₆ (A ^II =Ba, Sr, Ca; B ^II =Mg, Ni, Co, Cd, Ca; B ^VI =W, Mo, W _0.5 Mo _0.5 ) have been synthesized and their crystal structures investigated by powder X-ray diffraction measurements. In order to discriminate the crystal systems of the double perovskites, a fitness factor is proposed which corresponds to the size matching between the A cation and the cubo-octahedral cavity formed by eight BO ₆ octahedrons. The fitness factor can discriminate the crystal systems of obtained compounds more exactly than the well known tolerance factor.