DOUBLE PEROVSKITES CONTAINING HEXAVALENT MOLYBDENUM AND TUNGSTEN - SYNTHESIS, STRUCTURAL INVESTIGATION AND PROPOSAL OF A FITNESS FACTOR TO DISCRIMINATE THE CRYSTAL SYMMETRY

Double perovskites An(2)(II)B(II)B(VI)O(6) (A(II) = Ba, Sr, Ca; B-II = Mg, Ni, Co, Cd, Ca;B-VI = W, Mo, W0.5M0.5) have been synthesized and their crystal structures investigated by powder X-ray diffraction meas urements. In order td discriminate the crystal systems of the double p erovskites, a fitness factor is proposed which corresponds to the size matching between the A cation and the cubo-octahedral cavity formed b y eight BO6 octahedrons. The fitness factor can discriminate the cryst al systems of obtained compounds more exactly than the well known tole rance factor.