Thermal expansion of Cr2xFe2−2xMo3O12, Al2xFe2−2xMo3O12 and Al2xCr2−2xMo3O12 solid solutions

The transition temperature from monoclinic to orthorhombic and the thermal expansion of the orthorhombic phase were investigated for three systems of the family A2M3O12: Cr2xFe2−2xMo3O12, Al2xFe2−2xMo3O12 and Al2xCr2−2xMo3O12. It was possible to obtain a single-phase solid solution in all studied samples (x=0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1). A linear relationship between the transition temperature and the fraction of A3+ cations (x) was observed for each system. In all orthorhombic solid solutions studied here the observed thermal expansion was anisotropic. These anisotropic thermal expansion properties of crystallographic axes a, b and c result in a low positive or near-zero overall linear coefficient of thermal expansion (αl=αV/3). The relationship between the size of A3+ cations in A2M3O12 and the coefficient of thermal expansion is discussed. Near-zero thermal expansion of Cr2Mo3O12 is explained by the behavior of Cr–O and Mo–O bond distances, Cr–Mo non-bond distances and Cr–O–Mo bond angles with increasing temperature, estimated by Rietveld analysis of synchrotron X-ray powder diffraction data.