Thermal expansion of Cr 2xFe 2−2xMo 3O 12, Al 2xFe 2−2xMo 3O 12 and Al 2xCr 2−2xMo 3O 12 solid solutions

Abstract

The transition temperature from monoclinic to orthorhombic and the thermal expansion of the orthorhombic phase were investigated for three systems of the family A 2M 3O 12: Cr 2 x Fe 2−2 x Mo 3O 12, Al 2 x Fe 2−2 x Mo 3O 12 and Al 2 x Cr 2−2 x Mo 3O 12. 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 A 3 + 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 ( α l= α V /3). The relationship between the size of A 3 + cations in A 2M 3O 12 and the coefficient of thermal expansion is discussed. Near-zero thermal expansion of Cr 2Mo 3O 12 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.