Crystal structure and high temperature phase relations of monoclinic and orthorhombic pyroxene-compounds MnGeO_3: a comparison

摘要

The pyroxene-type compound MnGeO_3 crystallizes in both, the monoclinic Cllc and the orthorhombic Pbca modification in flux growth experiments using MnCl_2. The structure of both forms was determined and refined at room temperature using single crystal X-ray diffraction data and compared, both among each other and with other MeGeO_3 compounds (Me = Mg, Co and Fe). In solid state sintering and tempering experiments it is shown that between 800 °C and 1050 °C only the orthorhombic form is obtained, while at T> 1050 °C also monoclinic MnGeO_3 is found, however, the total amount of the Cllc phase does not exceed 18(1) wt. %. During in situ X-ray diffraction measurements on a powdered sample from the flux growth experiment, a Cllc to Pbca phase transition takes place around 860 °C.