Crystal structure of the thortveitite-related M phase, (MnZn)VO (0.75 < x < 0.913): a combined synchrotron powder and single-crystal X-ray study.

The determination of the crystal structure of the M phase, (MnZn)VO (0.75 < x < 0.913), in the pseudobinary MnVO-ZnVO system for x ≃ 0.8 shows that the previously published triclinic unit-cell parameters for this thortveitite-related phase do not describe a true lattice for this phase. Instead, single-crystal X-ray data and Rietveld refinement of synchrotron X-ray powder data show that the M phase has a different triclinic structure in the space group P-1 with Z = 2. As prior work has suggested, the crystal structure can be described as a distorted version of the thortveitite crystal structure of β-MnVO. A twofold superstructure in diffraction patterns of crystals of the M phase used for single-crystal X-ray diffraction work arises from twinning by reticular pseudomerohedry. This superstructure can be described as a commensurate modulation of a pseudo-monoclinic basis structure closely related to the crystal structure of β-MnVO. In comparison with the distortions introduced when β-MnVO transforms at low temperature to α-MnVO, the distortions which give rise to the M phase from the β-MnVO prototype are noticeably less pronounced.