Supporting information: this article has supporting information at journals.iucr.org/c Crystal structure of the thortveitite-related M phase, (Mn x Zn 1-x ) 2 V 2 O 7 (0.75 < x < 0.913): a combined synchrotron powder and single-crystal X-ray studyThe determination of the crystal structure of the M phase, (Mn x Zn 1-x ) 2 V 2 O 7 (0.75 < x < 0.913), in the pseudobinary Mn 2 V 2 O 7 -Zn 2 V 2 O 7 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 P1 with Z = 2. As prior work has suggested, the crystal structure can be described as a distorted version of the thortveitite crystal structure of -Mn 2 V 2 O 7 . 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 -Mn 2 V 2 O 7 . In comparison with the distortions introduced when -Mn 2 V 2 O 7 transforms at low temperature to -Mn 2 V 2 O 7 , the distortions which give rise to the M phase from the -Mn 2 V 2 O 7 prototype are noticeably less pronounced. research papers 1080 Knowles et al. Crystal structure of a thortveitite-related M phase Knowles et al. Crystal structure of a thortveitite-related M phase 1087 supporting information sup-1