Crystal structure, infrared spectroscopy and thermodynamic properties of a manganese member of the ellenbergerite family

Single-crystals of manganese hydroxide hydrogen vanadate divanadate, Mn6.75(OH)3H1.20(VO4)4-2z(V2O7)z (z = 0.15), have been synthesized hydrothermally. Its hexagonal crystal structure, sp. gr. P63mc, related to mineral ellenbergerite, has been determined by single-crystal X-ray diffraction. It features a three-dimensional framework of vertex- and face-sharing Mn2+-centered octahedra with defective Mn2+-octahedral chains and disordered V2O7, HVO4 and H2VO4 groups located in hexagonal and triangular type channels. The presence of these groups has been confirmed by Fourier transform infrared spectroscopy. According to magnetization and specific heat data, Mn6.75(OH)3H1.20(VO4)4-2z(V2O7)z (z = 0.15) experiences ordering into a canted antiferromagnetic state at TN = 42.5 K. We report on the structural features of Mn6.75[H1.20(VO4)]4-2z(V2O7)z (z = 0.15), which clarified the mechanism of negative charge compensation of the framework. The compound undergoes phase transition at TN = 42.5 K into canted AF state.