Synthesis and characterization of new fluoride-containing manganese vanadates A₂Mn₂V₂O₇F₂ (A=Rb, Cs) and Mn₂VO₄F

Large single crystals of A₂Mn₂V₂O₇F₂ (A=Rb, Cs) and Mn₂VO₄F were grown using a high-temperature (~600 °C) hydrothermal technique. Single crystal X-ray diffraction and powder X-ray diffraction were utilized to characterize the structures, which both possess MnO₄F₂ building blocks. The A₂Mn₂V₂O₇F₂ series crystallizes as a new structure type in space group Pbcn (No. 60), Z=4 (Rb₂Mn₂V₂O₇F₂: a=7.4389(17) Å, b=11.574(3) Å, c=10.914(2) Å; Cs₂Mn₂V₂O₇F₂: a=7.5615(15) Å, b=11.745(2) Å, c=11.127(2) Å). The structure is composed of zigzag chains of edge-sharing MnO₄F₂ units running along the a-axis, and interconnected through V₂O₇ pyrovanadate groups. Temperature dependent magnetic susceptibility measurements on this interesting one-dimensional structural feature based on Mn²⁺ indicated that Cs₂Mn₂V₂O₇F₂ is antiferromagnetic with a Neél temperature, T_N=~3 K and a Weiss constant, θ, of -11.7(1) K. Raman and infrared spectra were also analyzed to identify the fundamental V-O vibrational modes in Cs₂Mn₂V₂O₇F₂. Mn₂(VO₄)F crystalizes in the monoclinic space group of C2/c (no. 15), Z=8 with unit cell parameters of a=13.559(2) Å, b=6.8036(7) Å, c=10.1408(13) Å and β=116.16(3)°. The structure is associated with those of triplite and wagnerite. Dynamic fluorine disorder gives rise to complex alternating chains of five-and six-coordinate Mn²⁺. These interpenetrating chains are additionally connected through isolated VO₄ tetrahedra to form the condensed structure.