Crystal Structures of New Pyrovanadates A2MnV2O7 (A = Rb, K)

The new compounds A₂MnV₂O₇ (A = K, Rb) with structures related to the melilite-type have been synthesized by a solid state reaction route. The crystal structures of K₂MnV₂O₇, Rb₂MnV₂O₇ and KRbMnV₂O₇ have been determined using X-ray single crystal diffraction data. The compound K₂MnV₂O₇ crystallizes with a melilite-type structure with tetragonal unit cell parameters a = 8.609(3), c = 5.538(4) A and space group P42₁m. The structures of Rb₂MnV₂O₇ and KRbMnV₂O₇ are derived from the melilite-type structure with space group P42/mnm and unit cell parameters a = 8.577(6), c = 11.809(6) A , and a = 8.530(6), c = 11.466(5) A , respectively. The three structures consist of [MnV₂O₇]²− layers perpendicular to the c axis separated by A⁺ layers. The [MnV₂O₇]²− layers feature corner-sharing MnO₄ tetrahedra and V₂O₇ pyrovanadate units, the linkage leading to rings of five tetrahedra. The doubling of the c parameter for Rb₂MnV₂O₇ or RbKMnV₂O₇ is explained by the existence of a mirror plane perpendicular to the [001] direction between two [MnV₂O₇]²− layers. The A⁺ alkali cations occupy distorted square antiprisms of oxygen atoms in K₂MnV₂O₇ and distorted square prisms of oxygen atoms in Rb₂MnV₂O₇and RbKMnV₂O₇.