Crystal Structures and Lattice Distortions ofSIGMA-Type Layered Vanadium Bronzes:SIGMA-M0.25V2O5·H2O (M=Mg, Co, Ni)

New-type layered vanadium bronzesM0.25V2O5·H2O were hydrothermally synthesized for divalent metals ofM=Mg, Co, Ni in addition to the first-reportedM=Zn compound. The-type structures, consisting of V2O5layers with three octahedral and one tetrahedral V atoms and interlayerM(H2O)4rectangles, were confirmed by single-crystal X-ray diffractometry. The Ni compound crystallizes in the sameP phase as the Zn compound while the Mg and Co compounds crystallize in the newP21/aphase of higher symmetry. TheP21/aphases is an undistorted form of theP phases, where the distortion is caused by the locations of interlayer water molecules. Among four V atoms one octahedral atom is tetravalent and others are pentavalent, being consistent with the average V4.75+valence. TheM(H2O)4is held by two apical oxygens of VO4tetrahedra to form a regularMO6octahedron and thusM2+ions must fit for the octahedral oxygen coordination.