Synthesis of New Mixed-Metal Ammonium Vanadates: Cation Order versus Disorder, and Optical and Photocatalytic Properties

Two new ammonium vanadate hydrates, i.e., M3(H2O)2V8O24·2NH4 (M = Mn and Co, I and II, respectively) were synthesized using hydrothermal reaction conditions, and their structures were determined by single crystal X-ray diffraction [I: P2/m (No. 10), Z = 1, a = 8.2011(2) A, b = 3.5207(1) A, c = 9.9129(3) A, β = 110.987(2)°; II: C2/m (No. 12), Z = 2, a = 19.4594(6) A, b = 6.7554(2) A, c = 8.4747(3) A, β = 112.098(2)°]. Interestingly, the two structures are homeotypic, with the structure of I exhibiting an uncommon type of structural disorder between locally-bridging Mn(H2O)22+ (i.e., part of the oxide framework) and nonbridging NH4+ cations over the same site (1:2 ratio), wherein two NH4+ ions occupy the same site as the two H2O molecules when Mn(II) is vacant. The amount of Mn(II) in the formula of I was determined by a combination of techniques, including electron paramagnetic resonance, while the relative amounts of NH4+/H2O in its structure were determined by combined thermogravimetric-mass spectrometry...