Syntheses, Structures, Anomalous Phase Transition and Optical Properties of Two New Polymorphic α- and β-LiMoPO6
New polymorphous phases, α- and β-LiMoPO6 have been successfully synthesized by high temperature solution technique and their structures are determined by the single crystal X-ray diffraction. α-LiMoPO6 crystallizes in orthorhombic space group Pbcn (No.60) with a = 12.1141(4) Å, b = 8.9251(3) Å and c = 8.7389(3) Å, while β-LiMoPO6 crystallizes in the monoclinic space group P21/c (No.14) with a = 6.3441(3) Å, b = 7.5949(3) Å, c = 9.3430(4) Å and β = 94.546(4)°. Both structures exhibit a similar three-dimensional (3D) framework composed of MoO6 octahedra and PO4 tetrahedra. Interestingly, an anomalous phase transition, i.e. the high-temperature β-LiMoPO6 phase exhibits larger density than low-temperature α-LiMoPO6 has been observed. The detailed structural analyses show that the anomalous phase transition mainly originates from the ‘straightened’ effect of the zigzag [MoO3O3PO]∞ chains in their structures. Further, the optical and thermal properties of two compounds are also reported.