Structural chemistry of NaCoPO4

Sodium cobalt phosphate, NaCoPO4, occurs as two different polymorphs which transform reversibly at 998 K. The crystal structures of both polymorphs have been determined by single crystal X-ray diffraction. The low-temperature form alpha-NaCoPO4 crystallizes in the space group Pnma with cell parameters: a = 8.871(3), b = 6.780 (3), c = 5.023 (1) Angstrom and Z = 4 [wR(F-2) = 0.0653 for all 945 independent reflections]. The alpha-phase contains octahedrally coordinated Co and Na atoms and tetrahedrally coordinated P atoms, and is isostructural with maracite, NaMnPO4. The structure of high-temperature beta-NaCoPO4 is hexagonal with space group P6(5) and cell parameters: a = 10.166(1), c = 23.881 (5) Angstrom and Z = 24 [wR(F-2) = 0.0867 for 4343 unique reflections]. The beta-phase belongs to the large family of stuffed tridymites, with the P and Co atoms occupying tetrahedral sites and the Na atoms located in the cavities of the tetrahedral framework. The long c axis corresponds to a 3 x superstructure of the basic tridymite framework (c similar or equal to 8 Angstrom) and is caused by the displacement of the Na atoms, tetrahedral tilts and strong distortions of the CoO4 tetrahedra. A bend-valence analysis of these phases reveals that the polymorphism in NaCoPO4 is due in part to over-/underbonding of the Na atom in the low/high-temperature structures, respectively.