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 α-NaCoPO₄ crystallizes in the space group Pnma with cell parameters: a = 8.871 (3), b = 6.780 (3), c = 5.023 (1) Å, and Z = 4 [wR(F²) = 0.0653 for all 945 independent reflections]. The α-phase contains octahedrally coordinated Co and Na atoms and tetrahedrally coordinated P atoms, and is isostructural with maracite, NaMnPO₄. The structure of high-temperature β-NaCoPO₄ is hexagonal with space group P6₅ and cell parameters: a = 10.166 (1), c = 23.881 (5) Å, and Z = 24 [wR(F²) = 0.0867 for 4343 unique reflections]. The β-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 × superstructure of the basic tridymite framework (c ≃ 8 Å) and is caused by the displacement of the Na atoms, tetrahedral tilts and strong distortions of the CoO₄ tetrahedra. A bond-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.

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