Structural chemistry of NaCoPO4

被引:51
作者
Hammond, R
Barbier, J
机构
[1] Department of Chemistry, McMaster University, Hamilton, Ont. L8S 4M1
来源
ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE | 1996年 / 52卷
关键词
D O I
10.1107/S0108768195016259
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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.
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页码:440 / 449
页数:10
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