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The antiperovskite crystal structure is similar to the perovskite structure that is common in nature. The key difference is that the positions of the cation and anion constituents are reversed in the unit cell structure. As opposed to perovskite, antiperovskite consists of two anions coordinated with a cation that bond with both. Antiperovskite compounds are an important class of materials since synthesized antiperovskites demonstrate interesting and useful physical properties. ==Structure== The crystal structure of antiperovskite parallels the perovskite structure, with the exception of switching anion and cation positions. The typical perovskite structure is ABX3, where A is a monovalent cation, and B is a divalent metallic cation and X is a monovalent anion. For antiperovskite, the ABX3 formula is reversed. The X position is an electropositive cation (such as an alkali metal), while the A is a monovalent anion and the B is a divalent anion. In the idealized cubic cell, the A anion is at the center of the cube, the B anion at the octahedral corners, and the X cation is at the faces of the cube. Thus the A anion has a coordination number of 12, while the B anion sits in the center of an octahedron with a coordination number of 6. While the crystal structure can be idealized as a cube, antiperovskite compounds are known to have orthorhombic and tetragonal phases. Whether a compound will form an antiperovskite structure depends not only on chemical formula, but also the relative sizes of ionic radii of the constituents. Antiperovskite structures will form only when the constituent ions radii meet the Goldschmidt tolerance factor. The equation for the radii of the A, B, and X, ions (ra, rb & rx) is: Tolerance factor = For the antiperovskite structure to be structurally stable, the tolerance factor calculated must be between .71 and 1. If between .71 and .9, the crystal will be orthorhombic or tetragonal. If between .9 and 1, it will be cubic. By mixing the B anions with another element of the same valence but different size, the tolerance factor can be altered. Different combinations of elements dictates which phase it will be in and the thermodynamic stability of the crystal. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Antiperovskite (structure)」の詳細全文を読む スポンサード リンク
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