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Abstract:
Single crystals as well as microcrystalline powders of K3NiO2 were obtained via the azide/nitrate route, starting from stoichiometric mixtures of KN3, KNO3 and NiO, at 923 K. According to temperature dependent X-ray investigations, K3NiO2 exhibits a phase transition at approx. 423 K. Single crystal X-ray analysis at 500 K has shown that the high temperature modification (beta-K3NiO2, tP12) crystallizes in P42/mnm (Z = 2, a = 6.0310(9), c = 7.156(1) angstrom, R1 = 0.037, R2 = 0.105). The ambient temperature modification (a-K3NiO2, tP24) was refined as a racemic twin (P41212/P43212; a = 6.012(4), c = 13.843(8) angstrom, R1 = 0.029, R2 = 0.070 at 100 K; a = 6.0300(9), c = 14.065(3) angstrom, R1 = 0.032, R2 = 0.082 at 298 K) yielding nearly equal volumes for both enantiomorphs. The structural relationship within the A3MX2 family is analyzed and displayed as a Barnighausen tree. The essential feature of the low and high temperature phases are isolated NiO23dumbbells, which are linked by potassium atoms to infinite chains.
