K 3 NiO 2 Revisited, Phase Transition and Crystal Structure Refinement

Single crystals as well as microcrystalline powders of K 3 NiO 2 were obtained via the azide/nitrate route, starting from stoichiometric mixtures of KN 3 , KNO 3 and NiO, at 923 K. According to temperature dependent X‐ray investigations, K 3 NiO 2 exhibits a phase transition at approx. 423 K. Single crystal X‐ray analysis at 500 K has shown that the high temperature modification ( β ‐K 3 NiO 2 , tP 12) crystallizes in P 4 2 / mnm ( Z = 2, a = 6.0310(9), c = 7.156(1) Å, R 1 = 0.037, R 2 = 0.105). The ambient temperature modification ( α ‐K 3 NiO 2 , tP 24) was refined as a racemic twin ( P 4 1 2 1 2/ P 4 3 2 1 2; a = 6.012(4), c = 13.843(8) Å, R 1 = 0.029, R 2 = 0.070 at 100 K; a = 6.0300(9), c = 14.065(3) Å, R 1 = 0.032, R 2 = 0.082 at 298 K) yielding nearly equal volumes for both enantiomorphs. The structural relationship within the A 3 MX 2 family is analyzed and displayed as a Bärnighausen tree. The essential feature of the low and high temperature phases are isolated NiO 2 3– dumbbells, which are linked by potassium atoms to infinite chains.