Zur Polymorphie von SrNi2P2 sowie zur Kristallstruktur von BaNi2P2

About Polymorphism of SrNi₂P₂ and Crystal Structure of BaNi₂P₂ SrNi₂P₂ and BaNi₂P₂ were prepared by heating mixtures of the elements and investigated by single crystal X-ray methods. The Sr compound at room temperature crystallizes in a superstructure of the ThCr₂Si₂ type (NT-phase; Immm; Z = 6; a = 3.951(2), b = 11.853(2), c = 10.432(2) Å), which is caused by displacements of the atoms from the ideal positions; the P? P distances are 2.45 and 3.28 Å. With increasing temperature at 45°C (ambient pressure) and increasing pressure at 4 kbar (room temperature) respectively the compound undergoes first order phase transitions and crystallizes after that in the undistorted ThCr₂Si₂ type (I4/mmm; Z = 2). While the P atoms of the high temperature phase (HT-SrNi₂P₂: a = 3.948(1), c = 10.677(3) Å; 100°C) are isolated from each other (d_{p p}: 3.12 Å) they most probably form pairs in the high pressure phase (HD-SrNi₂P₂: a = 4.003(1), c = 9.761(2) Å; ca. 4 kbar). This will be discussed on the basis of band structure calculations. BaNi₂P₂ (a = 3.947(1), c = 11.820(1) Å) also crystallizes in the ThCr₂Si₂ type structure, the P? P distance is extended to 3.71 Å.