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 r oom 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)Angs trom), which is caused by displacements of the atoms from the ideal po sitions; the P-P distances are 2.45 and 3.28 Angstrom. With increasing temperature at 45 degrees C (ambient pressure) and increasing pressur e at 4 kbar (room temperature) respectively the compound undergoes fir st order phase transitions and crystallizes after that in the undistor ted ThCr2Si2 type (I4/mmm; Z = 2). While the P atoms of the high tempe rature phase (HT-SrNi2P2: a = 3.948(1), c = 10.677(3) Angstrom; 100 de grees C) are isolated from each other (d(P-P): 3.12 Angstrom) they mos t probably form pairs in the high pressure phase (HD-SrNi2P2: a = 4.00 3(1), c = 9.761(2)Angstrom; ca. 4 kbar). This will be discussed on the basis of band structure calculations. BaNi2P2 (a = 3.947(1), c = 11.8 20(1)Angstrom) also crystallizes in the ThCr2Si2 type structure, the P -P distance is extended to 3.71 Angstrom.