BANI2P4 - DIMORPHISM BY PEIERLS INSTABILI TY

BaNi2P4 was prepared by heating a mixture of the elements and investig ated by means of single crystal X-ray methods. At T greater than or eq ual to 100 degrees C the compound crystallizes in the tetragonal BaPd2 P4-type structure (alpha phase: I4/mmm; a = 6.553(1) Angstrom, c = 5.7 69(1) Angstrom; Z = 2). Chains of edge-shared NiP4 tetrahedra are orie ntated to each other, so that the P atoms form P-4 rings (P-P distance : 2.23 Angstrom). These are connected with the Ni atoms to Ni8P16 cage s in the form of compressed truncated octahedra with Ba atoms in the c entres. Below 100 degrees C alpha-BaNi2P4 gradually undergoes a phase transition and forms an orthorhombic variant of the structure (beta-ph ase; Immm; a = 6.620(1) Angstrom, b = 6.470(1) Angstrom, c = 5.785(1) Angstrom; Z = 2). In the course of this the Ni-Ni distances of alpha-B aNi2P4 alternately split up into shorter and longer ones. Extended Huc kel calculations show, that the structure of beta-BaNi2P4 is stabilize d by a Peierls distortion of the chains of tetrahedra.