Dimorphic ThNi2P2 with BaCu2S2 and CaBe2Ge2 type structure

Two modifications of ThNi2P2 were prepared in a tin flux at 850 C (α-ThNi2P2) and 1000 C (β-ThNi2P2). The crystal structures of both modifications were refined from single-crystal X-ray data. α-ThNi2P2 (BaCu2S2 type structure): Pnma, a=819.69(5), b=394.28(3), c=981.54(7) pm, R=0.028 for 32 variables and 654 structure factors; β-ThNi2P2 (CaBe2Ge2 type structure): P4/nmm, a=408.5(1), c=908.0(3) pm, R=0.033 for 15 variable parameters and 261 F values. Although the two structures are closely related, they can be transformed into each other only by a reconstructive phase transformation. The differences and similarities of the two structures are discussed. The high temperature form has higher symmetry, a smaller number of variable positional parameters, and a tendency for higher coordination numbers. (orig.)