Birgit Fehrmann and Wolfgang Jeitschko

Anorganisch-Chemisches Institut, Universität Münster, Wilhelm-Klemm-Straße 8, D-48149 Münster, Germany

Keywords: Crystal Structure, Phosphidosilicates, Polyphosphides, Twinning

The preparation of LaSiP3 has been reported by Hayakawa, Sekine and Ono [1]. The crystal structure, however, has not been determined at that time. Th2SiP5 has first been prepared by Hofmann [2]. We have determined the crystal structures of both compounds from four-circle X-ray diffracto-meter data of twinned crystals. The crystal structure of LaSiP3 was first solved for an F centered orthorhombic cell, space group Fmmm, a = 582.3 pm, b = 597.0 pm and c = 2525.1 pm. With the refinement in this space group most light-atom positions were found with fractional occupancy. A refinement with a monoclinic cell (Cm, a = 597.2(1) pm, b = 2525.5(7) pm, c = 416.8(1) pm, b= 135.71(2)°, Z = 4, R = 0.059 for 1469 structure factors and 30 variable parameters), accounting for twinning, resulted in a structure, where all atomic positions are nearly fully occupied. The structure can be described to a first approximation by the formula La+3(SiP2)-2P-1, where silicon and phosphorus atoms alternate, thus forming puckered hexagonal rings, which are condensed to two-dimensionally infinite nets

Fig. 1. The crystal structures of LaSiP3 and Th2SiP5.

(Fig. 1). In addition to the three phosphorus neighbours within one net, each silicon atom has a terminal fourth phosphorus neighbour, resulting in the formula (SiP2)-2. The remaining phosphorus atoms form chains. However, these phosphorus positions are not fully occupied. Nevertheless, because the P-P distances in these chains (240 and 245 pm) are somewhat greater than the typical two-electron bond distances, these phosphorus atoms can still be assigned a formal charge of -1. The exact formula of the compound is LaSiP2.91(5).

The structure of Th2SiP5 is pseudo-tetragonal. It was solved first from a twinned crystal assuming monoclinic symmetry (Cm, a = 574.4 pm, b = 569.4 pm, c = 1028.1 pm, b= 90.22°). However, in this refinement the silicon atoms within the linked SiP4 tetrahedra have occupancy parameters of one half. A fully ordered structure was refined assuming twinning of a triclinic cell: P1, a = 404.3(1) pm, b = 404.5(1) pm and c = 1027.9(2) pm, a = 90.09(2)°, b = 90.09(2)° and g = 89.50(2)°, Z = 1, R = 0.047 for 1276 structure factors and 38 variable parameters. In this structure the SiP4 tetrahedra are linked via corners, thus forming chains, which can be represented by the formula (SiP3). The remaining phosphorus atoms form a polyphosphidic layer in the form of a distorted square grid with one short P-P distance of 235 pm, which is emphasized in Fig. 1.


  1. H. Hayakawa, T. Sekine and S. Ono, J. Less- Common Met. 41 (1975) 197-210.
  2. W. K. Hofmann, Diplomarbeit, Universität Gießen (1979).