**THE
PHOSPHIDOSILICATE-POLYPHOSPHIDES LaSiP**_{3}**
AND Th**_{2}**SiP**_{5}

__Birgit Fehrmann__**
and Wolfgang Jeitschko**

*Anorganisch-Chemisches
Institut, Universität Münster, Wilhelm-Klemm-Straße 8,* *D-48149 Münster, Germany
E-mail: *

**Keywords: **Crystal Structure,
Phosphidosilicates, Polyphosphides, Twinning

The preparation of LaSiP_{3} has been
reported by Hayakawa, Sekine and Ono [1]. The crystal structure,
however, has not been determined at that time. Th_{2}SiP_{5}
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 LaSiP_{3} 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}(SiP_{2})^{-2}P^{-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 LaSiP_{3} and Th_{2}SiP_{5}.

(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 (SiP_{2})^{-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 LaSiP_{2.91(5)}.

The structure of Th_{2}SiP_{5}
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 SiP_{4}
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 SiP_{4} tetrahedra are linked via corners, thus
forming chains, which can be represented by the formula (SiP_{3}).
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.

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