The two-dimensional thiophosphate CsCrP2S7

The quaternary title compound, caesium chromium(III) heptathiodiphosphate(V), CsCrP2S7, has been synthesized using the reactive halide flux method. It is isotypic with other AMP2S7 (A = alkali metal; M = Cr, V or In) structures and consists of two-dimensional ∞ 2[CrP2S7]− layers extending parallel to (001) which are separated from each other by Cs+ ions (symmetry 2). The layer is built up from slightly distorted octahedral [CrS6] units (symmetry 2) and bent [P2S7] units consisting of two corner-sharing [PS4] tetrahedra. The [CrS6] octahedra share two edges and two corners with the [PS4] tetrahedra. There are only van der Waals interactions present between the layers. The Cs+ ions are located in this van der Waals gap and stabilize the structure through weak ionic interactions. The classical charge balance of the title compound can be expressed as [Cs+][Cr3+][P5+]2[S2−]7.

The quaternary title compound, caesium chromium(III) heptathiodiphosphate(V), CsCrP 2 S 7 , has been synthesized using the reactive halide flux method. It is isotypic with other AMP 2 S 7 (A = alkali metal; M = Cr, V or In) structures and consists of two-dimensional 1 2 [CrP 2 S 7 ] À layers extending parallel to (001) which are separated from each other by Cs + ions (symmetry 2). The layer is built up from slightly distorted octahedral [CrS 6 ] units (symmetry 2) and bent [P 2 S 7 ] units consisting of two corner-sharing [PS 4 ] tetrahedra. The [CrS 6 ] octahedra share two edges and two corners with the [PS 4 ] tetrahedra. There are only van der Waals interactions present between the layers. The Cs + ions are located in this van der Waals gap and stabilize the structure through weak ionic interactions.

Comment
During an attempt to prepare new chromium thiophosphates with the use of halide fluxes, a new compound was isolated.
Here we report the synthesis and structure of the new layered quaternary thiophosphate, CsCrP 2 S 7 .
The title compound is a new member of the previously reported isotypic AMP 2 S 7 family (A = alkali metal; M = Cr, V, or In) (Kopnin et al., 2000;Durand et al., 1993;Gutzmann et al., 2005, Sekizawa et al. , 2004 (Derstroff et al., 2002), each Cr atom is surrounded by six S atoms in a (slightly distorted) octahedral arrangement. In the title compound they share two edges and two corners with the [PS 4 ] tetrahedra to form the two-dimensional infinite layer extending parallel to (001) (Fig. 1). There are only van der Waals interactions between the layers and the Cs + ions in this van der Waals gap stabilize the structure through weak ionic interactions (Fig. 2).
While both the [CrS 6 ] octahedron and the [PS 4 ] tetrahedron show angular distortions, the Cr-S and P-S distances are rather regular and in good agreement with those found in other related phases (e.g. Coste et al., 2001). Atom S4 is bridging two P atoms in the [P 2 S 7 4-] units. The bridging P-S4 bond is longer than those of the terminal bonds, a characteristic feature for two condensed PS 4 tetrahedra (Toffoli et al., 1982) or PO 4 tetrahedra (Wang et al., 1989).
The Cs + ion is surrounded by twelve S atoms if an arbitrarily choosen cut-off of 4.2 Å for the Cs-S bonding interactions is used. The anharmonic behavior of the alkali metal ion, as observed in the isotypic K or Rb analogues (Kopnin et al., 2000;Durand et al., 1993), is not found here. The harmonic behavior of the Cs + ion in the title compound could be due to the larger ionic radius and hence to a larger coordination number (Gutzmann et al., 2005)

Experimental
The title compound, CsCrP 2 S 7 , was prepared by the reaction of the elemental Cr, P and S with the use of the reactive alkali metal halide flux technique. A combination of the pure elements, Cr powder (CERAC 99.95%), P powder (CERAC 99.5%) and S powder (Aldrich 99.999%) were mixed in a fused silica tube in a molar ratio of Cr: P: S = 1: 2: 6 with CsCl/LiCl. The mass ratio of the reactants and the alkali halide flux was 1: 3. The tube was evacuated to 0.133 Pa, sealed and heated gradually (50 K/h) to 923 K, where it was kept for 72 h. The tube was cooled to 473 K at 3 K/h and then was quenched to room temperature. The excess halides were removed with distilled water and dark brown needle shaped crystals were obtained. The crystals are stable in air and water. Semi-qualitative analysis of the crystals with XRF indicated the presence of Cs, Cr, P, and S. No other element was detected.