Calcium platinum aluminium, CaPtAl

A preliminary X-ray study of CaPtAl has been reported previously by Hulliger [J. Alloys Compd (1993), 196, 225–228] based on X-ray powder diffraction data without structure refinement. With the present single-crystal X-ray study, we confirm the assignment of the TiNiSi type for CaPtAl, in a fully ordered inverse structure. All three atoms of the asymmetric unit have .m. site symmetry. The structure features a ∞ 3[AlPt] open framework with a fourfold coordination of Pt by Al atoms and vice versa. The Ca atoms are located in the large channels of the structure.

A preliminary X-ray study of CaPtAl has been reported previously by Hulliger [J. Alloys Compd (1993), 196, 225-228] based on X-ray powder diffraction data without structure refinement. With the present single-crystal X-ray study, we confirm the assignment of the TiNiSi type for CaPtAl, in a fully ordered inverse structure. All three atoms of the asymmetric unit have .m. site symmetry. The structure features a 1

3
[AlPt] open framework with a fourfold coordination of Pt by Al atoms and vice versa. The Ca atoms are located in the large channels of the structure.

Related literature
For a previous X-ray powder diffraction study of CaPtAl, see: Hulliger (1993). For related compounds, see: Dascoulidou-Gritner & Schuster (1994); Merlo et al. (1996). For structural systematics and properties of the TiNiSi structure type, see: Kussmann et al. (1998) a tripling of the a-axis of the of the TiNiSi-type basic cell (i 3 , minimal isomorphic subgroup of index 3) and a complete ordering of the atomic sites (Ponou & Lidin, 2008;Ponou, 2010). This phase has previously been reported by Merlo et al. (1996) in the KHg 2 structure type with Ag and Ge atoms randomly distributed on the Hg site. Hence, precice single crystals X-ray diffraction measurements is of importance in this family of compounds (Kussmann et al., 1998;Banenzoué et al., 2009) to access both the superstructure and a possible structure inversion with respect to the Ni and Si positions in the three-dimensional framework of fourfold interconnected atoms. Such a structure inversion is observed, for example, in CaGaPt, with Ga atoms at the orginal Ni position and Pt at the original Si position with respect to the prototype TiNiSi structure type (Dascoulidou-Gritner & Schuster, 1994). In 'normal' TiNiSi phases like CaPtGe, Ge occupies the Si position and Pt the Ni position (Evers et al., 1992;Nuspl et al., 1996). Here we report the structure refinement of CaPtAl from single-crystal X-ray diffraction data.
The structure of the title compound was first investigated by Hulliger (1993) from X-ray powder data and assigned to the orthorhombic TiNiSi structure type, however, without a detailed structural refinement. In the present study, the CaPtAl structure was successfully refined as an inverse TiNiSi type. Hence, Ca, Al and Pt atoms are found at Ti, Ni and Si atomic positions, respectively. The origin of the inversion in this structure type is generally ascribed to the relative electronegativity of the framework constituent elements, here Pt and Al. The more electronegative atom (here Pt) is found at the Si position in a strongly distorted tetrahedral (rather pyramidal) coordination whereas the coordination of the less electronegative atom at the Ni position is only slightly distorted from its idealized tetrahedral values (Hoffmann & Pöttgen, 2001). The Pt-Al interactomic distances range from 2.574 (3) Å to 2.675 (3) Å which is comparable with the sum of atomic radii of these elements, i.e. 2.62 Å (Al: 1.25 and Pt: 1.37 Å; Pauling, 1960), indicating a covalent character of the bonding. The first coordination sphere of the Ca atoms consists of two counter-tilted Pt 3 Al 3 hexagons with Ca-Al and Ca-Pt distances ranging from 3.143 (3) Å to 3.489 (3) Å and from 2.978 (2) Å to 3.1179 (16) Å, respectively, also in agreement with the sum of atomic radii (Ca: 1.97 Å).

Experimental
Single crystals of the title compound, suitable for X-ray diffraction studies, were obtained from a mixture of the elements (Ca ingots (99.5%), Al chunk (99.9%) and Pt pieces (99%), all from ABCR) with a molar ratio Ca:Pt:Al = 2:2:1, by heating in a Nb ampoule at 1253 K for one hour and then slowly cooling at a rate of 6 K/h to room temperature. The product is air-stable, silver-grey with metallic lustre.

Refinement
The refined unit cell parameters are quite close (slighly lower) to those obtained from a Guinier camera by Hulliger (1993) with a = 7.1722 (6), b = 4.2885 (4) and c = 7.7760 (7) Å. The refinement in the inverse TiNiSi structure model was straightforward. Full/mixed occupancies were checked for all atomic sites by freeing the site occupation factor for each given individual atom, while keeping that of the other atoms fixed. The residual map shows highest peak/deepest hole of 2.61/-2.95 e Å -2 at 0.75/0.60 Å from Pt1, respectively. Fig. 1. A perspective view of the CaPtAl structure with displacement ellipsoids drawn at the 99% probability level. Ca, Al, and Pt atoms are drawn as white-crossed, grey and black spheres, respectively.   (4)