Refinement of the crystal structure of dipalladium gallium, Pd2Ga

Abstract

GaPd2, orthorhombic, Pnma (no. 62), a = 5.4829(8) A, b = 4.0560(4) A, c = 7.7863(8) A, V = 173.2 A, Z = 4, Rgt(F) = 0.022, wRref(F) = 0.029, T = 295 K. Source of material Starting from powder of Pd (99.9 %, Chempur) and granules of Ga (99.9999%, Chempur) in a ratio of 2:1 and 1.5 mg/ml GaI3 as transport agent (99.999%, Chempur), needle-shaped single crystals of Pd2Ga were synthesized by an exothermal chemical transport reaction in a temperature gradient from 673 K (source) to 873 K (sink) [1]. Experimental details Lattice parameters of the title compound were determined by least-squares fitting of 24 reflections from powder X-ray diffraction data obtained from ground single crystals (Huber Image Plate Guinier camera G670, CuK41 radiation, , = 1.54056 A, LaB6 as internal standard, a = 4.15692 A). Due to the significant homogeneity range reported for Pd2+xGa1–x [2], the occupancy values together with the anisotropic displacement parameters were allowed to vary in separate series of the structure refinement while the overall scale factor was fixed. The resulting occupancies were equal to unity within one e.s.d. 0.998(4), 1.003(4) and 1.006(5) for Pd1, Pd2 and Ga sites, respectively, confirming the 2:1 composition of the investigated specimen. In the final refinement series full occupancies were assumed for all positions. Discussion The crystal structure of Pd2Ga adopts an atomic arrangement of theCo2Si type of structure [3]. The present investigation provides a more precise refinement of the atomic as well as the displacement parameters of the previously reportedmodel obtained on the basis of photographic single crystal data [4] and X-ray powder diffraction [2]. The title compound represents the stoichiometric (x = 0) composition of Pd2+xGa1–x and thus falls into the reported homogeneity range of –0.04 ( x ( 0.02 at 873 K [2,4]. All atoms in the crystal structure of Pd2Ga are situated on one of the two mirror planes perpendicular to [010]. A clear gap at !3.00 A separates the first coordination sphere of each atom in the structure of Pd2Ga. Each Pd1 atom has distorted tetrahedral coordination by four Ga atoms with distances varying between 2.54 A and 2.56 A. One additional Ga atom is quite far away at a distance of 2.96 A. The environment of Pd2 is different: nonplanar trigonal coordination by three Ga atomswith d(Pd—Ga) = 2.56A – 2.62A and two other Ga atoms at a considerably longer distance of 2.84 A. The closest Pd—Ga contacts are comparable with the sum of the single bond radii of Pd (1.28 A) and Ga (1.25 A) [5]. The coordination sphere of each palladium site is completed by eight Pd atomswith d(Pd—Pd) between 2.82A and 2.99A, thus increasing the coordination number of both positions to 13. These contacts are slightly longer than the interatomic distance of 2.75A in ccpPdmetal [6]. Gallium atoms are surrounded exclusively by ten palladium species. The shortest Ga···Ga distance of 3.43 A is significantly longer than the average interatomic distance of 2.70 A in the 4-modification of Ga [6]. The environment of gallium atoms is formed by seven Pd atoms in the range of 2.54 A – 2.62 A and three Pd atoms at distances 2.84 A (2×) and 2.96 A (1×). Z. Kristallogr. NCS 223 (2008) 7-8 / DOI 10.1524/ncrs.2008.0004 7 © by Oldenbourg Wissenschaftsverlag, Munchen