‘Pd20Sn13’ revisited: crystal structure of Pd6.69Sn4.31

The crystal structure of the title compound, previously reported as ‘Pd20Sn13’ on basis of powder X-ray data, was redetermined on basis of single-crystal X-ray data, resulting in a model with higher precision and accuracy.

The crystal structure of the title compound was previously reported with composition 'Pd 20 Sn 13 ' [Sarah et al. (1981). Z. Metallkd,72,[517][518][519][520]. For the original structure model, as determined from powder X-ray data, atomic coordinates from the isostructural compound Ni 13 Ga 3 Ge 6 were transferred. The present structure determination, resulting in a composition Pd 6.69 Sn 4.31 , is based on single crystal X-ray data and includes anisotropic displacement parameters for all atoms as well as standard uncertainties for the atomic coordinates, leading to higher precision and accuracy for the structure model. Single crystals of the title compound were obtained via a solid-state reaction route, starting from the elements. The crystal structure can be derived from the AlB 2 type of structure after removing one eighth of the atoms at the boron positions and shifting adjacent atoms in the same layer in the direction of the voids. One atomic site is partially occupied by both elements with a Pd:Sn ratio of 0.38 (3):0.62 (3). One Sn and three Pd atoms are located on special positions with site symmetry 2. (Wyckoff letter 3a and 3b).

Chemical context
In the context of investigations of the binary system Pd-Sn, Nowotny et al. (1946) observed a phase with approximate composition Pd 3 Sn 2 , which was later addressed as 'Pd 20 Sn 13 ' (Sarah et al., 1981). According to powder XRD measurements, this compound was found to be isotypical to Ni 13 Ga 3 Ge 6 (Nover & Schubert, 1981). Up to now, no further detailed structure examination has been published. In the course of our experiments, aiming at ternary Zintl phases containing tetrel elements (Hlukhyy et al., 2012), single crystals of the title compound have been obtained in significant amounts and were subjected to a closer structural investigation.

Structural commentary
The crystal structure of the title compound can be described as a defect variant of the AlB 2 structure type, where 1/8 of the boron atoms are missing. The symmetry reduction from P6/mmm to P3 2 21 with respect to AlB 2 results in 13 different crystallographic positions for the Pd and Sn atoms instead of only two, and a more complicated stacking of atomic planes including six differently packed layers for each of the former two, as shown in Fig. 1. The remaining atomic sites of the B atoms in AlB 2 are now substituted by seven independent atoms (Pd6, Pd7, Pd8, Sn2, Sn3, Sn4, and Sn5), the 'Al' layers are substituted alternatingly by Sn1, Pd3, Pd5, (layers 'Al1', 'Al3', 'Al5' in Fig. 1), and by Pd1, Pd2, and Pd4 ('Al2', 'Al4', 'Al6'), respectively.
The layered character of the Pd 6.69 Sn 4.31 structure is much less pronounced than in the parent AlB 2 type of structure, as indicated by the mixed substitution of both the Al and B sites ISSN 2056-9890 of the AlB 2 type by Pd as well as by Sn atoms, respectively. Accordingly, there are similar, in average slightly shorter interatomic distances within the planes (2.6407 (19) À 2.755 (2) Å ) than between them [2.7259 (18)-3.309 (2) Å ]. Nevertheless, the layers are clearly distinguishable and only marginally puckered. The distorted honeycomb lattice is obvious if the voids in the 'B' layer are considered (Fig. 2). The distortion results from a shift of the neighbouring Sn atoms within the boron layer (Sn2, Sn3 and Sn5) in the direction of the voids.
For Sn1 a partial occupation by Pd (Pd9) was found. A full occupation of the (Sn1/Pd9) site (Fig. 3a) by the element Sn would result in the composition Pd 13 Sn 9 as suggested by the isostructural compound Ni 13 Ga 3 Ge 6 . However, the occupancy of this position (in contrast to all other Pd and Sn sites) deviates significantly from 100% if only Sn (refined to 96%) or Pd (refined to 107%) is considered. It has to be noticed that this site is the only one in both kinds of 'Al' layers that is not close to a void in the 'B' layers ( Fig. 3). Consequently, the coordination number (CN) of the (Sn1/Pd9) site is 14, which is higher than that of all other Sn (CN = 10) and Pd atoms (CN = 11-13) in Pd 6.69 Sn 4.31 .
In the previous structure report of 'Pd 20 Sn 13 ' by Sarah et al. (1981), the atomic parameters were adopted from the isostructural compound Ni 13 Ga 3 Ge 6 , and the occupation of The crystal structure of Pd 6.69 Sn 4.31 , emphasizing the relationship to the AlB 2 structure type. The 'Al n' layers represent planes which are occupied by Al atoms in AlB 2 , the 'B n' layers those with B atoms, respectively. Anisotropic displacement ellipsoids are drawn at the 90% probability level.

Figure 2
The 'B1' layer (see Fig. 1) in Pd 6.69 Sn 4.31 . To illustrate the relationship to the AlB 2 structure type, the voids are drawn as empty squares and are connected to the neighbouring Sn atoms by dashed lines. Anisotropic displacement ellipsoids are drawn at the 90% probability level.

Synthesis and crystallization
Single crystals of the title compound were obtained from experiments aiming at a ternary alloy in the chemical system K-Pd-Sn, with similar conditions as reported by Hlukhyy et al. (2012). 23.4 mg K (99.9%, Riedel de Haë n), 71 mg Sn (99.999%, ChemPur), and 20.6 mg of PdSn, prefabricated by arc melting of the elements, were filled into a niobium crucible, which was sealed, placed in a silica glass tube, annealed under vacuum for 20 h at 1273 K and subsequently for 72 h at 873 K, and finally quenched with liquid nitrogen. As a by-product, K 4 Sn 4 (Hewaidy et al., 1964) was found.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. In contrast to the previously reported structure model, which was described in P3 1 21 (based on powder X-ray data; Sarah et al., 1981), the crystal under investigation adopts the inverted structure, as indicated by the refined Flack parameter (Flack, 1983;Parsons et al., 2013). Therefore space group P3 2 21 was chosen for the current refinement. It should be noted that the value and the corresponding standard uncertainty for the Flack parameter are rather high. However, the cause for this behaviour remains unclear. For the Sn1 site a partial occupation by Pd (Pd9) was found, with a refined occupation of 62 (3)% Sn and 38 (3)% Pd. All atoms were refined with anisotropic displacement parameters. The remaining maximum and minimum electron densities are located 2.08 Å from Sn2 and 0.46 Å from Pd8, respectively.  (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).