High-pressure synthesis and crystal structure of SrGa4As4

SrGa4As4 was synthesized under high-pressure/high-temperature conditions. The crystal structure comprises a network of GaIIIAs4 tetrahedra and ethane-like GaII 2As6 groups with Ga—Ga bonds.


Chemical context
The ternary systems A-Tr-As (A = Ca, Sr or Ba; Tr = Ga or In) contain numerous compounds with different crystal structures based on TrAs 4 tetrahedra which occur isolated (Kauzlarich & Kuromoto, 1991), as dimers, as chains (Stoyko et al., 2015;He et al., 2012), condensed to ethane-like Tr 2 As 6 groups (Mathieu et al., 2008;Goforth et al., 2009;He et al., 2011) or as large supertetrahedral units (Weippert et al., 2019). SrGa 4 As 4 is the first high-pressure compound in this system and contains an unprecedented layer-like framework, thus expanding the structural variety of the A-Tr-As family.

Structural commentary
SrGa 4 As 4 crystallizes in the space group P3 2 21 (No. 154) and constitutes a new structure type. Strontium is coordinated in a quadratic antiprismatic manner by eight As atoms (Fig. 1). The antiprisms are slightly distorted, with their quadratic planes twisted by $34 relative to each other instead of 45 for an ideal quadratic antiprism. Sr-As distances range from 3.2665 (4) to 3.4560 (4) Å . The SrAs 8 polyhedra are connected through common corners, each As atom shared by two quadratic antiprisms, building up a three-dimensional (3D) framework. A similar structural motif is known for RbAg 2 SbS 4 , which crystallizes in the space group P3 1 21 (Schimek et al., 1996). The surrounding construct in the two crystal structures differs however. SrGa 4 As 4 contains a 3D Ga/ As framework that can be subdivided into two types of layers with an AB stacking sequence along the c axis. The first type is built up from corner-and edge-sharing GaAs 4 tetrahedra forming sheets with triangular voids (Fig. 2). The tetrahedra are distorted, with angles in the range of 100.790 (19)-127.996 (19) , and have typical Ga-As distances of 2.4384 (5)-2.5470 (5) Å . The second layer type consists of distorted ethane-like Ga 2 As 6 groups with nearly eclipsed conformations. The Ga 2 As 6 groups are connected via common corners, forming a honeycomb-like sheet (Fig. 3). The Ga1A and Ga1B positions of the Ga-Ga dumbbell are disordered and were treated with split positions having an occupancy of 50% each (Fig. 4). The coordination of each of these Ga sites consists of three As atoms and one Ga atom forming trigonal pyramids, showing torsion angles of 114.5 (1) for As1 vi -Ga1A-Ga1A i -As1 iv and 119.3 (1) for As2 v -Ga1B-Ga1B i -As2 vii (for symmetry codes, see Fig. 4). The Ga-Ga distances range between 2.542 (8) and 2.572 (8) Å and are considered as Ga-Ga bonds, which is consistent with a charge-neutral compound. Ga-As distances between 2.477 (4) and 2.694 (2) Å for Ga1A are near to the covalent radii sum of 2.46 Å (Pauling, 1960). In comparison, the trigonal pyramid around Ga1B is elongated, with Ga-As distances of 2.415 (4)-2.845 (2) Å .

Synthesis and crystallization
The starting material SrAs was synthesized by heating stoichiometric amounts of Sr (Sigma-Aldrich, 99.95%) and As (Alfa Aesar, 99.99999+%) in alumina crucibles, sealed in silica ampules under an atmosphere of purified argon for 20 h at 1223 K. The title compound was obtained via high-pressure synthesis using a modified Walker-type multianvil set-up driven by a 1000 t hydraulic press (Voggenreiter, Mainleus, Germany). Corner-sharing Ga 2 As 6 dumbbells with disordered Ga positions forming a honeycomb-like layer viewed along [001].

Figure 4
Ga 2 As 6 groups with disordered Ga positions having an occupancy of 50%. Displacement ellipsoids are drawn at the 95% probability level.

Figure 1
The unit cell of SrGa 4 As 4 , viewed along [110], with the quadratic antiprismatic strontium coordination spheres shown as red polyhedra.

Figure 2
Edge-and corner-sharing GaAs 4 tetrahedra forming a layer with triangular voids viewed along [001].
rials SrAs (73.4 mg, 0.452 mmol), Ga (66.5 mg, 0.953 mmol, Alfa Aesar, 99.999%) and As (60.1 mg, 0.802 mmol) were mixed in a glove-box (H 2 O, O 2 <1 ppm) and filled into the octahedron assembly. The reaction was carried out at 8 GPa and 1573 K, with a dwell time of 3 h. The temperature was increased and decreased over a period of 1 h. The assembly was opened in a glove-box, revealing crystals with a metallic luster. The composition of SrGa 4 As 4 was verified by EDX measurements using a a Carl Zeiss EVO-MA 10 instrument with a Bruker Nano EDX detector. The experimental values [Sr 12 (1) at%, Ga 44 (2) at% and As 45 (1) at%] are in excellent agreement with the expected values (Sr 11.1 at%, Ga 44.4 at% and As 44.4 at%) within the typical error of the method, and confirm the composition obtained from singlecrystal X-ray diffraction data.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The Ga1A and Ga1B positions were introduced as half-occupied split positions since one fully occupied position with a prolate ellipsoid caused residual densities in the order of 2.2 e Å À3 . Upon exclusion of the Ga1A/Ga1B positions, the contour difference map in PLATON (Spek, 2009) shows two clearly separated maxima justifying this approach. Structural data were standardized with STRUCTURE-TIDY (Gelato & Parthé, 1987).  program(s) used to solve structure: SUPERFLIP (Palatinus & Chapuis, 2007) and EDMA (Palatinus et al., 2012); program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2014); software used to prepare material for publication: PLATON (Spek, 2009).

Strontium tetragallate(II,III) tetraarsenide
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.