Ca4As3 – a new binary calcium arsenide

The binary compound Ca4As3crystallizes in the Ba4P3 structure type and is thus a homologue of isotypic Sr4As3. The As atoms are connected by a single bond thus this calcium arsenide is a Zintl phase.


Chemical context
Six binary compounds have been reported so far in the binary phase diagram Ca-As: CaAs 3 (Brice et al., 1976), Ca 2 As 3 (Deller & Eisenmann, 1976), CaAs (Iandelli & Franceschi, 1973), Ca 16 As 11 (Leon-Escamilla et al., 1997), Ca 5 As 3 (Hü tz & Nagorsen, 1975) and Ca 2 As (Hü tz & Nagorsen, 1974). In the binary phase system, the following trend is observed: with increasing As-content the number of covalent As-As bonds increases. Ca 2 As and Ca 5 As 3 are reported as intermetallic phases. In the Ca-richest compound Ca 2 As, the Ca atoms in the first coordination sphere of As adopt a monocapped square-antiprismatic geometry (CN = 9), while the Ca atoms are situated inside cuboctahedra (eight Ca and four As atoms) or 13-vertex polyhedra (eight Ca and five As atoms). In Ca 5 As 3 , nine Ca atoms form deformed monocapped square antiprisms around As while the coordination polyhedra of Ca atoms are formed by bicapped-hexagonal antiprismatic (eight Ca and six As atoms) and 15-vertex polyhedra (ten Ca and five As atoms). No covalent bonds are found in either compound. The other four Ca-As compounds are Zintl phases containing polyanionic As-substructures. The polyarsenic substructure varies with the atomic percentage of Ca. Compounds with 50-59.3 at.% of Ca (CaAs, Ca 16 As 11 and the title compound Ca 4 As 3 ) all contain [As 2 ] 4dumbbells as a structure motif. Ca 2 As 3 contains two types of As chains: [As 4 ] 6and [As 8 ] 10-. The structure of the CaAs 3 compound with the highest As content contains a two-dimensional [As 3 ] 2network as a polyarsenic substructure besides the three bonded [As] 0 and two bonded [As] 1atoms in a ratio of 1:2.

Structural commentary
The unit cell of the title compounds is shown in Fig. 1. The phase Ca 4 As 3 (Z = 8) with 57 at.% Ca fulfils the 8-N rule according to a salt-like compound: the charge of 32 Ca 2+ cations are counterbalanced by 16 isolated As 3anions and four [As 2 ] 4dumbbells (two As2-As2 dumbbells and two As5-As5 dumbbells) per unit cell. The dumbbells formed by the As2 anions, with an interatomic distance of of 2.507 (2) Å , lie in the ab plane. The second type of dumbbells of the As5 ISSN 2056-9890 anions, with d(As5-As5) of 2.527 (2) Å , lie along the c-axis direction, thus the two dumbbells are oriented perpendicular with respect to each other. Both As-As distances are in the range of covalent single bonds observed in elemental As and other binary Ca-As compounds (2.44-2.57 Å ). Each As atom of the dumbbells is coordinated by eight Ca cations. Six Ca cations form a distorted trigonal prism while two Ca cations cap two of the rectangular faces of the prism; the third rectangular prism face is capped by the covalently bonded As atom (Fig. 2b,d). The two trigonal prisms around As2 or As5 share their tetragonal faces, each with the As dumbbell in the center of the eight-vertex polyhedron of Ca atoms. Two of the As 3anions (As1 and As3) that are not bonded to further As atoms are coordinated by Ca atoms in form of distorted trigonal prisms (Fig. 2a,c). For As1, two faces of the prism are capped while for As3, three faces are capped with Ca atoms. The trigonal-prismatic coordination polyhedra of As1 and As3 are connected by sharing edges. In contrast to the other As atoms, As4 possesses a different coordination sphere having also the highest coordination number (CN = 10) of Ca atoms, forming a polyhedron with 14 faces (Fig. 2e). The coordination sphere can be described as an icosahedron with two removed adjacent vertices.
The coordination around the Ca cations is formed by six or seven As atoms and eight to ten Ca atoms (Fig. 3). Distorted octahedra are formed by six As atoms around Ca1, Ca4, Ca5 and Ca6. For Ca4 and Ca5, one edge is formed by an As5 dumbbell. The faces of the octahedra are capped by Ca atoms. In most cases d(Ca-Ca) is longer than 3.5 Å ; however, a rather short distance of 3.289 (2) Å is observed between Ca4 and Ca5. Those two Ca atoms are coordinated by the As5 dumbbells (Fig. 3d,e). The distorted As octahedra around Ca4 and Ca5 share a common face (As1-As1-As4) in the ab plane. Ca2 and Ca3 are surrounded by seven As atoms (Fig. 3b,c). In both cases, the coordination polyhedron resembles a distorted pentagonal bipyramid. For Ca2, one edge of the pentagon is an As2 dumbbell. Each of the trigonal faces is capped by Ca atoms.

Comparison with isostructural compounds
Comparison of Ca 4 As 3 with the isostructural Sr 4 As 3 (Somer et al., 1995) and Ba 4 P 3 (Hadenfeldt et al., 1993) show that the lattice parameters increase in accordance with the cation size. The distances in the As-As dumbbells for Sr 4 As 3 are 2.52 and 2.55 Å , which is slightly longer than observed in dumbbells of Ca 4 As 3 [2.507 (2) Å and 2.527 (2) Å , respectively]. The lattice parameters for Ba 4 P 3 are further increased due to the larger Ba atoms. However, the distances in the dumbbells [d(P-P) of 2.25 and 2.32 Å ] are shorter than in the As compounds due to the smaller covalent radius of P.

Figure 2
The coordination polyhedra of As atoms. The Ca atoms are shown in gray and the As atoms in magenta as anisotropic displacement ellipsoids with a 90% probability level. The As-As dumbbell bonds are emphasized in magenta.

Figure 1
Crystal structure of Ca 4 As 3 shown along the a axis. The Ca atoms are shown in gray and the As atoms in magenta as anisotropic displacement ellipsoids with a 90% probability level. The As-As dumbbell bonds are shown in magenta.

Synthesis and crystallization
Single crystals of the title compound were obtained from experiments aiming at an alloy with the nominal composition of 12Ca:10Fe:10As:4Rh:8Si. A mixture of Ca (2.35 mmol), Fe (1.96 mmol), As (1.96 mmol) and pre-prepared 'Rh:2Si' precursor (0.78 mmol) was placed in an alumina crucible which was sealed in a tantalum ampoule under an argon atmosphere. The ampoule was heated in a resistances furnace to 1373 K and held for 24 h. Afterwards, the temperature was reduced to 1248 K at a rate of 0.1 K min À1 and held there for a week. Single crystals of the title compound could be isolated from the product. Energy-dispersive X-ray analysis (EDX) of the crystals showed an atomic ratio of Ca/As close to 4:3 in all analysed crystals. No impurity elements heavier than sodium were observed. The binary Ca 4 As 3 phase was subsequently synthesized from the pure elements.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All atoms were refined with anisotropic displacement parameters. The remaining maximum and minimum electron densities are located 1.29 Å from As2 and 0.03 Å from As5, respectively. The coordination polyhedra of Ca atoms. The Ca atoms are shown in gray and the As atoms in magenta as anisotropic displacement ellipsoids with a 90% probability level. The As-As dumbbells are emphasized in magenta.  (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2012).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.