Undecaeuropium hexazinc dodecaarsenide

The title compound, Eu11Zn6As12, crystallizes with the Sr11Cd6Sb12 structure type (Pearson’s symbol mC58). The complex monoclinic structure of the first arsenide to form with this type features chains made of corner-sharing ZnAs4 tetrahedra, separated by Eu atoms. There are a total of 15 unique positions in the asymmetric unit. Except for one Eu atom with site symmetry 2/m, all atoms are located on mirror planes. An usual aspect of the structure are some Zn—As distances, which are much longer than the sum of the covalent radii, indicating weaker interactions.


Related literature
The growing interest in ternary pnictides of alkaline-and rareearth metals with group 12 metals has feen fueled by the recent discovery of superconductivity (Rotter et al., 2008). Such compounds have also been investigated because of their promising behaviour as materials with high thermoelectric conversion efficiency (Snyder & Toberer, 2008). Our own exploratory studies revealed a wealth of new compounds with diverse crystal structures, including Ca 2 CdSb 2 and Yb 2 CdSb 2 (Xia & Bobev, 2007a), A 9 Cd 4+x Pn 9 and A 9 Zn 4+x Pn 9 (A = Ca, Sr, Eu, Yb; Pn = Sb, Bi) (Xia & Bobev, 2007b), A 11 Cd 6 Sb 12 (A = Sr, Ba, Eu) and Eu 11 Zn 6 Sb 12 (Park & Kim, 2004;Xia & Bobev, 2008b;Saparov et al., 2008a), A 21 Cd 4 Pn 18 (A = Sr, Ba, Eu; Pn = Sb, Bi) (Xia & Bobev, 2008a), Ba 3 Cd 2 Sb 4 (Saparov et al., 2008b), Ba 2 Cd 2 Pn 3 (Pn = As, Sb) (Saparov et al., 2010). The title compound is the As-analog of Eu 11 Zn 6 Sb 12 (Saparov et al., 2008a). For covalent radii, see: Pauling (1960).  electron bond. Analogously longer than normal Cd3-Sb5 and Zn3-Sb5 distances have been reported in Eu 11 Cd 6 Sb 12 and Eu 11 Zn 6 Sb 12 (Saparov et al., 2008a). We refer to the theoretical studies on Sr 11 Cd 6 Sb 12 and Ba 11 Cd 6 Sb 12 (Xia & Bobev, 2008b) for a more detailed discussion of the bonding interactions in this structure type. d-metal centered tetrahedra of the pnicogen elements are recurring motifs in the structural chemistry of such solid-state compounds, as evidenced by a number of reports (Rotter et al., 2008;Snyder & Toberer, 2008;Xia & Bobev, 2008a;Saparov et al., 2008b;Saparov et al., 2010). Sr 11 Cd 6 Sb 12 , the first structurally characterized phase with this monoclinic structure, was synthesized from a high temperature reaction of elements using Sn as metal flux (Park & Kim, 2004). In this report, the crystal structure was described as being composed of "double pentagonal tubes". A slightly different description of the structure was given in the light of the very long Cd3-Sb5 bond in Ba 11 Cd 6 Sb 12 (Xia & Bobev, 2008b). Therein, the authors performed comprehensive electronic structure calculations aimed at full understanding of the bonding in Sr 11 Cd 6 Sb 12 and Ba 11 Cd 6 Sb 12 . From these computational results, and from earlier results pertaining to related materials such as Yb 2 CdSb 2 (Xia & Bobev, 2007a), A 9 Cd 4+x Pn 9 and A 9 Zn 4+x Pn 9 (A=Ca, Sr, Eu, Yb; Pn= Sb, Bi) (Xia & Bobev, 2007b), it can be expected that the Eu cations in Eu 11 Zn 6 As 12 will be divalent, and the spins of the Eu's 7 unpaired electrons may couple magnetically at low temperatures. We were unable to experimentally confirm this conjecture because the title compound was not isolated as a pure phase, but magnetic property measurements on the isotypic europium antimonides Eu 11 Cd 6 Sb 12 and Eu 11 Zn 6 Sb 12 (Saparov et al., 2008a) confirmed Eu 2+ cations (4f 7 state). These measurements also suggested antiferromagnetic ordering in Eu 11 Cd 6 Sb 12 below T N =7.5 K. The temperature dependent electrical resistivity measurements carried out on a single crystal of Eu 11 Cd 6 Sb 12 suggested poorly metallic behavior, as expected from band structure calculations performed for Sr 11 Cd 6 Sb 12 and Ba 11 Cd 6 Sb 12 (Xia & Bobev, 2008b).

Experimental
The starting materials, Eu, Zn, As, and Pb, with stated purity greater than 99.9%, were purchased from Alfa or Aldrich, and used as received. Elements were loaded into an alumina crucible in a Eu:Zn:As:Pb=2:1:2:10 molar ratio inside an argon-filled glove-box. The alumina crucible was then sealed under vacuum in a silica tube. The reaction mixture was heated supplementary materials sup-2 to 1223 K, kept at this temperature for 20 hours, and then slowly cooled to 723 K at a rate of 3 K/hour. Finally, the Pb-flux was removed by centrifugation at this temperature. Together with irregular-shaped crystals with hitherto unknown structure, black needle shaped crystals of Eu 11 Zn 6 As 12 were also obtained.

Refinement
The collected data were successfully refined using the coordinates of Eu 11 Zn 6 Sb 12 (Saparov et al., 2008a) as a starting model. The maximum peak and deepest hole are located 0.97 Å away from Eu6 and 1.47 Å away from Zn1, respectively. Fig. 1. A plot of the Eu 11 Zn 6 As 12 structure viewed down the b-axis. Displacement ellipsoids are drawn at the 95% probability level. Color key: Eu -red, Zn -green, All As atoms, excluding As5 -blue. As5, which form dimers are shown in yellow. The long As5-Zn3 bonds are depicted as thiner solid lines. The unit cell is outlined.

Special details
Experimental. Selected in the glove box, crystals were put in a Paratone N oil and cut to the desired dimensions. The chosen crystal was mounted on a tip of a glass fiber and quickly transferred onto the goniometer. The crystal was kept under a cold nitrogen stream to protect from the ambient air and moisture.
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.
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 Rfactors(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.