Dibarium tricadmium bismuthide(-I,-III) oxide, Ba2Cd3−δBi3O

Ba2Cd2.13Bi3O, a new bismuthide(-I,-III) oxide, crystallizes with a novel body-centered tetragonal structure (Pearson code tI36). The crystal structure contains eight crystallographically unique sites in the asymmetric unit, all on special positions. Two Ba, one Cd and two Bi atoms have site symmetry 4mm, the third Bi atom has mmm. and the O atom has m2 symmetry; the second Cd site (2mm. symmetry) is not fully occupied. The layered structure is complex and can be considered as an intergrowth of two types of slabs, viz. BaCdBiO with the ZrCuSiAs type and BaCd2Bi2 with the CeMg2Si2 type.

bismuthide(-I,-III) oxide Ba 2 Cd 2.13 Bi 3 O. It crystallizes in space group I4/mmm in what appears to be a structure with a previously unreported structure type.
The crystal structure of the title compound is shown schematically in Figure 1. In this representation, the layered nature of the structure and the basic building blocks are emphasized. As seen from the plot, it can be readily described as consisting of PbO-type layers of fused [CdBi 4 ] tetrahedra, running parallel to the ab plane and which are alternately stacked along the c axis with BaO slabs and Bi square-nets ( Figure 1). The actual structure is more complicated due to the partially occupied Cd2 site. The Cd2 atoms cap the Bi square-nets from above and below and link these fragments to the CdBi slabs. Figure   2 shows a representation with anisotropic displacement ellipsoids.
Being a new structure type, it is important to relate the structure of the title compound to the structure(s) of previously reported phases with known structure types (Villars & Calvert, 1991). A good starting point for a discussion is BaCdBi 2 (Brechtel et al., 1981), reported with the ZrAl 3 type (Villars & Calvert, 1991). Coincidentally, BaCdBi 2 also crystallizes in space group I4/mmm and with cell parameters a = 4.77 Å and c = 23.6 Å. This structure features the very same PbO-type CdBi layers, stacked along the c-axis in alternating order with Bi square-nets. Not considering the partially occupied Cd2 site (for simplicity), one can then immediately reason that replacing every other BaBi slab in BaCdBi 2 with a BaO slab will yield a hypothetical Ba 2 Cd 2 Bi 3 O compound. The latter can be considered as a super-structure of BaCdBi 2 with doubled periodicity along the stacking detection, i.e., the c axis. Another way to relate the structure under consideration to other structure types is to consider the Cd2 site fully occupied and rationalize the structure of such an ordered Ba 2 Cd 3 Bi 3 O compound as an intergrowth of two types of slabs -BaCdBiO with the ZrCuSiAs type and BaCd 2 Bi 2 with the CeMg 2 Si 2 type, respectively. This line of thinking is schematically illustrated in Figure 1.

supplementary materials sup-2 Experimental
Handling of the reagents was done in an argon-filled glove box or under vacuum. All metals were with a stated purity higher than 99.9% (metal basis). They were purchased from Alfa, kept in a glove box, and were used as received.
The flux reaction was carried out in a 2 cm 3 alumina crucible, using a mixture of elemental Ba and Cd in a molar ratio 3 : 2 and ca 2.1 grams of Bi. The reaction was aimed at growing crystals of Ba 3 Cd 2 Bi 4 , a hitherto unknown phase with the Ba 3 Cd 2 Sb 4 structure (Saparov et al., 2008), using excess of bismuth as a metal flux. The crucible was subsequently enclosed and flame-sealed in an evacuated fused silica ampoule, and then was heated at 200Kh -1 to 973 K, homogenized at 973 K for 20 h, cooled at a rate of -5Kh -1 to 723 K, where the excess Bi was removed by decanting it, leaving behind some irregularly shaped silver pieces and a few dark-to-black plates. The former were confirmed (via single-crystal and powder X-ray diffraction) to be Ba 2 Cd 3 Bi 4 (Xia & Bobev, 2006b) and the latter turned out to be the title compound.
After the structure of the new compound was solved from single-crystal X-ray diffraction data, it was realized that an unadventurous exposure of the starting materials to air has led to the formation of Ba 2 Cd 2.13 Bi 3 O (minor product), alongside the intermetallic phase (major product). Subsequent attempts to produce Ba 2 Cd 2.13 Bi 3 O in quantitative yields from reactions of Ba, Cd, Bi and BaO 2 (Acros, 95%) were not successful, suggesting it might be a metastable phase.

Refinement
The observed reflections satisfied the systematic extinction conditions for a body-centered cell, and the centrosymmetric space group I4/mmm (No. 139) was chosen based on intensity statistics. The structure was successfully solved by direct methods, which located six atomic positions -the two alkaline-earth metals, the three Bi atoms and one Cd atom. Subsequent structure refinements by full matrix least-squares methods on F 2 showed the location of the oxygen atom in a tetrahedral void of Ba atoms with Ba-O distances of 2.6736 (14) Å. The difference Fourier map, however, also showed a residual peak of about 15 e -Å -3 , located ca. 2.7 Å away from Bi. At first, we attempted to refine this as oxygen, however, there were serious problems with this model: 1) the electron density was much higher than a fully occupied O 2-; 2) such coordination is inconsistent with the bonding requirements of oxygen; 3) the electron count was clearly implausible, viz. (Ba 2+ ) 2 (Cd 2+ ) 2 (Bi 3-) 2 (Bi 1-)(O 2-) 2 . Here, the polyanionic networks features bismuth in two different coordination modes, which require different formal charges. The Bi atoms in the square-net are hypervalent, thus formally Bi 1-, as analyzed computationally elsewhere (Papoian & Hoffmann, 2000). Therefore, this additional site was modeled as a partially occupied Cd atom (Cd2). The formal electron count taking into account the ca. 1/8 occupied Cd2 site is then (Ba 2+ ) 2 (Cd 2+ ) 2.13 (Bi 3-) 2 (Bi 1-)(O 2-), rendering this model much more reasonable (despite the shortcoming of the shorter Cd2-Bi distances, vide supra) The occupancy of Cd2 was fixed at 12.5%. After including the partially occupied Cd2 site, the refinement converged at low residuals, accompanied with a flat final difference Fourier map -the maximum residual electron density lies 0.74 Å from Bi1, and the minimum residual electron density lies 2.33 Å from O.
In the final refinement cycles, all atoms were refined with anisotropic displacement parameters and with coordinates standardized using the software STRUCTURE-TIDY (Gelato & Parthε, 1987).

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 R-supplementary materials sup-5 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.