Complex pnictide-oxide structures have been investigated for nearly two decades [1], however they have garnered more interest lately because of the surprising discovery of high temperature superconductivity within their realm [2]. For example, Fe-based arsenic-oxides, such as SmFeAsO_∼0.85, have displayed superconductivity at temperatures as high as 55 K [3]. Over the past seven years, our research group has systematically explored the alkaline-earth metal and rare-earth metal antimonide systems. The resulting new compounds have often been Zintl phases [4-6] with complex crystal and electronic structures, and unusual properties [7-20]. Although oxide research is not our focus, it is sometimes the case that adventurous oxygen molecules find their way into our experiments and surprise us with an atypical compound, such as the previously reported Ba₅Cd₂Sb₅O_x (0.5 < x < 0.7) [17].

In a more recent experiment, serendipitous oxidation during the synthesis of a reaction containing Ba, Cd, and Sb occurred. Upon inspection, several atypical (for an intermetallic compound) crystals were identified among the reaction's main product, BaCdSb₂ [21]—these turned out to be crystals of the novel antimonide oxide, Ba₅Cd₂Sb₄O₂, which is the subject of this paper.

Ba₅Cd₂Sb₄O₂ crystallizes in the monoclinic space group C2/m (No. 12), and its crystal structure contains three unique barium positions, one unique cadmium position, two unique antimony positions, and one unique oxygen position in the asymmetric unit (Tables 1 and 2). The structure is best described as polyanionic [ Cd 2 Sb 4 ] ∞ 2 6 − layers made up of edge-shared CdSb₄ tetrahedra extending along the b axis, with O²⁻ anions and Ba²⁺ cations between them (Figure 1). The structure is similar to that of the previously reported Zintl phase Ba₃Cd₂Sb₄ (also with space group C2/m) [12]. In fact, their lattice parameters are quite similar—a = 17.247(7) Å; b = 4.9279(18) Å; c = 12.240(5) Å; β = 132.558(4)° for the former and a = 17.835(2) Å, b = 4.8675(5) Å, c = 7.6837(7) Å, and β = 112.214(1)° for the latter. Recognizing this relationship and noting the expansion in the c axis and β angle, we can consider the Ba₅Cd₂Sb₄O2 structure to be a 1:2 intergrowth of Ba₃Cd₂Sb₄ and BaO fragments, i.e., the formula can be broken down as follows: Ba 5 Cd 2 Sb 4 O 2 = Ba 3 Cd 2 Sb 4 + 2 × BaOA schematic representation of this idea, also showing how the parent Ba₃Cd₂Sb₄ structure distorts to accommodate the O²⁻ anions and the additional Ba²⁺ cations is shown in Figure 2.

The length of the Cd–Sb bonds in Ba₅Cd₂Sb₄O₂ range from 2.8190(13) Å to 2.9463(9) Å, with Sb–Cd–Sb angles varying from 101.76(3)° to 116.52(4)°. These distances and angles are very similar to those reported for Ba₃Cd₂Sb₄. More specifically, the length of the Cd–Sb bonds in Ba₃Cd₂Sb₄ range from 2.9125(4) Å to 2.9531(5) Å, with Sb–Cd–Sb angles ranging from 101.95(1)° to 113.36(2)°.

Just like in Ba₃Cd₂Sb₄, the CdSb₄ tetrahedra within the polyanionic [ Cd 2 Sb 4 ] ∞ 2 6 − layers of Ba₅Cd₂Sb₄O₂ are “bridged” through Sb–Sb bonds, which measure 2.8615(16) Å. Compared with Ba₃Cd₂Sb₄, where the Sb–Sb bond length is 2.8114(8) Å, one sees a slight elongation, consistent with the idea that the Cd₂Sb₄ layers in the parent structure have been “stretched” in order to accommodate the Ba-O slabs. We also draw attention to the fact that in both Ba₅Cd₂Sb₄O₂ and Ba₃Cd₂Sb₄, the Sb–Sb distance is shorter than the 2.908 Å distance found in elemental Sb [22], indicating a strong covalent character for that bonding interaction.

The Ba–O fragments do not resemble the actual structure of BaO (NaCl type). The “BaO” layers in Ba₅Cd₂Sb₄O₂ are composed of Ba2 and O atom positions (Table 2), and are inserted into the Ba₃Cd₂Sb₄ structure template between the existing Ba1 and Ba3 atom positions. In this case, the neighboring Ba atoms form a distorted, square pyramidal, five-coordinate bonding environment around the O, with the Ba–O bond lengths on the order of 2.565(5) Å to 3.040(6) Å. These distances are comparable to the Ba–O bond lengths of 2.628(6) Å to 2.766(4) Å found in a similar compound, Ba₅Cd₂Sb₅O_x (0.5 < x < 0.7) [17]. As seen in Figure 3, the O²⁻ ions in Ba₅Cd₂Sb₅O_x (0.5 < x < 0.7) exist in a tetrahedral environment, where the Ba₄O-tetrahedra share corners to form an infinite chain. In Ba₅Cd₂Sb₄O₂, however, the O²⁻ ions are found in a five-coordinate environment of Ba²⁺ cations. The shape of such a coordination polyhedron resembles a distorted square pyramid; the latter share edges and corners to form an infinite puckered ribbon.

Ba₅Cd₂Sb₄O₂ is an electron precise, salt-like compound, where the electron count conforms to the Zintl-Klemm rules [4,5]. In addition to the notion of it being a “double-salt”, as described above, the formula can be rationalized as: 5 × [ Ba 2 + ] + 2 × [ Cd 2 + ] + 2 × [ Sb 2 − ] + 2 × [ Sb 3 − ] + 2 × [ O 2 − ] = 0where the two different types of antimony atoms are the ones that are connected through a homoatomic Sb–Sb bond, and the ones that are only bound to Cd atoms, respectively. This simplistic structure rationalization is nicely supported by the electronic structure calculations, which show a band gap of about 0.3 eV at the Fermi level, indicative of a closed-shell system. As shown in Figure 4, just below the Fermi level, the main contributors to the DOS are the Sb p bands, admixed with states originated from Cd and Ba orbitals. The oxygen p bands are filled and situated ca. 2 eV below the top of the valence band, corroborating the assignment of the oxygen atoms as oxo-anions.

Based on electron counting and DOS calculations, and in an agreement with the dark-colored and brittle habits of the Ba₅Cd₂Sb₄O₂ crystals, it is likely that this new antimonide oxide will exhibit semiconducting behavior. Regrettably, resistivity measurements were impeded because of the material's extreme sensitivity to air, and we were unable to experimentally confirm this.

Handling of all materials was performed inside an argon-filled glove-box with oxygen and moisture levels below 0.5 ppm, or under vacuum. Ba, Cd, Sb, and Pb were purchased from Alfa or Aldrich with stated purity greater than 99.9% and were used as received. Single crystals of Ba₅Cd₂Sb₄O₂ were synthesized using the metal flux method. Ba, Cd, and Sb were measured in the elemental ratio of 6:4:7 and combined in an alumina crucible with an excess of Pb (ca. 10×, based on molar Ba content) and sealed in an evacuated fused silica jacket. The ampoule was heated in a programmable furnace from 100 °C to 1000 °C at a rate of 100 °C/h, allowed to homogenize at this temperature for 29 h, then cooled to 850 °C at a rate of −1.5 °C/h, and then cooled to 600 °C at a rate of −10 °C/h. At 600 °C, the ampoule was removed from the furnace, inverted, and spun in a centrifuge for 30 seconds to separate the Pb from the crystals. The ampoule was opened inside the argon-filled glove-box for inspection.

From this reaction, the majority phase was identified as BaCdSb₂, however several plank-like single crystals of Ba₅Cd₂Sb₄O₂ were identified and selected for single-crystal X-ray diffraction. Crystals of Ba₅Cd₂Sb₄O₂ removed from the glove-box and exposed to the air and moisture in the laboratory decomposed and became tarnished, pockmarked, and brittle over 36 h. Because of the air-sensitivity of these crystals, no property measurements were attempted.

Single-crystal X-ray diffraction was performed on a single crystal of Ba₅Cd₂Sb₄O₂ selected inside the argon-filled glove-box and placed in Paratone-N oil. This crystal was then quickly removed from the glove-box and cut to suitable dimensions (less than 0.1 mm) under the microscope and placed on the tip of a glass fiber on the goniometer of a Bruker SMART CCD-based diffractometer [23]. The crystal was checked for quality, and full spheres of diffraction data were collected in four batch runs. Although the crystal was air-sensitive, the Paratone-N oil coating the surface of the crystal and the cold stream of nitrogen gas (ca. 200(2) K) protected it, and no sign of deterioration during the data collection was observed. SAINTPLUS was used for reduction and integration of the raw data [24]. Semi-empirical absorption correction based on equivalents (i.e, multi-scan) was applied with the SADABS software [25]. The SHELXTL software package was used for structure solution and refinement [26]. Direct methods found the position of all seven unique atoms in the C2/m space group. The atomic coordinates were standardized using STRUCTURE TIDY, and anisotropic displacement parameters were included in the final least squares refinements and resulted in a featureless difference Fourier map [27]. Selected details of the data collections and structure refinement parameters are summarized in Table 1. The refined atomic coordinates and equivalent isotropic displacement parameters are given in Table 2.

Additional details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247808-666, E-Mail: crysdata@fiz-karlsruhe.de) on quoting the depository numbers CSD-423458.

TB-LMTO-ASA electronic structure calculations for Ba₅Cd₂Sb₄O₂ were performed using the LMTO-4.7 package [28]. This program is based on the tight-binding linear muffin-tin orbital (LMTO) method in the local density (LDA) and atomic sphere (ASA) approximations [29-32]. Atomic sphere radii used in the calculations were chosen using an automated procedure, and interstitial spheres were inserted to achieve the space filling automatically [33]. The Fermi level was selected as the energy reference (ε_F = 0 eV).

Ba₅Cd₂Sb₄O₂ crystallizes in the monoclinic space group C2/m and its crystal structure can be derived from that of a similar compound, Ba₃Cd₂Sb₄, which has been distorted and augmented. The Cd–Sb, Sb–Sb, and Ba–O bond lengths and Sb–Cd–Sb bond angles agree with those of similar compounds. The frailty of Ba₅Cd₂Sb₄O₂ limits its utility to that of a laboratory curiosity, however there are undoubtedly more robust, and more interesting pnictide-oxide compounds waiting to be synthesized.