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Introduction

In the last decade, there have been numerous reports on intermetallic compounds of the pnicogen elements with the alkali or alkaline-earth metals and many of the 3d-metals. More recently, the interest in such materials has further increased, largely due to the unexpected discovery of superconductivity in potassium-doped (Ba_1-xK_x)Fe₂As₂, RbFe₂As₂ and CsFe₂As₂, LiFeAs, LiCu₂P₂, etc. The appeal of many antimonides for use in thermoelectrics, such as (Ca_1-xYb_x)Zn₂Sb₂, Yb₁₄MnSb₁₁, SrZn₂Sb₂, EuZn₂Sb₂, BaZn₂Sb₂, among others, has also contributed to the growing attention in this sub-field of chemistry.

The past research efforts in our group have been closely aligned with research on the crystal and electronic structures of new Zintl phases with complex structures, both from a fundamental and practical standpoint. Inspired by their promise for solid-state energy conversion, we have already reported on more than a dozen novel ternary pnictides with the d⁵- and d¹⁰-metals, whose bonding characteristics could be advantageous in the efforts to optimize the thermoelectric efficiency. Most of the previous work has been focused on A-Cd-Pn phases, where A = Ca, Sr, Ba, Eu, Yb; and Pn = As, Sb, Bi. Extending our research into the zinc-pnictides, we first undertook systematic syntheses in the Ba-Zn-Pn ternary systems, which seem sporadically and insufficiently mapped out. So far, only the following four phases have been synthesized and structurally characterized—BaZn₂P₂ (ThCr₂Si₂ type), the dimorphic BaZn₂As₂ (BaCu₂S₂ and ThCr₂Si₂ type), BaZn₂Sb₂ (BaCu₂S₂ type), BaZnSb₂ and BaZnBi₂ (ZrAl₃ type). Herein we report the synthesis and the crystal structures of three new Zintl phases Ba₂ZnAs₂, Ba₂ZnSb₂ and Ba₂ZnBi₂, effectively doubling the number of ternary compounds in each of the corresponding phase diagrams.

Results

Because the title compounds are isotypic and isoelectronic, for the sake of simplicity, the focus of the structural discussions will be primarily on one of them, Ba₂ZnSb₂. In the following two paragraphs, we will briefly describe the important characteristics of its structure. We will also depict some useful structural relationships and examine in greater detail the chemical bonding.

Structure. Ba₂ZnSb₂ crystallizes in the centrosymmetric space group Ibam (Figure 1) with three crystallographically unique atoms in the asymmetric unit, all in special positions.

Figure 1. (a) Combined ball-and-stick and polyhedral view of the Ba₂ZnSb₂ structure, projected down the c-axis. The representation emphasizes the isolated [ZnSb₂] polyanionic chains, "solvated" by the cations. The orthorhombic unit cell is outlined. (b) A close-up view of a segment of the [ZnSb₂] chains, depicting the edge-shared ZnSb₄ tetrahedra. The Ba cations, which cap the triangular faces in m₃-fashion are also shown. Color code: Zn – yellow; Sb – red; Ba – blue, respectively.

Formally, the structure belongs to the known K₂SiP₂ type (Pearson's code oI20), where Ba takes the K site, and the Zn atoms are at the Si site. Ba₂ZnSb₂ is also isostructural and isoelectronic to the ternary chalcogenides A₂MnQ₂ and A₂CoQ₂ (A = K, Rb, Cs; Q = S, Se, Te). Noteworthy, all previously known compounds with this structure contain alkali metals as cations, while Ba₂ZnPn₂ (Pn = As, Sb, Bi) appear to be the first phases formed with alkaline-earth metals cations; they are also the first antimonides and bismuthides in this series.

Following the differences in electronegativity between the constituting elements, the Ba₂ZnSb₂ structure (Figure 1a) can be "broken down" to Ba²⁺ cations and isolated polyanionic chains, [ZnSb_4/2]^4–, running parallel to the c-axis. They are made of edge-shared ZnSb₄ tetrahedra (Figure 1b), and are isosteric to the [SiS_4/2] chains in the binary SiS₂. The chains are indeed well-separated from one another with closest contacts between adjacent Sb-vertices on the order of 5.3  (center-to-center separation is ca. 7.9 ). Within the ZnSb₄ tetrahedra, Zn–Sb interactions appear to be typical 2-center 2-electron bonds with distances that measure 2.7744(3) . These values compare well with those reported for other intermetallic compounds with similar bonding patterns: d_Zn-Sb = 2.675-2.924  in Ca₉Zn_4.5Sb₉, d_Zn-Sb = 2.725  in Yb₁₄ZnSb₁₁, d_Zn-Sb = 2.660-2.810  in BaZn₂Sb₂, d_Zn-Sb = 2.722  in NaZnSb, d_Zn-Sb = 2.710-2.727  in REZn_1–xSb₂ (RE = La-Tb), etc. The tetrahedral angles deviate slightly from the ideal 109.5°, but nothing out of the ordinary. Similar comparisons can be made between the Zn–As (Zn–Bi) distances and angles in Ba₂ZnAs₂ (Ba₂ZnBi₂) with the metrics of other published structures.

Ba-coordination is distorted octahedral with "normal" Ba–Sb distances. Four Ba²⁺ cations are positioned around each ZnSb₄ tetrahedron in such a way (Figure 1b) that each of its faces is capped. The "solvation" of the anions by cations has also been discussed by Corbett on the example of K₂SiAs₂. Such an arrangement of the cations and anions results in Ba–Zn contacts, which are shorter than the sum of the Ba and Zn metallic radii (r_Ba = 2.215 ; r_Zn = 1.339 ), and shorter than the observed Ba–Zn distances in BaZn₅, BaZn₁₃, and all known BaZnPn₂ and BaZn₂Pn₂ compounds. Although the above might be suggestive of a certain covalency of the Ba–Zn interactions, the electronic structure calculations discussed later (vide infra), indicate very weak Ba–Zn bonding.

The last point of relevance to the structural description we touch upon concerns the Zn–Zn distances. Inspection of the structures of the ternary pnictides BaZn₂Pn₂ and BaZnPn₂ shows that they feature anionic networks made of edge-sharing or combinations of edge- and corner-sharing of ZnPn₄ tetrahedra. As a result, Zn–Zn distances in these compounds are relatively shorter than those in Ba₂ZnPn₂. For comparison, Zn–Zn contacts in BaZn₂Sb₂ measure 3.241 , while  in Ba₂ZnSb₂ d_Zn–Zn = 3.480 , respectively. This elongation can be related to the reduced dimensionality of the anionic sub-structure in Ba₂ZnPn₂, compared to BaZn₂Pn₂, and the unique linear geometry of the [ZnSb_4/2]^4– chains. Therefore, the contribution of the Zn–Zn interactions to the overall bonding is expected to be negligible, and the structure can be viewed as containing no close homoatomic bonds. Consequently, applying the simple valence rules, Ba₂ZnSb₂ can be rationalized as (Ba²⁺)₂(Zn²⁺)(Sb^3–)₂ or alternatively, as (Ba²⁺)₂[(4b-Zn^2–)(2b-Sb^1–)₂], i.e., a typical Zintl compound. Electronic structure calculations presented below confirm the description based on the classic Zintl formalism.