Quaternary Pnictides with Complex, Noncentrosymmetric Structures. Synthesis and Structural Characterization of the New Zintl Phases Na₁₁Ca₂Al₃Sb₈, Na₄CaGaSb₃, and Na₁₅Ca₃In₅Sb₁₂

Abstract

Three new Zintl phases, Na₁₁Ca₂Al₃Sb₈, Na₄CaGaSb₃, and Na₁₅Ca₃In₅Sb₁₂, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na₁₁Ca₂Al₃Sb₈ crystallizes with its own structure type (Pearson index <i>oP</i>48) with the primitive orthorhombic space group <i>Pmn</i>2₁ (No. 31). The structure is best viewed as [Al₃Sb₈]^15– units of fused AlSb₄ tetrahedra, a novel type of Zintl ion, with Na⁺ and Ca²⁺ cations that solvate them. Na₄CaGaSb₃ also crystallizes in its own type with the primitive monoclinic space group <i>Pc</i> (No. 7; Pearson index <i>mP</i>36), and its structure boasts one-dimensional [GaSb₃]^6– helical chains of corner-shared GaSb₄ tetrahedra. The third new compound, Na₁₅Ca₃In₅Sb₁₂, crystallizes with the recently reported K₂BaCdSb₂ structure type (space group <i>Pmc</i>2₁; Pearson index <i>oP</i>12). The Na₁₅Ca₃In₅Sb₁₂ structure is based on polyanionic layers made of corner-shared InSb₄ tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials