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

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

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

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