Complex Structural Disorder in the Zintl Phases Yb₁₀MnSb₉ and Yb₂₁Mn₄Sb₁₈

A systematic investigation of the ternary system Yb-Mn-Sb led to the discovery of the novel phase Yb₁₀MnSb₉. Its crystal structure was characterized by single-crystal X-ray diffraction and found to be complex and highly disordered. The average Yb₁₀MnSb₉ structure can be considered to represent a defect modification of the Ca₁₀LiMgSb₉ type and to crystallize in the tetragonal P4₂/mnm space group (No. 136) with four formula units per cell. The structural disorder can be associated with both occupational and positional effects on several Yb and Mn sites. Similar traits were observed for the structure of the recently reported Yb₂₁Mn₄Sb₁₈ phase (monoclinic space group C2/c, No. 15), which was reevaluated as part of this study as well. In both structures, distorted Sb₆ octahedra centered by Yb atoms and Sb₄ tetrahedra centered by Mn atoms form disordered fragments, which appear as the hallmark of the structural chemistry in this system. Discussion along the lines of how difficult, and important, it is to distinguish Yb₁₀MnSb₉ from the compositionally similar binary Yb₁₁Sb₁₀ and ternary Yb₁₄MnSb₁₁ compounds is also presented. Preliminary transport measurements for polycrystalline Yb₁₀MnSb₉ indicate high values of the Seebeck coefficient, approaching 210 μV K^-1 at 600 K, and a semiconducting behavior with a room-temperature resistivity of 114 mΩ cm.