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Syntheses and Structures of New Phases AeMxIn4-x (Ae = Sr, Ba; M = Mg, Zn):  Size Effects and Site Preferences in BaAl4-Type Structures

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posted on 15.10.2007, 00:00 by Bin Li, John D. Corbett
The title phases were synthesized via high-temperature solid-state methods and structurally characterized by single-crystal X-ray diffraction. The phase widths of both SrMgxIn4-x (0.85 ≤ x ≤ 1.53) and BaMgxIn4-x (0 ≤ x ≤ 1.79) are consistent with size matches between Ae and the encapsulating M−In cages. All compounds crystallize in body-centered tetragonal BaAl4-type structure (I4/mmm, Z = 2) except that SrMgxIn4-x, 0.85 ≤ x ≤ 1.24, occurs in the lower-symmetry space group (Im2, Z = 2) and the atoms are all ordered at the composition SrMgIn3. The substitution of the effectively smaller Mg or Zn atoms into the indium sublattice of the monoclinic SrIn4 (C2/m) dramatically changes the structure into the tetragonal BaAl4-type structure. Compared with the Mg compounds in which all Mg occupy the basal 4d crystallographic site, Zn occupies the other 4e (apical) site in both BaZn0.79(2)In3.21 and SrZn1.00(4)In3.00. These differences are related to the interplay of size and electronic factors. The latter may be expressed by not only electronegativities but also total energies according to linear muffin-tin-orbital calculations.