Syntheses and structures of new phases AeMxIn(4-x) (Ae = Sr, Ba; M = Mg, Zn): size effects and site preferences in BaAl4-type structures.

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 SrMg(x)In(4-x) (0.85 <or= x <or= 1.53) and BaMg(x)In(4-x) (0 <or= x <or= 1.79) are consistent with size matches between Ae and the encapsulating M-In cages. All compounds crystallize in body-centered tetragonal BaAl(4)-type structure (I4/mmm, Z = 2) except that SrMg(x)In(4-x), 0.85 <or= x <or= 1.24, occurs in the lower-symmetry space group (I4m2, Z = 2) and the atoms are all ordered at the composition SrMgIn(3). The substitution of the effectively smaller Mg or Zn atoms into the indium sublattice of the monoclinic SrIn(4) (C2/m) dramatically changes the structure into the tetragonal BaAl(4)-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 BaZn(0.79(2))In(3.21) and SrZn(1.00(4))In(3.00). 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.