Transformation of AeIn4 Indides (Ae=Ba, Sr) into an AeAu2In2 structure type through gold substitution

The title compounds were prepared from the elements by high-temperature solid-state synthesis techniques. X-ray structural analyses shows that BaAu2In2 (1) and SrAu2In2 (2) crystallize in a new orthorhombic structure, Pnma, Z=4 (a=8.755(2), 8.530(2) A; b=4.712(1), 4.598(1) A; c=12.368(3), 12.283(4) A, respectively). Gold substitutes for 50% of the indium atoms in the tetragonal BaIn4 and monoclinic SrIn4 parents to give this new and more flexible orthorhombic structure. The Ae atoms in this structure are contained within chains of hexagonal prisms built of alternating In and Au that have additional augmenting atoms around their waists from further condensation of parallel displaced chains. The driving forces for these structural changes are in part the shorter Au-In distances (2.72 and 2.69 A) relative to d(In-In) in the parents, presumably because of relativistic contractions with Au. Generalities about such centered prismatic building blocks and their condensation modes in these and related phases are described. Band structure calculations (EHTB) demonstrate that the two compounds are metallic, which is confirmed by measurements of the resistivity of 1 and the magnetic susceptibilities of both.