Evidence of incipient bond-stretching isomerism in Sr_2.04(1)Ca_0.96(1)Sn₅ from variable-temperature structural studies

Abstract

The title compound was found to crystallize in the Pu₃Pd₅ structure type (SG Cmcm) with cell dimensions of a = 10.5179(9) Å, b = 8.4789(8) Å, and c = 10.7623(10) Å. The structure consists of isolated Sn₅⁶⁻ square-pyramidal units surrounded by cations that seem to play a crucial role in stabilizing the Zintl polyanions. The square pyramids contract at low temperatures (100 K) leading to the shortening of the basal intracluster Sn-Sn bond (2.74 Å), while the intercluster bonds become very large, indicating features of bond stretching isomerism as is known for Ba₃Ge₄. A study of different crystals shows a slight variation in the lattice parameters, suggesting the presence of a definite phase width which was substantiated by the successful synthesis of monophasic samples of Sr_3−xCa_xSn₅ (0.5 ≤ x ≤ 2.5). However, all attempts to obtain the pure Ca phase, Ca₃Sn₅ have been unsuccessful. The compounds show a weakly metallic behavior, as shown by the electrodeless Q method. Magnetic studies show a very low susceptibility (nearly temperature independent till 5 K). Our studies suggest that the "Sn" polyanions in Ca_2.04Sr_0.96Sn₅ may be described as an arachno-Sn₅⁻⁶ Zintl cluster.