Evidence of incipient bond-stretching isomerism in Sr2.04(1)Ca0.96(1)Sn5 from variable-temperature structural studies

The title compound was found to crystallize in the Pu3Pd5 structure type (SG Cmcm) with cell dimensions of a = 10.5179(9) A, b = 8.4789(8) A, and c = 10.7623(10) A. The structure consists of isolated Sn5(6-) 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 A), 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-x)CaxSn5 (0.5