“Nanoscale Zippers” in the Crystalline Solid. Structural Variations in the Giant Magnetocaloric Material Gd5Si1.5Ge2.5

The magnetocaloric material Gd 5 Si 1.5 Ge 2.5 has been synthesized and its crystal structures at 292 and 163 K are reported from single-crystal X-ray diffraction experiments. At room temperature, orthorhombic Sm 5 Ge 4 -type and twinned, monoclinic Gd 5 Si 2 Ge 2 -type phases coexist in single crystal specimens. This phenomenon is mainly due to the covalent bond breaking and formation of (Si,Ge)−(Si,Ge) dimers during the crystallographic phase transition. We suggest an atomic-level model for the interface of the two distinct domains. A detailed TEM analysis also confirms the coexistence of both phases in a polycrystalline sample. The structural relationship between such nanoscale zippers and other known phases is identified.