“Nanoscale Zippers” in the Crystalline Solid. StructuralVariations in the Giant Magnetocaloric MaterialGd 5 Si 1.5Ge 2.5

Abstract

The magnetocaloric material Gd5Si1.5Ge2.5 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 SM5Ge4-type and twinned, monoclinic Gd5Si2Ge2-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.