Nanoscale zippers in Gd5(SixGe1-x)4:: Symmetry and chemical influences on the nanoscale zipping action

One critical parameter influencing the structural nature of the phase transitions in magnetocaloric materials Gd-5(SixGe1-x)(4) is the Si/Ge ratio (x/1 - x), because transition temperatures and structures depend crucially on this value. In this study, single-crystal X-ray diffraction indicates that Si and Ge atoms are neither completely ordered nor randomly mixed among the three crystallographic sites for these elements in these structures. Ge atoms enrich the T sites linking the characteristic slabs in these structures, while Si atoms enrich the T sites within them. Decomposition of the total energy into site and bond energy terms provides a rationale for the observed distribution, which can be explained by symmetry and electronegativity arguments. For any composition in Gd-5(SixGe1-x)(4), a structure map is presented that will allow for a rapid assessment of the specific structure type.