Making and breaking covalent bonds across the magnetic transition in the giant magnetocaloric material Gd₅(Si₂Ge₂)

A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si₂Ge₂), across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si, Ge) dimers that are richer in Ge increase their distances by 0.859(3) Å and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the Gd₅(Si_xGe_1-x)4.