Evidence for a short-range chemical order of Ge atoms and its critical role in inducing a giant magnetocaloric effect in Gd₅Si_1.5Ge_2.5

Crystal structure and magnetic properties of Gd₅Si_1.5Ge_2.5 and Gd_4.9Zr_0·1Si_1.5Ge_2.5 were investigated using high-energy X-ray diffraction and magnetic measurements. Results showed that a Zr substitution for 2% Gd reduces unit cell volumes of a room-temperature monoclinic and a low-temperature orthorhombic lattice and a difference between them at a magnetostructural transition. At a microscopic level, the Zr substitution increases length of disconnected interlayer T–T bonds of the monoclinic lattice at the expense of length of connected interlayer T–T bonds (T = Si, Ge). These opposing changes of the interlayer T–T bonds provided evidence for existence of a short-range chemical order of Ge atoms in lattices of Gd₅Si_1.5Ge_2.5 and its weakening by the Zr substitution. Magnetic measurements revealed that the Zr substitution brings about a change of the magnetic structure and a reduction of a giant magnetocaloric effect of Gd₅Si_1.5Ge_2.5. Based on such structural and magnetic changes due to the Zr substitution, we propose a relation between the short-range chemical order and the total entropy change at the magnetostructural transition. Using this relation, a giant magnetocaloric effect and an annealing effect observed over a wide range of Gd₅(Si,Ge)₄ composition can be explained quantitatively.