LAUSR

Abstract Crystal structure and magnetic properties of Gd5Si1.5Ge2.5 and Gd4.9Zr0·1Si1.5Ge2.5 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 Gd5Si1.5Ge2.5 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 Gd5Si1.5Ge2.5. 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.