Influence of oxygen on the giant magnetocaloric effect of Gd5Si1.95Ge2.05

The influence of oxygen on the 5:4 (i.e. 5Gd:4[Si + Ge]) monoclinic Gd5Si1.95Ge2.05 was investigated. Five oxygen-containing alloys, Gd5Si1.95Ge2.05Ox, were prepared with x varying from 0.05 to 0.5. The addition of small amounts of oxygen, x ≤ 0.10, stabilizes the orthorhombic form at high temperature, such that both phases are present in the arc-melted alloys. At higher oxygen contents, x ≥ 0.15, oxygen tends to favor the decomposition of the Gd5Si1.95Ge2.05 phase into the GdSiyGe1−y (1:1) and the Gd5SizGe3−z (5:3) phases. The solubility of oxygen in the two polymorphic 5:4 phases is less than x = 0.05. The presence of oxygen in the monoclinic form destroys the giant magnetocaloric effect, apparently by preventing the monoclinic phase from transforming to the orthorhombic modification at the Curie temperature giving rise to a second order magnetic transformation.