Effect of Si doping and applied pressure upon magnetostructural properties of Tb5(SixGe1-x)4 magnetocaloric compounds

The composition- and pressure-dependent magnetostructural properties of Tb5(SixGe1-x)4 (x = 0.4, 0.485, 0.625, and 0.7) were investigated using x-ray powder diffraction and x-ray magnetic circular dichroism in a diamond anvil cell, respectively. Substituting the smaller-size Si for Ge stabilizes a single-phase, ferromagnetic (FM) orthorhombic O(I) structure for x ? 0.7. Similarly, application of external pressure causes a canted antiferromagnetic orthorhombic O(II) sample (x = 0.4) to transform into an FM O(I) phase at 4 GPa. The element- and orbital-specific x-ray absorption data indicate that the Tb 4f orbital occupation changes with external pressure, likely through 4f-5d electronic mixing, yet no changes in Tb 4f electronic structure are observed with Si doping. The results point to different mechanisms behind the enhancement of FM exchange interactions in Tb5(SixGe1-x)4 with chemical and applied pressure, respectively.