Preparation, crystallography, magnetic and magnetothermal properties of Ce<sub>5</sub>Si<sub>x</sub>Ge<sub>4-x</sub> alloys

Abstract

An investigation of the crystal structure and the phase relationships in the Ce5Si4-xGex system has been carried out. The crystal structures of the single phase intermetallics were characterized using X-ray powder diffraction and subsequent refinement employing the Rietveld analysis technique was performed. The intermetallic system was found to crystallize in three distinct crystal structures. The Ce5Si4-based solid solution extends from x = 0 to x = 2.15 and it was found to crystallize in the well-known Zr5Si4-type tetragonal structure. The germanium rich alloys, where 3.1 ≤} x ≤ 4, crystallized in the Sm5Ge4-type orthorhombic structure. The crystal structure of the intermediate phase, when 2.35 ≤ x ≤ 2.8, was found out to be of the Gd5Si2Ge2-type monoclinic structure. Microhardness tests were conducted on the samples in order to probe the trend in mechanical properties in this alloy system as a function of Ge concentration. The magnetic, thermal and magnetocaloric properties of the Ce5Si4-xGex alloy system have been investigated for x = 0, 1.0, 1.8, 2.5, 2.8, 3.5, 3.8 and 4.0. The phases with x = 0, 1.0 and 1.8 crystallize in the tetragonal Zr5Si4 structure and those with x = 2.5, 2.8 form in the Gd5Si2Ge2-type monoclinic structure. The alloys with x = 3.5, 3.8 and 4.0 crystallize in the Sm5Ge4-type orthorhombic structure. The Curie temperature of the tetragonal phases increases with increasing Ge content. The ordering temperatures of the monoclinic and orthorhombic phases remain nearly unaffected by the composition, with the Curie temperatures of the latter slightly higher than those of the former. All the alloys display evidence of antiferromagnetic interactions in the ground state. The orthorhombic and the monoclinic alloys behave as ferromagnets whereas the Si-rich tetragonal phase acts more like an antiferromagnet at high fields (1 T and higher). The maximum isothermal magnetic entropy change occurs at ~11 K in the monoclinic and orthorhombic phases with the highest value being 14.7 J/kg.K for Ce5Ge4 for a field change of 10 T which is considered moderate.