Structural, magnetic, and thermal characteristics of the phase transitions in Gd5Ga x Ge4− x magnetocaloric materials

Temperature-dependent, single crystal and powder X-ray diffraction studies as well as magnetization, and heat capacity measurements were carried out on two phases of the Gd5Ga x Ge4− x system: for x=0.7 and 1.0. Gd5Ga0.7Ge3.3 shows three structure types as a function of temperature: (i) from 165K to room temperature, the orthorhombic Sm5Ge4-type structure exists; (ii) below 150K, it transforms to a orthorhombic Gd5Si4-type structure; and (iii) a monoclinic Gd5Si2Ge2-type component is observed for the intermediate temperature range of 150K≤T≤165K. This is the first time that all these three structure types have been observed for the same composition. For Gd5Ga1.0Ge3.0, the room temperature phase belongs to the orthorhombic Pu5Rh4-type structure with interslab contacts between main group atoms of 2.837(4)Å. Upon heating above 523K, it transforms to a Gd5Si4-type structure with this distance decreasing to 2.521(7)Å before decomposing above 573K.