Structural, magnetic, and thermal characteristics of the phase transitions in Gd5GaxGe4−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 Gd5GaxGe4−x system: for x=0.7 and 1.0. Gd5Ga0.7Ge3.3 shows three structure types as a function of temperature: (i) from 165 K to room temperature, the orthorhombic Sm5Ge4-type structure exists; (ii) below 150 K, it transforms to a orthorhombic Gd5Si4-type structure; and (iii) a monoclinic Gd5Si2Ge2-type component is observed for the intermediate temperature range of 150 K≤T≤165 K. 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 523 K, it transforms to a Gd5Si4-type structure with this distance decreasing to 2.521(7) Å before decomposing above 573 K.