Crystal structures and magnetism of the hydrides of Tb2T2Ga and Tb3Co3Ga (T = Co, Ni)

Abstract

Hydrogenation effects on the crystal structures and magnetic properties of Tb2Co2Ga, Tb2Ni2Ga, and Tb3Co3Ga were studied. Tb2Co2Ga, Tb2Ni2Ga (both structure type W2CoB2), and Tb3Co3Ga (structure type W3CoB3) are structurally related belonging to inhomogeneous linear structure series based on the TlI- (also referred to as CrB type) and YAlGe- (ordered derivative of the binary type UPt2) type structure motives. The hydrides Tb2Co2GaH6.2, Tb3Co3GaH9.7, and Tb2Ni2GaH5.1 were synthesized at room temperature with hydrogen pressures of 52–53 ​kPa. Hydrogenation results in strongly anisotropic lattice volume expansion ΔV/V ​= ​19.2–20.0%. It also leads to a decrease of the Curie temperature in ferromagnetic Tb2Co2Ga and Tb3Co3Ga and practically suppresses the antiferromagnetic ordering in Tb2Ni2Ga. A model of the crystal structure of the hydride Tb3Co3GaH10 with four hydrogen positions was suggested based on stronger effect of hydrogenation on the TlI-type slabs. The more complex character of the magnetic phase transitions for Tb2Co2Ga and Tb3Co3Ga presumes involvement of Co in the magnetism of these compounds, while Tb2Ni2Ga acts as a typical 4f-electron system with dominating RKKY exchange interactions.