Hydrogen assisted order–disorder transformations in Cu–Sn sublattices of the (La,Ce)CuSn–D 2 systems

Abstract

The equiatomic RTX intermetallic compounds ( R = rare earth metal ; T = Fe , Co , Ni ; X = non-transition element ) exhibit interesting crystal structures, hydrogenation and magnetic properties. The hexagonal intermetallics are frequently formed in these systems and can be obtained via internal and external deformation of the AlB 2-type metal sublattice accompanied by a partial substitution of T by X. This work was focused on studies of hydrogen interaction with LaCuSn and CeCuSn intermetallics. Original compounds crystallise with the hexagonal “ a *2 c” type structures characterised by ordering of Cu and Sn with a slight internal deformation. ( c / a ) hex was found to be significantly higher for R = La . The structural phase transformations were studied by synchrotron XRD and powder neutron diffraction. The hydrogenation is accompanied by a small volume expansion (0.8–1.1%) proceeding exclusively along [0 0 1] and leading to a rebuilding of the structures of (La,Ce)CuSnD 0.3 –0.5 into deformed AlB 2 type (Cu + Sn disordered; “ a * c” cells). Deuterium atoms partially occupy trigonal bipyramidal R 3Sn 2 sites with D slightly shifted from the R 3 planes towards the R 3Sn tetrahedra. Thermal desorption properties studied by the vacuum TDS technique show that D desorption leads to a reversible formation of the initial intermetallics that restores the LiGaGe (Cu + Sn ordered) type structure.