Hydrogen assisted order-disorder transformations in Cu-Sn sublattices of the (La,Ce)CuSn-D2 systems

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 AlB2-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 *2c” 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)CuSnD0.3-0.5 into deformed AlB2 type (Cu + Sn disordered; “a *c” cells). Deuterium atoms partially occupy trigonal bipyramidal R3Sn2 sites with D slightly shifted from the R3 planes towards the R3Sn 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.