The Hydrogenation of the Zintl Phase NdGa Studied by in situ Neutron Diffraction

Abstract

The hydrogenation of the Zintl phase NdGa was studied by in situ neutron powder diffraction. We find a compositional range of 0.1 < x < 0.8 in NdGaH1+x. Hydrogen atoms are located in two different positions, in HNd4 tetrahedra, and close to the polyanionic chains. For the latter, the Ga–H distance in NdGaH1.66 is quite long (ca. 200 pm) with a trigonal bipyramidal Nd3Ga2 surrounding of hydrogen atoms. Hydrogen poor NdGaH<1 phases as known for similar systems were not observed. The changing hydrogen content shows no measureable effect on the unit cell volume, but on lattice parameter ratios. Superstructures occur for 0.53 < x < 0.66 and 0.73 < x < 0.8, leading to a doubling or tripling of the lattice parameter a. They are probably caused by partial hydrogen ordering. The threefold superstructure contains a 1∞[(Ga–H–Ga–H–Ga)6–] moiety with hydrogen bridging two gallium chains.