Crystal chemistry and metal-hydrogen bonding in anisotropic and interstitial hydrides of intermetallics of rare earth (R) and transition metals (T), RT3 and R2T7

Abstract

Abstract Hydrides of the i 3- and R2Ni7-based (R = light rare earth element) intermetallics exhibit novel structural features. Structures of these hydrides, including CeNi3D2.8, La2Ni7D6.5, LaNi3D2.8, and Ce2Ni7D4.7, are formed via a huge volume expansion occurring along a single crystallographic direction. Unique structural features during the formation of the hydrides include: (a) The lattice expansion proceeds exclusively within the RNi2 slabs leaving the RNi5 slabs unmodified. Such expansion, about 60% along [001] for the Laves layers, is associated with occupation by D atoms of these slabs; (b) New types of interstitial sites occupied by D are formed; (c) An ordered hydrogen sublattice is observed. In the present work we give (a) a review of the crystal chemistry of the conventional, interstitial type hydrides formed by RT3 and R2T7 intermetallic compounds (R = rare earths; T = Fe, Co, Ni) as compared to the unusual features of the crystal chemistry of anisotropic hydrides formed by the RNi3 and R2Ni7 intermetallics and (b) studies of the interrelation between structure and bonding in anisotropic hydrides by performing density functional calculations for CeNi3 and Ce2Ni7 intermetallic alloys and their corresponding hydrides. These studies allowed obtaining an understanding of the bonding mechanism in the hydrogenated compounds which causes a complete anisotropic rebuilding of their structures. From DOS analysis, both initial intermetallics and their related hydrides were found to be metallic. Bader topological analysis for the non-hydrogenated intermetallics showed that Ce atoms donate in average of almost 1.2 electrons to the Ni sites. Hydrogenation increases electron transfer from Ce; its atoms donate 1.2–1.6 electrons to Ni and H. Charge Density Distribution and Electron Localization Function for Ce2Ni7D4.7 phase clearly confirm that the interaction between the Ce and Ni does not have any significant covalent bonding. Ni is bonded with H via forming spatial frameworks –H–Ni–H–Ni– where H atoms accumulate an excess electron density of about 0.5e–. Thus, the tetrahedral or open saddle-type NiH4 coordination observed in the structures of the hydrides is not associated with the formation of [Ni0H4 1–]4– complexes containing a hydrido-ion H–1. In the structural frameworks there are terminal bonds Ni–H, bridges Ni–H–Ni, and the bonds where one H is bound to three different Ni. These spatial ordered frameworks stand as the principal reason for the anisotropic changes in the structural parameters on hydrogenation. Another unique feature of anisotropic hydrides is the donation of electrons from nonhydrogenated RNi5 parts to hydrogen in RNi2 slabs stabilising these fragments.