Hydrogenation induced structure and property changes in GdGa

Abstract

Hydrides GdGaHx were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50bar and temperatures from 50 to 500°C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50bar and temperatures below 500°C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) Å, b=12.024(2) Å, c=4.1009(6) Å) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200°C affords a hydride with x≈1.3, which is stable up to 500°C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200°C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300°C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 μB which is typical for compounds containing Gd3+ions.