Hyperfine spectroscopic study of Hf2Fe hydrides and their thermal stability

Abstract

Perturbed angular correlation (PAC) spectroscopy has been used to investigate the hydrides of the intermetallic compound Hf2Fe. Aiming at information on the hydrogen diffusion, measurements of the electric quadrupole interaction of 181Ta on Hf sites of Hf2FeHx have been carried out in the concentration range 0.8≤x≤4.8. PAC measurements as a function of temperature show that for hydrogen concentrations x>1.5 these hydrides are thermally unstable and decompose at T≈650 K. The time constant of the decomposition at 1100 K was determined by PAC spectroscopy to be τ=1.0(2) h for x=3.75. The cubic Hf hydride δ-HfHx with x≈1.6 and the intermetallic compound HfFe2 were identified as well crystallized segregation products by X-ray diffraction and 57Fe Moessbauer spectroscopy. The activation energy for hydrogen jumps in the decomposition product δ-HfHx was determined to be Ea = 0.50(3) eV. The thermal stability increases towards lower hydrogen concentrations. For x≤1 the PAC spectra of Hf2FeHx were found to be fully reversible between 290 K and 1100 K. Hf2FeHx has been reported to show magnetic order at large hydrogen concentrations and low temperatures. PAC measurements for large x values and temperatures between 290 K and 9 K, however, gave no evidence for a magnetic hyperfine field at the Hf sites. Moessbauer measurements suggest that the magnetic order temperature of Hf2FeH3.9 is less than 50 K.