Perturbed angular correlation study of static and dynamic hyperfine interactions at181Tain the Laves-phase hydridesErFe2Hx

Abstract

The static magnetic hyperfine interaction and the hydrogen-induced dynamic electric quadrupole interaction (QI) at the nuclear probe ${}^{181}\mathrm{Ta}$ on cubic Er sites in magnetically ordered $C15$ Laves-phase hydrides ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{x}$ have been investigated by perturbed angular correlation (PAC) spectroscopy as a function of temperature $(10 \mathrm{K}<~T<~600 \mathrm{K})$ and hydrogen concentration $(0<~x<~3.2).$ At room temperature and concentrations $x<2,$ the PAC spectra, supported by x-ray diffraction measurements, indicate the coexistence of two hydride phases ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{x}$ with $x<~0.1$ and $x\ensuremath{\sim}1.6--1.7,$ respectively, and a linear increase of the fraction of the high-concentration phase with increasing x. At low temperatures the PAC spectra of the high-concentration hydride reflect a broad distribution of strong static hyperfine interactions. The dynamic QI caused by rapidly diffusing H atoms becomes observable at $T>400 \mathrm{K}.$ The resulting ${}^{181}\mathrm{Ta}$ nuclear quadrupole relaxation rates show an Arrhenius behavior with an activation energy of ${E}_{a}=0.39(3) \mathrm{eV}$ for $x=1.5$ and 2.0 and permit an estimate of the effective charge associated with the diffusing H atom of ${Z}^{\ensuremath{'}}e\ensuremath{\approx}0.095e.$ The vanishing time average of the nuclear quadrupole interaction in the fast-fluctuation region allows the separation of the magnetic and electric hyperfine interactions and the determination of the ${}^{181}\mathrm{Ta}$ magnetic hyperfine field ${B}_{\mathrm{hf}}$ as a function of temperature and H concentration. At a given temperature $T>~450 \mathrm{K}$ in the fast-fluctuation region, ${B}_{\mathrm{hf}}$ decreases with increasing x, indicating a decrease of the s conduction electron polarization at the probe site. The temperature dependence of ${B}_{\mathrm{hf}}$ in the fast-fluctuation region is also affected by the hydrogen concentration: at $x=1.1,$ ${B}_{\mathrm{hf}}$ was found to decrease, at $x=1.5$ and 2.0 to increase with increasing T. Irreversible changes of the PAC spectra indicate that the thermal stability of crystalline ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{2}$ encapsulated under vacuum is limited to $T<~550 \mathrm{K}.$ ${\mathrm{ErH}}_{2}$ was identified by x-ray diffraction as one of the dissociation products, but there was no evidence for the precipitation of elemental Fe and a hydrogen-induced amorphization step.