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The static magnetic hyperfine interaction and the hydrogen-induced dynamic electric quadrupole interaction ͑QI͒ at the nuclear probe 181 Ta on cubic Er sites in magnetically ordered C15 Laves-phase hydrides ErFe 2 H x have been investigated by perturbed angular correlation ͑PAC͒ spectroscopy as a function of temperature (10 KрTр600 K) and hydrogen concentration (0рxр3.2). At room temperature and concentrations x Ͻ2, the PAC spectra, supported by x-ray diffraction measurements, indicate the coexistence of two hydride phases ErFe 2 H x with xр0.1 and xϳ1.6-1.7, respectively, and a linear increase of the fraction of the highconcentration 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 TϾ400 K. The resulting 181 Ta nuclear quadrupole relaxation rates show an Arrhenius behavior with an activation energy of E a ϭ0.39(3) eV for xϭ1.5 and 2.0 and permit an estimate of the effective charge associated with the diffusing H atom of ZЈeϷ0.095e. 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 Ta magnetic hyperfine field B h f as a function of temperature and H concentration. At a given temperature Tу450 K in the fast-fluctuation region, B h f decreases with increasing x, indicating a decrease of the s conduction electron polarization at the probe site. The temperature dependence of B h f in the fast-fluctuation region is also affected by the hydrogen concentration: at xϭ1.1, B h f was found to decrease, at xϭ1.5 and 2.0 to increase with increasing T. Irreversible changes of the PAC spectra indicate that the thermal stability of crystalline ErFe 2 H 2 encapsulated under vacuum is limited to Tр550 K. 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.

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