Anomalous proton-spin--lattice relaxation at high temperatures in zirconium dihydrides

Measurements of the proton-spin--lattice relaxation rate R1 are reported for zirconium dihydrides, ZrHx (1.58≤x≤1.98), with emphasis on the behavior of R1 at high temperatures (to 1300 K). An anomalously sharp increase in R1 is found at all hydrogen concentrations at temperatures above ∼900 K. At the onset temperature Tc for anomalous R1 behavior, defined as that of the intermediate R1 minimum, the hydrogen hopping rate νc congruent 109--1010 s-1, decreasing with increasing hydrogen concentration x. The anomalous behavior depends only weakly on resonance frequency and shows no correlation with the pronounced peak in the electronic density of states at x congruent 1.80. No differences were found in the behavior of pure and impurity-doped (Cr, Mn, Fe) samples. The anomalous contribution to R1 is described by R1x=A' exp(-U/kBT), with U congruent 0.4--1.0 eV/atom