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 less-than-or-equal-to x less-than-or-equal-to 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 approximately 900 K. At the onset temperature T(c) for anomalous R1 behavior, defined as that of the intermediate R1 minimum, the hydrogen hopping rate v(c) congruent-to 10(9)-10(10) 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-to 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/k(B)T), with U congruent-to 0.4 - 1.0 eV/atom.