NMR evidence for change in the local structure ofZrCr2Hx

Abstract

We report here measurements of ${T}_{1\ensuremath{\rho}}^{\ensuremath{-}1},$ the spin-lattice relaxation rate in the rotating frame, for protons from 4 to 300 K in two samples of ${\mathrm{ZrCr}}_{2}{\mathrm{H}}_{x},$ with $x=0.2$ and 0.5. We find, especially for $x=0.2,$ that the maximum rate ${(T}_{1\ensuremath{\rho}}^{\ensuremath{-}1}{)}_{\mathrm{MAX}}$ is substantially larger than expected from the laboratory frame spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ by comparison with a model that successfully fits the ${T}_{1}^{\ensuremath{-}1}$ data. Further, a dimensionless parameter R is formed that expresses the relative strength of ${(T}_{1}^{\ensuremath{-}1}{)}_{\mathrm{MAX}}$ after correction for the different measurement frequencies. The measured values of R for $x=0.2$ are larger by a factor of 2 or more than that of any simple model, indicating that the dipolar second moment ${M}_{2}$ (primarily proton-proton) increases as temperature decreases, a surprising result. The increasing second moment is confirmed by direct measurement from dipolar echoes (magic echoes) at low temperature. The increased second moment reflects either decreasing average H-H distances below 150 K (partial clustering, a change in the local structure) or the progressive freeze-out of motional averaging of ${M}_{2}$ by high-frequency local motions.