NMR EVIDENCE FOR CHANGE IN THE LOCAL-STRUCTURE OF ZRCR2HX

We report here measurements of T-1 rho(-1), the spin-lattice relaxatio n rate in the rotating frame, for protons from 4 to 300 K in two sampl es of ZrCr2Hx, with x = 0.2 and 0.5. We find, especially for x = 0.2, that the maximum rate (T-1 rho(-1))(MAX) is substantially larger than expected from the laboratory frame spin-lattice relaxation rate T-1(-1 ) by comparison with a model that successfully fits the T-1(-1) data. Further, a dimensionless parameter R is formed that expresses the rela tive strength of (T-1(-1))(MAX) after correction for the different mea surement 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 tha t the dipolar second moment M-2 (primarily proton-proton) increases as temperature decreases, a surprising result. The increasing second mom ent is confirmed by direct measurement from dipolar echoes (magic echo es) at low temperature. The increased second moment reflects either de creasing average H-H distances below 150 K (partial clustering, a chan ge in the local structure) or the progressive freeze-out of motional a veraging of M-2 by high-frequency local motions. [S0163-1829(98)04417- 8].