Co-59 NMR studies of metallic NaCo2O4

Co-59 NMR studies have been performed in metallic NaCo2O4 in the temperatur e range 350-100 K. The powder patterns predominantly correspond to typical second-order quadrupolar splitted central transition of two inequivalent co balt sites. Features of magnetic interaction are also present. The analysis of the NMR line shape shows that the electric field gradient (EFG) experie nced by one of the two sites(site 1) is independent of temperature, whereas around 47% increment in EFG has been observed for site 2 as the temperatur e is lowered from 350 to 100 K. Co-59 NMR shift for each site is almost iso tropic in character which varies linearly with susceptibility and basically consists of two parts. Both the sites suffer a large temperature independe nt shift due to the contribution from orbital paramagnetism (chi(vv)) which is analogous in metals to the Van Vleck paramagnetic susceptibility. The t emperature-dependent Knight shift for site 1 reveals a negative hyperfine c oupling constant (A(hf)(d)) arising from inner s core polarization by the d . electrons which are itinerant in character. A positive A(hf)(d) observed for site 2 cannot be interpreted directly in a metallic system such as NaCo 2O4. A comparison of the magnitude of A(hf)(d) with those of other cobalt o xides suggests that the nucleus which is being probed by NMR, belongs to a diamagnetic cobalt state. Thus the present NMR results together with the su sceptibility indicate the presence of Co3+ ion with S = 0 and the magnetic Co4+ ion. [S0163-1829(99)01314-4].