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].