98 research outputs found
59Co NMR study of the Co states in superconducting and anhydrous cobaltates
Co NMR spectra in oriented powders of NaCoO and in its
hydrated superconducting phase (HSC) NaCoO,1.3HO reveal a
single electronic Co state with identical independent NMR shift tensor.
These phases differ markedly from NaCoO, in which we resolve 3
types of Co sites. The large T variation of their spin susceptibilities and the anisotropy of the orbital susceptibility allow us
to conclude that charge disproportionation occurs, in a non magnetic Co
and two magnetic sites with about 0.3 and 0.7 holes in the multiplet.
The data are consistent with those for the single Co site in the anhydrous and
HSC phase assuming the expected Co charge.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Complex magnetic differentiation of cobalts in NaCoO with 22K N\'eel temperature
Single crystals of sodium cobaltates NaCoO with
were grown by the floating zone technique. Using electrochemical Na
de-intercalation method we reduced the sodium content in the as-grown crystals
down to pure phase with 22 K N\'eel temperature and . The
Co NMR study in the paramagnetic state of the K phase
permitted us to evidence that at least 6 Co sites are differentiated. They
could be separated by their magnetic behaviour into three types: a single site
with cobalt close to non-magnetic Co, two sites with the most magnetic
cobalts in the system, and the remaining three sites displaying an intermediate
behaviour. This unusual magnetic differentiation calls for more detailed NMR
experiments on our well characterized samples.Comment: 5 pages, 4 figures, submitted to JETP Let
Phase segregation in NaxCoO2 for large Na contents
We have investigated a set of sodium cobaltates (NaxCoO2) samples with
various sodium content (0.67 \le x \le 0.75) using Nuclear Quadrupole Resonance
(NQR). The four different stable phases and an intermediate one have been
recognized. The NQR spectra of 59Co allowed us to clearly differentiate the
pure phase samples which could be easily distinguished from multi-phase
samples. Moreover, we have found that keeping samples at room temperature in
contact with humid air leads to destruction of the phase purity and loss of
sodium content. The high sodium content sample evolves progressively into a
mixture of the detected stable phases until it reaches the x=2/3 composition
which appears to be the most stable phase in this part of phase diagram.Comment: 5 pages, 4 figure
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