1202.4254 (H. Alloul et al.)

23Na NMR study of sodium order in NaxCoO2 with 22K Neel temperature    [PDF]

H. Alloul, I. R. Mukhamedshin, A. V. Dooglav, Ya. V. Dmitriev, V. -C. Ciomaga, L. Pinsard-Gaudart, G. Collin

We report a systematic study of the $c$ lattice parameter in the
Na$_{x}$CoO$_{2}$ phases versus Na content $x>0.5$, in which sodium always
displays ordered arrangements. This allows us to single out the first phase
which exhibits an AF magnetic order at a N\'eel temperature $T_{N}=$22 K which
is found to occur for $x\approx 0.77(1)$. Pure samples of this phase have been
studied both as aligned powders and single crystals. They exhibit identical
$^{23}$Na NMR spectra in which three sets of Na sites could be fully resolved,
and are found to display $T$ dependencies of their NMR shifts which scale with
each other. This allows us to establish that the $T$ variation of the shifts is
due to the paramagnetism of the Co sites with formal charge state larger than
3$^{+}$. The existence of a sodium site with axial charge symmetry and the
intensity ratio between the sets of $^{23}$Na lines permits us to reveal that
the 2D structure of the Na order corresponds to 10 Na sites on top of a 13 Co
sites unit cell, that is with $x=10/13\approx 0.77$. This structure fits with
that determined from local density calculations and involves triangles of 3 Na
sites located on top of Co sites (so called Na1 sites). The associated ordering
of the Na vacancies is quite distinct from that found for $x<0.75$.

View original: http://arxiv.org/abs/1202.4254