Nonmagnetic Insulator State in ${\mathrm{Na}}_1{\mathrm{CoO}}_2$ and Phase Separation of Na Vacancies

Crystallographic, magnetic, and NMR properties of a ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ single crystal with $x\simeq 1$ are presented. We identify the stoichiometric ${\mathrm{Na}}_1{\mathrm{CoO}}_2$ phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of ${\mathrm{Co}}^{3+}$ ions with $S=0$. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, ${\mathrm{Na}}_{0.8}{\mathrm{CoO}}_2$ phase. The importance of phase separation is discussed in the context of magnetic order for $x\simeq 0.8$ and the occurrence of a metal-insulator transition for $x\to 1$.

Figure 1

(a) ${}^{23}\mathrm{Na}$ NMR spectrum, showing the sharp (arrows) and broad signals. Experimental conditions were chosen in order to favor the broad signal. The sharp lines are thus comparatively much more intense than shown here. The gray area is the ${}^{23}\mathrm{Na}$ NMR spectrum in a ${\mathrm{Na}}_{0.8}{\mathrm{CoO}}_2$ single crystal [18]. Inset: detail of the ${}^{23}\mathrm{Na}$ central lines for $x=1$ and $x=0.8$ crystals. Note that the number of ${}^{23}\mathrm{Na}$ NMR lines and their shifts may somewhat vary from sample to sample with $x\simeq 0.8$ [18], so that strict one-to-one correspondence of ${}^{23}\mathrm{Na}$ NMR lines is not required to identify the $x\simeq 0.8$ phase here. (b) ${}^{59}\mathrm{Co}$ NMR spectrum in full scale (line) and in enlarged scale (dashed line) in order to evidence the broad ${}^{59}\mathrm{Co}$ signal. Grey area: ${}^{59}\mathrm{Co}$ NMR spectrum for $x=0.8$ [18].

Figure 2

(a) ${}^{23}\mathrm{Na}$ magnetic hyperfine shift for $H\parallel c$ (${}^{23}K_c$) for the sharp (filled symbols) and broad (open symbols) signals. Vertical bars are not error bars but the FWHM. Dashed lines: ${}^{23}K_c$ in ${\mathrm{Na}}_{0.8}{\mathrm{CoO}}_2$ [18]. (b) ${}^{59}\mathrm{K}_c$ for the sharp ${}^{59}\mathrm{Co}$ signal. FWHM is close to the symbol size. (c) Symbols: magnetization data ($H=1\mathrm{T}$, $\parallel c$). Line: fit explained in the text. Inset: same data in enlarged scale showing a hump at $T_{\mathrm{M}}=20\mathrm{K}$.

Figure 3

Top: $T_1^{-1}$ data for the broad (open symbols) and sharp (filled symbols) ${}^{23}\mathrm{Na}$ signals. For the latter, $T_1$ could be reliably extracted at two different positions only on the broad spectrum, while ${}^{23}T_1$ in ${\mathrm{Na}}_{0.8}{\mathrm{CoO}}_2$ (dashed lines) could be measured for three resolved groups of lines [18]. Inset: $T_2^{-1}$ for the sharp ${}^{23}\mathrm{Na}$ signal. Bottom: $T_1^{-1}$ and $T_2^{-1}$ (inset) for the sharp ${}^{59}\mathrm{Co}$ signal. Continuous lines are guides to the eye.