Mixed valency and charge ordering in α′-NaV2O5

We report ${}^{51}\mathrm{V}$ NMR measurements in ${\alpha }^{\prime }$-${\mathrm{NaV}}_2{\mathrm{O}}_5$, which has been considered as a spin-Peierls (SP) system. Above the transition temperature $T_{\mathrm{C}},$ only one set of V sites was observed in the NMR spectra, indicating that all the V sites are in a uniform electronic state with the average oxidation of ${\mathrm{V}}^{4.5+}.$ Below $T_{\mathrm{C}},$ there appear two inequivalent sets of V sites assigned to ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{5+}$ states. This result indicates that the phase transition in ${\alpha }^{\prime }$-${\mathrm{NaV}}_2{\mathrm{O}}_5$ is not a conventional SP transition but a charge ordering. Below $T_{\mathrm{C}},$ 1/$T_1$ at the ${\mathrm{V}}^{4+}$ sites has an activated T dependence, confirming a spin-singlet ground state with the excitation gap $\sim 108\hspace{0.5em}\mathrm{K}.$