NaV2O5as a Quarter-Filled Ladder Compound

A new x-ray diffraction study of the one-dimensional spin-Peierls compound ${\alpha }^{\prime }-\mathrm{Na}{\mathrm{V}}_2{\mathrm{O}}_5$ reveals a centrosymmetric $\left(\mathrm{Pmmn}\right)$ crystal structure with one type of V site, contrary to the previously postulated noncentrosymmetric ${P2\mathrm{}}_1\mathrm{mn}$ structure with two types of V sites ( $V^{+4\mathrm{}}$ and ${\mathrm{V}}^{+5\mathrm{}}$). Density-functional calculations indicate that $\mathrm{Na}{\mathrm{V}}_2{\mathrm{O}}_5$ is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of $\mathrm{Na}{\mathrm{V}}_2{\mathrm{O}}_5$ and its magnetic properties.