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<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>NaV</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>as a Quarter-Filled Ladder Compound
A new x-ray diffraction study of the one-dimensional spin-Peierls compound ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ reveals a centrosymmetric $(\mathrm{Pmmn})$ crystal structure with one type of V site, contrary to the previously postulated noncentrosymmetric ${P2}_{1}\mathrm{mn}$ structure with two types of V sites ( ${V}^{+4}$ and ${\mathrm{V}}^{+5}$). Density-functional calculations indicate that $\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ is a quarter-filled ladder …