NAV2O5 AS A QUARTER-FILLED LADDER COMPOUND

A new x-ray diffraction study of the one-dimensional spin-Peierls comp ound alpha'-NaV2O5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated noncent rasymmetric P2(1)mm structure with two types of V sites (V+4 and V+5). Density-functional calculations indicate that NaV2O5 is a quarter-fil led 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 insul ating behavior of NaV2O5 and its magnetic properties.