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.