IMPACT OF STRUCTURE ON MAGNETIC COUPLING IN CAV4O9

The strong corrugation of the V4O5 layer in the spin gap system CaV4O9 is examined for its impact on the exchange coupling constants between the spin-1/2 V ions. Local spin density calculations show that the V spin occupies a d(x2-y2) orbital ((x) over cap and (y) over cap are th e V-V directions) that would have vanishing second order exchange coup ling if the V4O5 layer were flat. The Kanamori-Goodenough superexchang e rules, and the small calculated energy difference of ferromagnetic a nd antiferromagnetic alignments of spins for the real structure, indic ate that second neighbor couplings dominate over nearest neighbor. Thi s suggests two coupled S = 1/2 metaplaquette systems, each tending tow ard singlet formation.