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 spin12 V ions. Local spin density calculations show that the V spin occupies a dx22y2 orbital (x̂ and ŷ are the V-V directions) that would have vanishing second order exchange coupling if the V4O5 layer were flat. The Kanamori-Goodenough superexchange rules, and the small calculated energy difference of ferromagnetic and antiferromagnetic alignments of spins for the real structure, indicate that second neighbor couplings dominate over nearest neighbor. This suggests two coupled S ­ 12 metaplaquette systems, each tending toward singlet formation. [S0031-9007(97)03862-3]