Analysis of the spin-spin interactions in layered oxides α′-NaV2O5, CaV2O5 and MgV2O5 and the spin-Peierls distortion in α′-NaV2O5 by molecular orbital, Madelung energy and bond valence sum calculations

The spin-spin exchange parameters of alpha'-NaV2O5, CaV2O5 and MgV2O5 were examined by calculating the spin-spin interaction energies of their spin dimers, and the probable pairing distortion leading to the (2a x 2b x 4c) superstructure of alpha'-NaV2O5 was investigated by performing Madelung energy and bond valence sum analyses. In the V2O5 layers of alpha'-NaV2O5, CaV2O5 and MgV2O5, ladder chains of vanadium atoms are arranged in a staggered manner. CaV2O5 and MgV2O5 have an unpaired spin at each vanadium site, while alpha'-NaV2O5 has one unpaired spin in each rung of the ladder chains. The latter makes alpha'-NaV2O5 behave as a spin 1/2 Heisenberg linear chain system. Madelung energy and bond valence sum calculations for alpha'-NaV2O5 show that the pairing distortion from the uniform spin lattice to the zigzag spin lattice proposed by Seo and Fukuyama is energetically favorable. In the zigzag spin lattice the paired spins do not reside in the ladder chains but in between the ladder chains, which makes alpha'-NaV2O5 different from a simple spin-Peierls system. (C) 1999 Elsevier Science Ltd. All rights reserved.