MAGNETISM AND THE ALPHA-BETA-PHASE TRANSITION IN SOLID OXYGEN

The electronic spectra of alpha and beta solid O2 were calculated in a full many-body approach for a cluster consisting of four O2 molecules with periodic boundary conditions. Only the partially occupied pi orb itals (sixteen spin-orbitals, eight electrons) were included. The lowe st 81 many-body states, which correspond to neutral, unexcited molecul es, describe accurately the magnetic excitations of the solids. The tw o phases have very different spectra resulting, even at very low tempe ratures, in a large difference in the (magnetic) entropy. The calculat ed entropy difference at 23.9 K (the alpha-beta phase transition tempe rature) agrees surprisingly well with the experimentally measured heat of transformation.