WEAK LONG-DISTANCE SUPEREXCHANGE INTERACTION AND ITS TEMPERATURE-VARIATIONS IN COPPER(II) COMPOUNDS STUDIED BY SINGLE-CRYSTAL EPR

In this paper we review experimental EPR data and theoretical models o f the weak superexchange interaction (J < 0.1 cm(-1)) mainly in copper (II) compounds single crystals. The different EPR techniques are descr ibed which allow one to determine the exchange coupling such weak as J = 0.0001 cm(-1). Such a weak interaction is detectable by EPR on dist ances up to 25 Angstrom and the presented data allowed us to establish empirical limits for a weak exchange coupling transmission for J < 0. 3 cm(-1) as \J(iim)\ = 5.9exp(-0.335R) where R (Angstrom) is the inter molecular distance. The weak superexchange coupling was found to be te mperature dependent and this dependence is very specific to a crystal or molecular structure. In most cases temperature dependence of supere xchange is governed by thermal crystal lattice contraction. A phenomen ological model of competing potential and kinetic exchange is presente d and used for description of copper(II) compounds with opposite tempe rature behaviour of superexchange coupling. Possible mechanisms drivin g temperature variations of superexchange are discussed.