Anomalous behaviour of the weak exchange coupling between water bibridged dimers in crystals. Temperature and pressure EPR studies

Angular, temperature (4.2-300 K) and pressure (0-400 MPa) variations of the EPR spectra of single crystals show that magnetic dimers of bibridged by water molecules exist with a triplet state produced by dipolar coupling between ions. This is consistent with preliminary structural x-ray diffraction data. Using a decoupling computer procedure the superexchange coupling parameter between dimers was determined as at room temperature. The parameter depends strongly on temperature and pressure with an anomalous minimum in J(T) at about 200 K. Such behaviour is discussed in terms of a phenomenological model of competing contributions to the effective superexchange coupling based on existing superexchange theories. We have found that the interdimer exchange coupling is antiferromagnetic, and leads to a magnetic ordering at very low temperatures as observed by the huge EPR line broadening.