Anomalous behaviour of the weak exchange coupling between water bibridged dimers in (9-aminoacridinium)(2)CuCl4 center dot H2O crystals. Temperature and pressure EPR studies

Angular, temperature (4.2-300 K) and pressure (0-400 MPa) variations of the EPR spectra of (9-aminoacridinium)(2)CuCl4. H2O single crystals show that magnetic dimers of CuCl4 bibridged by water molecules exist with a triplet state produced by dipolar coupling between Cu2+ ions. This is consistent wi th preliminary structural x-ray diffraction data. Using a decoupling comput er procedure the superexchange coupling parameter between dimers was determ ined as J(eff) = 0.0040 cm(-1) at room temperature. The parameter depends s trongly on temperature and pressure with an anomalous minimum in J(T) at ab out 200 K. Such behaviour is discussed in terms of a phenomenological model of competing contributions to the effective superexchange coupling based o n existing superexchange theories. We have found that the interdimer exchan ge coupling is antiferromagnetic, and leads to a magnetic ordering at very low temperatures as observed by the huge EPR line broadening.