REANALYSIS AND IDENTIFICATION OF AN RH2+ DIMER CENTER IN NACL BY COMBINED APPLICATION OF 9.5 AND 95 GHZ EPR

A combined 9.5 (X-band) and 95 GHz (W-band) EPR study of an Rh2+ dimer center in Bridgman-grown rhodium-doped NaCl is presented. A close-to- tetragonal symmetry, similar to that of the monomer centers, with domi nant symmetry axis along [001], the z-axis, is observed for this defec t. The dimer and monomer spectra, which strongly overlap in the X-band spectra, could be well separated in the W-band measurements due to th e higher g-resolution at higher magnetic fields. The anisotropy in the perpendicular plane, with [110] symmetry directions, is shown, from t he combined X-and W-band spectra, to derive predominantly from a held- independent contribution, i.e. an orthorhombic spin-spin interaction, S-1.D.S-2 (S-1 = S-2 = 1/2), while the g matrices are near to axial, a s in the case of the RhCl64-.2Vac monomer center (Vac = cation vacancy ). This dimer center is shown to consist of a square arrangement of tw o substitutional Rh2+ ions and two next-nearest neighbor cation vacanc ies in an {001}-plane, with the Rh2+ ions on opposite corners at a dis tance of ca. 7.6 Angstrom, as estimated from the magnitude of the dipo lar interaction. This work illustrates the ability of combined EPR exp eriments at widely different frequencies to discriminate field-indepen dent from field-dependent terms in the spin-Hamiltonian.