Reanalysis and identification of an Rh2+ dimer center in NaCl by combined application of 9.5 and 95 GHz EPR

Abstract

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 dominant symmetry axis along 〈001〉, the z-axis, is observed for this defect. The dimer and monomer spectra, which strongly overlap in the X-band spectra, could be well separated in the W-band measurements due to the higher g-resolution at higher magnetic fields. The anisotropy in the perpendicular plane, with 〈110〉 symmetry directions, is shown, from the combined X-and W-band spectra, to derive predominantly from a field-independent contribution, i.e. an orthorhombic spin–spin interaction, S₁·D·S₂ (S₁ = S₂ = 1/2), while the g matrices are near to axial, as in the case of the RhCl₆⁴⁻·2Vac monomer center (Vac = cation vacancy). This dimer center is shown to consist of a square arrangement of two substitutional Rh²⁺ ions and two next-nearest neighbor cation vacancies in an {001}-plane, with the Rh²⁺ ions on opposite corners at a distance of ca. 7.6 Å, as estimated from the magnitude of the dipolar interaction. This work illustrates the ability of combined EPR experiments at widely different frequencies to discriminate field-independent from field-dependent terms in the spin-Hamiltonian.