Exchange-coupled pairs of Cr3+ions in emerald (in EPR spectra)

The EPR spectra of synthetic and natural samples of chromium-doped beryllium aluminium silicate (emerald) show a large number of resonance lines in addition to those arising from isolated Cr 3+ ions occupying Al 3+ sites. The angular and temperature dependence of the spectra indicate that most of the additional lines arise from three types of exchange-coupled pairs of Cr 3+ ions occupying first, second and third nearest neighbour Al 3+ sites, with antiferromagnetic exchange coupling constants of approximately 2.260, 0.195 and 0.026 cm -1 respectively. The spectra have been fitted with a spin Hamiltonian which includes anisotropic bilinear exchange, Zeeman and crystal field terms. Dynamic strain experiments for the first nearest-neighbour Cr 3+ ion pair suggest that the magnetostriction contribution to the biquadratic exchange is negligible, and so the upper limit of 1% refers to intrinsic biquadratic exchange.