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

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Abstract
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 Cr3+ ions occupying Al3+ 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 Cr3+ ions occupying first, second and third nearest neighbour Al3+ 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 Cr3+ 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.
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