ESR of an S=2 centre in amethyst quartz and its possible identification as the d4ion Fe4+

Abstract

A paramagnetic centre having spin S=2 and C₂ symmetry has been detected in natural amethyst crystals. The only ESR transition observed was the Delta M=+or-4 transition between the lowest two energy levels M=+or-2. Line position measurements at 35 GHz and 16 GHz gave the g-tensor principal value for the C₂ direction g_zz=1.9876+or-0.0006 and the zero-field splitting of +or-2, Delta ₂=10.166+or-0.005 GHz (at 77K). The constants D and E of the second-order zero-field splitting operator DS_z²+E(S_x²-S_y²) could not be determined; however, -D must be greater than 80 GHz and mod 3E/D mod 4+ ion on the tetrahedral silicon site of the quartz lattice.