Several X-ray induced paramagnetic defects, which were stable up to room temperature, were generated when single crystals were irradiated at 77K. In previous electron paramagnetic resonance investigations, 2 of these stable defects (labelled C and Z) had been described as unusual and were thought to be coupled to each other by a ferrimagnetic interaction of about 23/cm at low temperatures. New electron paramagnetic resonance experiments on recently synthesized single crystals, performed at temperatures down to 4.2K, revealed that C and Z were not correlated. The Z centre was shown to be Cr3+ in a Si site, with an unusually high zero-field splitting constant of about 1.5T. This was suggested to be caused by the strong crystal field due to the two Zr4+ ions along the c-axis. The C centre was proposed to be an O hole centre in which the 2p hole orbital of O was oriented towards a nearest-neighbour Zr4+ vacancy which stabilized the hole via Coulomb forces. This explained the high thermal stability of the centre. Hyperfine interaction with one 29Si and one 91Zr neighbour was explained by near-complete orbital quenching.

Structural Models for Room Temperature Stable Radiation-Induced Centres in Zircon. R.F.C.Claridge, W.C.Tennant, S.Schweizer, J.M.Spaeth: Journal of Physics - Condensed Matter, 1999, 11[43], 8579-89