Optical and electron spin resonance measurements were performed on type-IIa diamond which had been electron-irradiated at room temperature and isochronally annealed at high temperatures. It was found that the electron spin resonance center, R4/W6, formed when the vacancy became mobile. Evidence was presented which supported a di-vacancy model for R4/W6.

M.A.Lea-Wilson, J.N.Lomer, J.A.Van Wyk: Philosophical Magazine B, 1995, 72[1], 81-9