A study was made of the electron paramagnetic resonance and optical (4f-4f photoluminescence and photoluminescence excitation) spectra which were due to Er centers in bulk crystals that had been grown by using the high-pressure Bridgman technique. As well as the usual, almost isotropic, signal (g = 5.94) which was attributed to isolated Er3+ on a Zn lattice site, 3 strongly anisotropic electron paramagnetic resonance spectra were observed. These so-called A, B and B' spectra were attributed to transitions in non-cubic Er3+ (A) and Er2+ (B and B') centers. The symmetry axes of these centers had directions which were close to <111> (A) and were parallel to <110> (B and B'). The angular dependences of the signals were affected by twinning effects. In the case of crystals which had been doped only with ErF3, only a single type of center produced photoluminescence. This was identical to the type-A electron paramagnetic resonance center. A crystal-field analysis of the corresponding electron paramagnetic resonance and optical spectra showed that this center had a Γ6-type ground level and was characterized by a crystal-field parameter ratio, A6<r6>/A4<r4>, that was equal to -0.22. It was thought that this center was a complex that consisted of Er3+ on a Zn site and F on a nearest-neighbor interstitial site. The g-factors which were found for the electron paramagnetic resonance signals, B and B', could be explained in terms of the non-Kramers doublet ground levels of E2+ 4f12 which resulted from the splitting of cubic Γ5 triplets, due to non-cubic crystal-field components. These 2 signals were attributed to Er2+ on the 2 zincblende-type interstitial sites; each of which formed a complex with some other kind of atom on its next-nearest neighbor interstitial site. The electron paramagnetic resonance spectra were detected only in the presence of Li2CO3 co-doping.

J.Dziesiaty, S.Müller, R.Boyn, T.Buhrow, A.Klimakow, J.Kreissl: Journal of Physics Condensed Matter, 1995, 7[22], 4271-82