High-resolution Fourier transform spectroscopy of Fe2+ luminescence and absorption in polymorphic material was used to monitor the energy transfer (cross-relaxation) mechanism between various polytypic Fe2+ centers. In order to obtain further insight into these processes, calorimetric absorption and transmission spectroscopy were used to determine quantitatively the non-radiative relaxation of the Fe2+ transition. Quantum efficiencies of 10 and 20% were found for the 5T2  5E transition of the cubic and 2 axially distorted Fe2+ centers, respectively. With increasing Fe concentration, this behavior changed and the radiative relaxation decreased for Fe2+ centers with a higher transition energy.

A.Hoffmann, L.Podlowski, P.Thurian, R.Heitz, I.Broser, F.Fuchs, P.Koidl: Materials Science Forum, 1994, 143-147, 411-6