with 𝜙 Ln as the Ln 3+ (Ln = Nd, Ho) activator content (𝜙 Ln = 15 mol%, 30 mol%), CET as the effective interaction strength for a Ln 3+ → Ln 3+ energy transfer and nj as the number of available sites for the acceptor ions in a shell j with a distance rj to the donor. Only contributions from Ln 3+ acceptors at distances up to a cutoff of rcutoff = 20 Å (30 shells) were considered. agreement between the fits and the experimental decay curves indicates the validity of the structural models and assumptions above. From the resulting transfer-specific parameters CET = (65.45 ± 5.61) Å 6 µs -1 (Ln = Nd) and CET = (131.74 ± 1.58) Å 6 µs -1 (Ln = Ho) also a critical distance rc for the cross-relaxation processes can be estimated,