The extent of cross-relaxation is dependent on the mutual distance between the luminescent lanthanoid activators. At higher activator fractions, the probability of the occupation of two nearest-neighbouring Gd sites by the incorporated activators increases and thus, makes an energy transfer gradually more probable than spontaneous (radiative + nonradiative) decay. It is, however, noteworthy that the luminescence is still not excessively quenched at nominal activator concentrations as high as 30 mol%. This feature is a general advantage of garnettype hosts and was one of the major reasons for the success of YAG:Nd 3+ as a lasing gain medium.