4. Interactions between rare earth ions

When the concentration of rare earth ions is increased in a host matrix, either for the same type of ion or for different types (codoping), ion-ion interactions occur. Generally speaking, the phenomenon takes place when the average distance between the two ions reaches some 2 nm, i.e. assuming a uniform distribution, when the ion concentration reaches some 10 ²⁷ m ^–3 . If the ions are found in clusters, interaction can occur as early as 10 ²³ ions · m ^–3 .