Structural defects in fluoride single crystals for laser applications

The introduction of activating $Yb^{3+}$ and $Еr^{3+}$ ions belonging to the right-hand subgroup of the lanthanide series is shown to affect the mechanisms of structure formation in the $BaY_2F_8$ and $LiYF_4$ fluoride matrices. For the optical applications, it is essential that these crystals exhibit two types of microtwin structures corresponding to two types of polymorphic transformation. Another important finding is the appearance in $Еr^{3+}$-doped crystals of active centers responsible for the formation of a structure essentially different from that of the $Yb^{3+}$ activated matrix.