MPb₂Br₅(M=K, Rb):RE crystals attract attention due to a possibility of laser generation beyond 4μm limit owing to their lower vibrational frequencies and higher quantum yields. MPb₂Hal₅ are transparent from 0.3 to 30 μm, have satisfactory mechanical properties, high chemical resistance, extremely low maximum phonon energy (~140 cm^-1) and as a result lower non-radiative rates. But their use as laser matrices is associated with some difficulties. KPb₂Br₅ (KPB) is monoclinic and contains 2 Pb positions: with coordination numbers (CN) 8 and 7 and Pb²⁺ radii 1.22Å and 1.10Å, respectively. The second position is suitable for RE doping. Unfortunately, KPB has a poor optical quality due to a phase transition during growth. RbPb₂Br₅ (RPB) don’t have phase transitions below the melting point and can be used to produce high quality and large size elements. RPB is tetragonal, Pb²⁺ occupy one position (CN=8, R_ion=1.22Å). Its size exceeds RE radii, so their distribution coefficient in RPB is low. RPB exhibits an expressed temperature expansion and optical properties anisotropy. There is no such effect in KPB because of significant difference in the unit cell parameters ratio a,b/c, a/c in KPB, RPB.

To obtain an efficient material with an optimal set of characteristics, authors obtained K_0.5Rb_0.5Pb₂Br₅:REE crystals (P2₁/c). They are characterized by minimal anisotropy of thermal and spectral parameters along different crystallographic directions. Transparency range and the dominance of the radiative relaxation mechanism up to 10μm, typical for RPB and KPB, are preserved, therefore K_0.5Rb_0.5Pb₂Br₅:REE is a potential mid-IR laser matrix.

This work was partly done on state assignment of IGM SB RAS, Ministry of Science and Higher Education of the Russian Federation and was supported by Russian Foundation for Basic Research (grant # 19-42-540012).