Ions of d elements in complex oxides with olivine and phenakite structures exhibit diverse structural localization and may incorporate into crystals in various oxidation states. The complexity of the structure of these materials requires a complex approach in investigation of their spectroscopic characteristics with regard to the type of optical center and the growth conditions. The method of estimation of the distribution of an activator ion over crystallographically non-equivalent cationic positions on the basis of the symmetry analysis of the structure of these positions with regard to the spatial distribution of the activator-ion electron density is described. The results of experimental studies of the spectroscopic characteristics of vanadium and chromium ions in the crystals with the olivine (Mg₂SiO₄, Ca₂GeO₄, and CaMgSiO₄) and phenakite (LiGaSiO₄, LiAlGeO₄, and Zn₂SiO₄) structures performed by us (some experimental data are reported for the first time) are analyzed with regard to the growth conditions. Data in the literature are also analyzed. In order to estimate the structural and valence states of the d ions, a number of other experimental techniques were used, and the experimental data were compared with the calculated energy-level diagrams for activator ions in centers of various types. This approach made it possible to adequately describe the possibilities of complex oxides Mg₂SiO₄, Ca₂GeO₄, CaMgSiO₄, LiGaSiO₄, LiAlGeO₄, and Zn₂SiO₄ doped with d ions, regarded as new laser media.

Kinetic order-disorder phase transitions (space group space group $ I\bar 4 $ ↔ space group I4₁/a) have been considered for nonactivated and activated scheelite compounds (Na_0.5Gd_0.5)WO₄ (NGW), (Na_0.5Gd_0.5)MoO₄ (NGM), (Na_0.5La_0.5)WO₄(NLW), and (Na_0.5La_0.5)MoO₄ (NLM) synthesized by the Czochralski method and structural and growth sources of crystal dissymmetrization have been suggested. For NGW, it was shown that an increase in the difference between the content of Gd and Na in two positions of the structure with space group $ I\bar 4 $ and their ratio leads to an increase in the deviation from centrosymmetry. On the basis of available literature data and our results, it was demonstrated that the degree of order depends on the initial composition of the reaction mixture, crystal growth and cooling rates, activator concentrations, and postgrowth treatment conditions. The inconsistency between X-ray diffraction data and asynchronous second harmonic generation studies was explained by the possibility of formation of centrosymmetric superstructures and/or local ordering of atoms.

Single crystals of scheelite-like disordered double molybdate NaLa(MoO₄)₂ doped with thulium ions with dopant concentrations of 3.0 × 10¹⁹ and 1.2 × 10²⁰ cm⁻³ are grown by the Czochralski method. Spectral and luminescent studies of crystals grown are performed in wavelength ranges of 800 nm (the transitions ³ H ₆↔³ H ₄) and 2 μm (the transitions ³ H ₆↔³ F ₄), including the measurements of the polarized absorption and luminescence spectra of these crystals and the decay kinetics of their excited states. A model that satisfactorily describes the decay kinetics of the luminescence from the ³ H ₄ level taking into account cross-relaxation interactions is proposed. The prospects for the use of these crystals as active media in two-micrometer solid-state lasers are discussed.