In recent years borate-based crystals has attracted substantial interest among the research community. The overall importance of this family of materials is reflected in miscellaneous articles and several reviews that have been published over the years. Crystalline borate materials exhibit numerous interesting physical properties, which make them promising for further practical applications. Diversity of functional characteristics results from their high structural flexibility caused in the linkage of planar/non–planar BO₃ groups and BO₄ tetrahedra, which can occur as isolated or condensed structural units. This report is a brief review on crystal chemistry and structure features of anhydrous/high-temperature borates. Polymorphism of boron-oxygen radicals has been considered basing on cations’ nature and synthesis conditions. Analysis of the laws governing borates structures and general principles of their systematics was discussed. As a result, an alternative classification of anhydrous compounds has been considered. It is based on four orders of their subdivision: (1) by the variety of anion formers, (2) by the cation charge, (3) by the N = N_M:N_B, i.e., ratio of metal atoms number to the ratio of boron atoms number (N-factor) value indicating the borate structural type (if it is known), (4) by the cation type and size.

Phase relationships in the Er:Yb_xLu_1−xAl₃(BO₃)₄–K₂Mo₃O₁₀–B₂O₃–(Lu,Er,Yb)₂O₃ (x = 0–0.2, Er = 1 at%) system have been studied in the temperature range from 1150 to 900 °C. Multicomponent Li₂WO₄–B₂O₃ and K₂Mo₃O₁₀–B₂O₃–Lu₂O₃ melts were used as reasonable fluxes for the high-temperature solution growth of LuAl₃(BO₃)₄ (LuAB) single crystals. The LuAB solubility in the complex flux composition has been found at 25 wt% with decreasing temperature in the temperature interval of 1130–900 °C. (Er,Yb):LuAB single crystals were grown on dipped “point” seeds. The absorption spectra of (Er,Yb):LuAB crystals were measured at room temperature. The emission spectrum and lifetime of the ⁴I_13/2 energy level were determined.

We have optimized conditions for the flux growth of the RAl₃(BO₃)₄ (R = Y, Nd, Gd, Lu) and RMgB₅O₁₀ (R = Y, La, Gd) rare-earth borates and measured the thermal conductivity of YAl₃(BO₃)₄, NdAl₃(BO₃)₄, GdAl₃(BO₃)₄, LuAl₃(BO₃)₄, and LaMgB₅O₁₀ crystals. The results strongly suggest that these materials are potentially attractive for use as laser gain elements.

A transparent Er³⁺,Yb³⁺:GdMgB₅O₁₀ single crystal with dimensions up to 24 × 15 × 12 mm was grown successfully by the high-temperature solution growth on dipped seeds technique from K₂Mo₃O₁₀-based solvent. The grown crystal was characterized using PXRD, DSC and ATR techniques. Differential scanning calorimetry measurements and SEM analysis of the heat-treated solids revealed Er,Yb:GdMgB₅O₁₀ to be an incongruent melting compound with an onset point of 1087 °C. The absorption edge of the Er,Yb:GMBO sample is located in the region of 245 nm, which approximates a value of 4.8 eV. Absorption and emission spectra, and luminescence kinetics, were studied. The energy transfer efficiency from ytterbium to erbium ions was determined. The laser operation in continuous-wave mode was realized and output characteristics were measured. The maximal output power of 0.15 W with a slope efficiency of 11% was obtained at 1568 nm.

TmMgB₅O₁₀ spontaneous crystals were synthesized via the flux-growth technique from a K₂Mo₃O₁₀-based solvent. The crystal structure of the compound was solved and refined within the space group P2₁/n. The first principles calculations of the electronic structure reveal that TmMg-pentaborate with an ideal not defected crystal structure is an insulator with an indirect energy band gap of approximately 6.37 eV. Differential scanning calorimetry measurements and powder X-ray diffraction studies of heat-treated solids show that TmMgB₅O₁₀ is an incongruent melting compound. A characteristic band of the Tm³⁺ cation corresponding to the ³H₆ → ¹D₂ transition is observed in the photoluminescence excitation spectra of TmMg-borate. The as-obtained crystals exhibit intense blue emission with the emission peaks centered at 455, 479, 667, and 753 nm. The most intensive band corresponds to the ¹D₂ → ³F₄ transition. TmMgB₅O₁₀ solids demonstrate the thermal stability of photoluminescence.