We have studied and optimized conditions for spontaneous flux growth of RMgB₅O₁₀ (R = Y, La, Gd) crystals and assessed the feasibility of seeded growth of such crystals. Using high-temperature solution growth with K₂Mo₃O₁₀ as a solvent, we have prepared optical-quality (Er,Yb):YMgB₅O₁₀ single crystals and measured their thermal conductivity and optical characteristics to assess their potential for use in diode-pumped lasers operating in the spectral range 1.5–1.6 μm. Absorption cross section spectra of an (Er,Yb):YMgB₅O₁₀ single crystal have been measured in the spectral ranges 800–1100 and 1400–1650 nm. The results demonstrate that (Er,Yb):YMgB₅O₁₀ single crystals are similar in characteristics to (Er,Yb):YAl₃(BO₃)₄, promising for use in advanced near-IR laser systems, and are capable of replacing RAl₃(BO₃)₄ materials in such devices because they are simpler to grow.

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.

A transparent Yb:YMgB₅O₁₀ single crystal with dimensions up to 25 × 23 × 25 mm and weight 10.337 g was grown using a high-temperature solution growth on dipped seeds technique with a K₂Mo₃O₁₀ solvent. The Yb³⁺ concentration was calculated to be 4.7 at.% (N_Yb = 3.71 × 10²⁰ atoms/cm³) with a distribution coefficient K_d of 0.59. The grown crystal was characterized by means of PXRD, TGA-DSC and ATR-FTIR techniques. The spectroscopic characteristics of the Yb:YMgB₅O₁₀ crystal were demonstrated. Absorption cross-section spectra were produced. The luminescence spectra of the Yb:YMgB₅O₁₀ crystal were measured in the spectral range of 950–1100 nm. The luminescence kinetics of the ²F_5/2 energy level were investigated and the lifetime was determined.

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.