[en] We have found conditions for reproducible spontaneous crystal growth of (R_xLa_{1 –x})Al_2.07(B₄O₁₀)O_0.6 (R = Ce, Pr; x = 0, 0.003, 0.03, 0.5, 1) dimetaborate solid solutions. The composition and morphology of the crystals have been studied and limitations on isomorphous substitutions on the rare-earth site within the stability range of their structure type have been found out. We have studied the thermal properties of lanthanum dimetaborate. According to differential thermal analysis, X-ray diffraction, and analytical scanning electron microscopy data, this compound melts incongruently in the range 1120–1200°C, with a maximum endothermic peak at 1165°C. The decomposition products have been shown to include Al₁₈B₄O₃₃, Al₄B₂O₉, and LaBO₃.