With the development of technology, the need for highly efficient functional materials is steadily increasing. Currently, borates attract the attention of researchers, as they are promising nonlinear materials. Potassium rubidium aluminum borate based on potassium aluminum borate (trigonal syngony, space group P321, Z = 3) was obtained by solid-phase synthesis. The individuality and purity of the borates were confirmed by X-ray diffraction. Analysis of differential scanning calorimetry and thermogravimetric method for K 2(1-x) Rb 2x Al 2 B 2 O 7 (x = 0.1-0.8) was performed in the temperature range of 25-1075°С. Potassium rubidium borates decompose in the temperature range of 900-1000°C. Differential scanning calorimetry, dielectric loss tangent, and second-harmonic generation data revealed phase transitions for K 0.6 Rb 1.4 Al 2 B 2 O 7. A significant SHG effect was found at room temperature for K 0.6 Rb 1.4 Al 2 B 2 O 7 (Q = 70). Then the SHG effect increases to Q = 85 at a temperature of 645°C and remains constant with a further increase in temperature. The new triple borate Cs 1.39 Tl 0.61 Al 2 B 2 O 7 was synthesized by the solid-phase synthesis, and its crystallographic parameters were obtained by the Rietveld method. This borate crystallizes in the monoclinic space group P2 1 / c with the unit cell parameters: Z = 2, a = 6.6669(3) А , b = 7.2991(3) А , c = 9.3589(4) А , β =116.6795(18)°, V = 406.94(3) А 3. The structure can be considered to be built up from the nearly planar [Al 2 B 2 O 10 ] rings, which are composed of two AlO 4 tetrahedra and two BO 3 triangles, connected, alternately to each other by corner-sharing.
