The crystal structures of NaK₂B₉O₁₅ (a = 9.1158 (1) Å, b = 6.6246 (1) Å, c = 20.6812 (3) Å, β = 94.080 (1) °, R_p = 0.047, R_wp = 0.059, R_B = 0.026), Na(Na_.17K_.83)₂B₉O₁₅ (a = 9.0559 (1) Å, b = 6.6170 (1) Å, c = 20.5833 (3) Å, β = 94.228 (2) °, R_p = 0.053, R_wp = 0.068, R_B = 0.026), and (Na_.80K_.20)K₂B₉O₁₅ (a = 9.1534 (1) Å, b = 6.6296 (1) Å, c = 20.7422 (3) Å, β = 94.071 (1) °, Z = 4, R_p = 0.041, R_wp = 0.052, R_B = 0.023) were refined in the monoclinic space groups P2₁/c(Z = 4) using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K₃B₉O₁₅. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK₂B₉O₁₅ decomposes at about 700 °C in accordance with the peritectic reaction NaK₂B₉O₁₅↔K₅B₁₉O₃₁+liquid. The thermal expansion of NaK₂B₉O₁₅ and Na(Na_.17K_.83)₂B₉O₁₅ is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK₂B₉O₁₅ is revealed: heating of NaK₂B₉O₁₅ by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na_1-xK_x)₃B₉O₁₅ by 0.04 at% K.