X-ray powder diffraction was used for the structural study of nonlinear optical borates K1−xNaxSr4(BO3)3 (x≤0.5). Results show that up to 50% K+ can be substituted by Na+ in orthorhombic K1−xNaxSr4(BO3)3. Isolated BO3 triangles in the Na-substituted compound constrict to adjust to a local distribution of alkali-metal atoms, which explains the large range of structural homogeneity. An expansion of the c axis in a unit cell with increasing Na substitution was found probably caused by the tilted BO3 triangles and asymmetric distortion of (K/Na)O8 polyhedra. As the ratio of ionic radii of alkaline-earth and alkali-metal cations decreases and the electronegative difference between alkaline-earth and alkali-metal cations increases, the crystal system of MM′4(BO3)3 borates changes from cubic to orthorhombic and then to monoclinic.
