Structural study of nonlinear optical borates K 1− x Na x Sr 4 (BO 3 ) 3 ( x ≤0.5)

X-ray powder diffraction was used for the structural study of nonlinear optical borates K 1− x Na x Sr 4 (BO 3 ) 3 ( x ≤0.5). Results show that up to 50% K + can be substituted by Na + in orthorhombic K 1− x Na x Sr 4 (BO 3 ) 3 . Isolated BO 3 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 BO 3 triangles and asymmetric distortion of (K/Na)O 8 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 (BO 3 ) 3 borates changes from cubic to orthorhombic and then to monoclinic.