Structural study of nonlinear optical borates K1-xNaxSr4(BO3)3 (x≤0.5)

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)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)(BO3)(3) borates changes from cubic to orthorhombic and then to monoclinic. (C) 2010 International Centre for Diffraction Data. [DOI: 10.1154/1.3478412]