(K_1-xNa_x)₂Al₂B₂O₇ with 0 ≤ x < 0.6: A Promising Nonlinear Optical Crystal

Up to about 60% K + in K 2 Al 2 B 2 O 7 (KABO), a well-known nonlinear optical material, can be substituted by Na + with the structure type being retained. AlO 4 tetrahedra and BO 3 triangles in the Na-substituted compound tilt to adjust to the local distribution of alkali-metal atoms, which explains the large range of homogeneity. When the Na + substitution is higher than 50%, the decomposition and volatility of KABO are strongly inhibited and the sample melts congruently at about 940 °C, which makes it possible to grow bulk crystals from the melt directly. The second harmonic generation property of KNaAl 2 B 2 O 7 is close to that of KABO. Taking the crystal growth into account, (K 1 - x Na x ) 2 Al 2 B 2 O 7 (0 ≤ x < 0.6) seems to be a more promising nonlinear optical material than KABO.