Review of Heteroleptic Tetrahedra as Birefringent or Nonlinear Optical Motifs

Birefringence and second harmonic generation are important to realizing the phase-matching capability and laser frequency conversion efficiency in a nonlinear optical (NLO) crystal. Heteroleptic tetrahedra have usually been used to design and explore birefringent or NLO materials, such as fluorooxo-heteroleptic tetrahedra BOxF4–x (x = 1, 2, 3), POxF4–x (x = 2, 3), SO3F and other groups SiO4–xNx (x = 1, 2, 3), PO4–xSx (x = 1, 2, 3). Generally, heteroleptic tetrahedra exhibit larger microscopic properties than those of normal oxy-tetrahedra MO4 (M = B, P, S, Si, etc.), which will improve the birefringence or NLO coefficients in a crystal. In this review, we systematically investigated the microscopic properties of heteroleptic tetrahedra which cover the deep-ultraviolet to the mid-infrared region, including SiO4–xNx (x = 1, 2, 3), PO4–xSx (x = 1, 2, 3), POS(NH2)2, PO2S(NH2), AsO4–xSx (x = 1, 2), PO4–xNx (x = 1, 2, 3), GaO4–xSx (x = 1, 2, 3), GeQ4–xTx (Q = O, S; T = OH, N, NH2; x = 1, 2, 3), SO3T (T = F, NH2, S), SiOS3, ZnO4–xSx (x = 2, 3), and BO3N. The structure-performance relationships of the related compounds with the above-mentioned heteroleptic tetrahedra were discussed through first-principles calculations. According to the computational results, some compounds with heteroleptic tetrahedra have good optical performances and can be used as potential ultraviolet or mid-infrared optical functional materials. This work will provide helpful insights into rational design and synthesis of new optical functional materials containing heteroleptic tetrahedra.