Second-order nonlinear optical crystals with mixed anions

Crystals with mixed anions refer to inorganic salts containing at least two types of anions or anionic groups, and these anions can be Q2− (Q = O, S, Se, Te) and X− (X = F, Cl, Br, I), and anionic groups include various B–O groups, (OH)−, (IO3)−, (CO3)2−, (SiO4)4−, (NO3)3− and (PO4)3−. Recently, in view of the insufficiency and strong requirements of second-order nonlinear optical (NLO) crystals, especially ones in the deep ultraviolet (DUV) and middle and far-infrared (MFIR) regions, the study of NLO crystals have received intensive interest from researchers. For a NLO crystal candidate, desirable properties include large NLO coefficients, high laser-induced damage threshold values, phase matchabilities and wide transparent windows. The availability of the former two properties is especially problematic since the structural factor influences them in reverse order. To balance them, it is necessary to introduce different anions into the structure to induce them to function differently. Under the guidance of anionic group theory and the functional moiety concept, many NLO crystals with mixed anions have been obtained. However, to date, a systematic survey of this topic has not been carried out. In this review, recently reported NLO crystals with mixed anions are summarized, the contents of which are mainly focused on their crystal structures and NLO behaviors, together with the relationship between these two aspects. It is hoped that this work will provide a useful perspective on the most promising NLO candidates.