Research on Nonlinear Optical Materials Based on BO₃ and BO₄ Hydroxy Tetraborates

Ultraviolet (UV) and deep ultraviolet (DUV) nonlinear optical (NLO) materials have received extensive attention due to their wide applications. Borate NLO crystals have become key materials in the field of UV and DUV laser frequency conversion because of their excellent NLO performance. The boron‐oxide framework formed by flexible assembly of [BO 3 ] planar triangular and [BO 4 ] tetrahedral elements provides a rich structural basis for materials with wide optical bandgaps, high birefringence performance, and strong second‐order NLO effects. Here, the [B 4 O 5 (OH) 4 ] 2− based compounds are reviewed for the following reasons: 1) Based on the B 4 O 5 (OH) 4 framework, flexibly assembled units from [BO 3 ] and [BO 4 ] may result in many compounds; 2) The four OH groups within [B 4 O 5 (OH) 4 ] 2− facilitate the formation of a complex and highly oriented hydrogen‐bonding network to promote the formation of NCS structure; 3) The four OH groups help to break them down into low‐dimensional structures to have large birefringence; 4) The incorporation of [B 4 O 5 (OH) 4 ] 2− unit into alkali or alkaline metal system is useful to make compounds with short UV cutoff edge; 5) Compounds with [B 4 O 5 (OH) 4 ] 2− unit may have suitable SHG effects. It is believed this review will assist researchers in finding more B 4 O 5 (OH) 4 based compounds and exploring new promising UV/DUV NLO borate materials.