A review on phase transition and structure-performance relationship of second-order nonlinear optical polymorphs

Inorganic second-order nonlinear optical (NLO) crystals have become increasingly important to promote the laser frequency-conversion technology and scores of them crystallize in the polymorphic forms. Note that polymorphs possess identical chemical composition but multiple phase-transitions and they have drawn extensive concern as promising NLO candidates owing to their inherent structural diversity and attractive properties. However, the systematic review on NLO polymorphs is absent so far, thus, studying the intrinsic relationship of “phase transition–structural change–NLO performance” on NLO polymorphs is urgent and necessary. In this review, we have done the systematic summary on the synthetic condition, phase-transition and physicochemical performances in 51 known NLO polymorphs (25 oxides, 5 halides and 21 chalcogenides) under two limited conditions: (i) one phase crystallizes in the NCS space group; (ii) experimental or/and theoretical NLO performances of one NCS phase should be reported. Survey result shows that the achievement of multiple phase changes (i.e. non-centrosymmetric (NCS) to CS or NCS to NCS) can be summarized as the regulating effect of pressure, temperature, molar ratio or mixed factors and incorporation of mixed anionic groups, halogen atoms or distorted SOJT/SALP cations centered ligands into crystal structures afford the enhanced possibility for the born of NLO polymorphs. Moreover, polymorphs reflected into crystal structures are mainly the inconformity of atomic coordination environments or/and motif arrangement modes that further result in alterable material performances, especially critical second-order NLO effect. This review indicates that exploring new polymorphs is one of important research directions to design and synthesize functional materials with tunable crystal structures and performances for the promising application in the NLO region.