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.

无机二阶非线性光学（NLO）晶体对于促进激光频率转换技术已经变得越来越重要，并且它们中的许多以多晶型形式结晶。请注意，多晶型物具有相同的化学组成，但具有多个相变，由于其固有的结构多样性和吸引力，它们作为有希望的NLO候选物引起了广泛关注。但是，到目前为止，还没有关于NLO多态性的系统评价，因此，研究NLO多态性的“相变-结构变化-NLO性能”的内在联系非常必要。在这篇综述中，我们对51种已知的NLO多晶型物（25种氧化物，（5个卤化物和21个硫属化物）在两个有限的条件下：（i）一相在NCS空间群中结晶；（ii）应报告一个NCS相的实验或理论NLO性能。调查结果表明，实现了多个阶段的变化（即与CS的非中心对称（NCS）或NCS到NCS）可以归纳为压力，温度，摩尔比或混合因素的调节作用，以及将混合的阴离子基团，卤素原子或扭曲的SOJT / SALP阳离子为中心的配体掺入晶体结构，为NLO的诞生提供了更大的可能性多态。此外，反射到晶体结构中的多晶型物主要是原子配位环境或/和基序排列模式的不符合，这进一步导致材料性能发生变化，尤其是关键的二阶NLO效应。这项审查表明，探索新的多晶型物是设计和合成具有可调晶体结构和性能的功能材料的重要研究方向之一，从而有望在NLO地区应用。