Temperature-Controlled Polymorphism in Organic and Semi-Organic Nonlinear Optical Crystals: A Critical Review of Thermal, Structural, Dielectric, and Mechanical Properties

The literature confirms that crystallization temperature is a critical, controllable parameter for selecting among polymorphic forms of organic and semi-organic NLO crystals, and that the resulting structural differences can substantially alter SHG efficiency, optical transparency, dielectric behaviour, and thermal stability. At the same time, the existing body of work is dominated by single-polymorph characterization rather than systematic, temperature-controlled comparison, and organic–semi-organic comparisons under matched growth conditions remain particularly sparse. Closing this gap — through slow evaporation solution growth under carefully controlled temperature conditions, paired with integrated structural, thermal, dielectric, and SHG characterization — offers a promising route toward predictive, rather than empirical, design of next-generation photonic materials [2,6].

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