The exploitable properties of many functional materials are intimately linked with symmetry-changing phase transitions. These include properties such as ferroelectricity, second harmonic generation, conductivity, magnetism and many others. We describe a new symmetry-inspired method for systematic and exhaustive evaluation of the symmetry changes possible in molecular systems using molecular distortion modes, and how different models can be automatically tested against diffraction data. The method produces a quantitative structural landscape from which the most appropriate structural description of a child phase can be chosen. It can be applied to any molecular or molecular-fragment containing material where a (semi) rigid molecule description is appropriate. We exemplify the method on 5,6-dichloro-2-methylbenzimidazole (DC-MBI), an important molecular ferroelectric. We show that DC-MBI undergoes an unusual symmetry-lowering transition on warming from orthorhombic Pca21 ( T ≲ 400 K) to monoclinic Pc. Contrary to expectations, the high temperature phase of DC-MBI remains polar.

中文翻译：

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使用对称适应畸变模式分析了解室温分子铁电体 5,6-二氯-2-甲基苯并咪唑的行为

许多功能材料的可利用特性与对称变化相变密切相关。这些包括诸如铁电性、二次谐波产生、导电性、磁性等特性。我们描述了一种新的受对称启发的方法，用于系统和详尽地评估使用分子畸变模式的分子系统中可能发生的对称变化，以及如何根据衍射数据自动测试不同的模型。该方法产生一个定量的结构景观，从中可以选择最合适的子阶段结构描述。它可以应用于任何适合（半）刚性分子描述的分子或包含分子片段的材料。我们举例说明了 5,6-二氯-2-甲基苯并咪唑 (DC-MBI) 的方法，一种重要的分子铁电体。我们表明，DC-MBI 在从正交晶系 Pca21 ( T ≲ 400 K) 到单斜晶系 Pc 变暖时经历了不寻常的对称性降低转变。与预期相反，DC-MBI 的高温阶段仍然是极性的。