Prediction and analysis of symmetry-raising transitions in anilinium tetrafluoroborate

We report the structural and dielectric characterization of anilinium tetrafluoroborate (AnBF₄), a molecular crystal identified as a candidate for symmetry-raising phase transitions using the FERROSCOPE routine. Variable-temperature powder X-ray diffraction and differential scanning calorimetry revealed a sequence of three structural phase transitions between room temperature and 490 K, yielding four polymorphs (A-D), three of which are previously unreported. All structures were solved either by single-crystal X-ray diffraction or ab initio from powder diffraction data. Single crystal experiments indicate diffuse scatter in polymorph C which can be related to the structural changes between the different polymorphs. The structural transitions have been rationalized using group-subgroup relationships. Dielectric measurements show a temperature-dependent response consistent with the structural changes, with the A to B transitions associated with a peak in the dielectric constant.