True Polymorphic Phase Transition or Dynamic Crystal Disorder?: An Investigation into the Unusual Phase Behavior of Barbituric Acid Dihydrate

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Crystalline barbituric acid dihydrate (BTADH) was previously thought to undergo a subtle temperature-dependent phase transition from a high-temperature orthorhombic Pnma space group to a nonmerohedrally twinned monoclinic P21/n phase below a transition temperature of ∼217 K. Questions remain on the true nature of the unusual transformation and the structure of the high-temperature crystal phase due to difficulties in resolving the Pnma structure from X-ray diffraction data and because of the subtlety of the structural transition. In this study, terahertz (THz) spectroscopy and solid-state density functional theory (DFT) were utilized to explore this suspected phase transition and uncover the principle physical mechanisms contributing to this anomalous transition. Our findings suggest that at temperatures above the previously reported phase transition temperature of 217 K, the crystal does not exist in the Pnma space group configuration. However, two equivalent favorable energetic states with P21/n symmetry related by the twinning plane lie on either side of the proposed Pnma structure. It is these same local potential energy minima that guide the crystal system to a nonmerohedrally twinned configuration identified in the lowtemperature crystal structures. At temperatures above 217 K, the system possesses the thermal energy necessary to readily transition between these degenerate states separated by the low-lying Pnma structural barrier. The evidence acquired by THz spectroscopy and DFT indicate the absence of a true temperature-dependent phase change, and rather a system that exists in a tenuous thermally disordered state above the previously alleged transition temperature.