Pressure-induced isosymmetric phase transition in biurea

Abstract

An isosymmetric phase transition of biurea has been found in which the molecules appear to “snap” from one conformer to another upon compression.

Suppression of isotopic polymorphism

“…The same behaviour has also been seen in biurea where a low-pressure form was stabilised via entropic contributions, before eventually being overcome by more favourable internal energy and packing in a closely-related conformational polymorph. 45 An assessment of a large range (475 pairs) of crystalline polymorphs indicated that vibrational energy contributions resulted in diverging free energy curves versus temperature for 38% of the structures investigated. 46 Thus, ranking relative polymorph stabilities on lattice energy alone can often be too simplistic.…”

Can We Predict the Pressure Induced Phase Transition of Urea? Application of Quantum Molecular Dynamics

“…One of the methods that can be used to obtain new polymorphs is exposing the molecular crystals to high-pressure in order to induce the phase transition [1]. Many studies deal with the experimental pressure-induced polymorphic transformations in molecular solids [2][3][4], in some cases the high-pressure polymorph may have the same space group symmetry as the original ambient-pressure form and even be isostructural with it, which is defined as isosymmetric phase transition [5,6]. However, in most cases the polymorphic phase transition is accompanied with the change of not only cell dimensions but also the crystal space group.…”

Can We Predict the Isosymmetric Phase Transition? Application of DFT Calculations to Study the Pressure Induced Transformation of Chlorothiazide

“…To be precise, it must be stated that some of the crystals that have been proven experimentally to undergo IPT were also modeled using the DFT methods. However, in those cases, the computational part was limited to the geometry optimization at the pressure at which the particular structure was obtained, followed sometimes by the thermodynamic parameters calculations [ 10 , 11 ].…”

“…Isosymmetric structural phase transition (IPT, type 0), in which there are no changes in the occupation of Wyckoff positions, the number of atoms in the unit cell, and the space group symmetry are relatively uncommon [5]. However, there are some well-known and recently discovered examples of such transitions: sodium oxalate [6], 1,3-cyclohexandione [7] L-serine [8], sulfamic acid [9], biurea [10], and α-glycylglycine [11].…”