Abstract
Ambient-pressure trigonal phase α of selenourea SeC(NH2)2 is noncentrosymmetric, with high Z' = 9. Under high pressure it undergoes several intriguing transformations, depending on the pressure-transmitting medium and the compression or recrystallization process. In glycerine or oil, α-SeC(NH2)2 transforms into phase β at 0.21 GPa; however in water, phase α initially increases its volume and can be compressed to 0.30 GPa due to the formation of α-SeC(NH2)2·xH2O. The single crystals of α-SeC(NH2)2 and of its partial hydrate α-SeC(NH2)2·xH2O are shattered by pressure-induced transitions. Single crystals of phase β-SeC(NH2)2 were in situ grown in a diamond-anvil cell and studied by X-ray diffraction. The monoclinic phase β is centrosymmetric, with Z' = 2. It is stable to 3.20 GPa at least, but it cannot be recovered at ambient conditions due to strongly strained NH...Se hydrogen bonds. No hydrogen-bonding motifs present in the urea structures have been found in selenourea phases α and β.
Highlights
IntroductionSelenourea SeC(NH2) is intriguing, because it forms crystals of enantiomorphic spacegroup symmetry, either P31 or P32, with nine independent molecules in the structure
By crystallographic standards, selenourea SeC(NH2)2 is intriguing, because it forms crystals of enantiomorphic spacegroup symmetry, either P31 or P32, with nine independent molecules in the structure
We have established that high pressure destabilizes the structure of -SeC(NH2)2 and reduces its exceptionally high Z0 number from 9 to 2
Summary
Selenourea SeC(NH2) is intriguing, because it forms crystals of enantiomorphic spacegroup symmetry, either P31 or P32, with nine independent molecules in the structure. Number increases to 3; the urea (Olejniczak et al, 2009; Roszak & Katrusiak, 2017) crystal transforms from the ambient-pressure phase I (Z0 = 0.25) to phase III (Z0 = 1) at 0.48 GPa. On the other hand, it can be argued that high pressure increases the potential energy Ep for intermolecular interactions, which differ for independent molecules. High pressure increases intermolecular interactions and the potential energy Ep of the crystal can be reduced by eliminating the independent molecules with the highest Ep (Le Chatelier’s principle), which affects the crystal symmetry and reduces Z0 Numerous examples of such a behaviour were reported: 4,40-bipyridinium perchlorate undergoes a phase transition from space group P1 (Z0 = 2) at 0.1 MPa to space group Cmc at 0.30 GPa, Z0 = 0.5 (Anioła & Katrusiak, 2017); -CD-MOF-1 space group R32 (Z0 = 1) at 0.1 MPa to space group I432 at 0.16 GPa, Z0 = 0.25 (Patyk-Kazmierczak et al, 2017) and others (Olejniczak & Katrusiak, 2008; Marciniak et al, 2016; Andrzejewski & Katrusiak, 2017).
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