A new high-pressure polymorph of phosphoric acid

Abstract

The high-pressure structural behaviour of phosphoric acid is described. A compression study of the monoclinic phase, using neutron powder diffraction and X-ray single-crystal diffraction, shows that it converts to a previously unobserved orthorhombic phase on decompression. Compression of this new phase is reported up to 6.3 GPa. The orthorhombic phase is found to be more efficiently packed, with reduced void space, resulting in a larger bulk modulus. Molecule-molecule interaction energies reveal a more extensive network of increased attractive forces in the orthorhombic form relative to the monoclinic form, suggesting greater thermodynamic stability.

Effect of High Pressure on the Crystal Structures of Polymorphs of l-Histidine

“…More data are available by spreading the net beyond organic solids. Neutron powder diffraction experiments performed on the monoclinic polymorph of phosphoric acid showed that it converts to a more efficiently packed, and thus harder, orthorhombic phase during pressure release at 1.97 GPa; the bulk moduli being 13.81(15) and 17.61(36) GPa, respectively, following the expected trend . A similar study performed on the orthorhombic form of Sc 2 W 3 O 12 showed that it undergoes a phase transition between 0.25 and 0.3 GPa to a softer monoclinic phase, with bulk moduli respectively of 32(2) and 11.8(8) GPa .…”

“…Neutron powder diffraction experiments performed on the monoclinic polymorph of phosphoric acid showed that it converts to a more efficiently packed, and thus harder, orthorhombic phase during pressure release at 1.97 GPa; the bulk moduli being 13.81(15) and 17.61(36) GPa, respectively, following the expected trend. 71 A similar study performed on the orthorhombic form of Sc 2 W 3 O 12 showed that it undergoes a phase transition between 0.25 and 0.3 GPa to a softer monoclinic phase, with bulk moduli respectively of 32(2) and 11.8(8) GPa. 72 A Raman and synchrotron X-ray diffraction study on three polymorphs of alane, AlH 3 (orthorhombic γ, trigonal α, cubic β), found their bulk moduli to be 42(2), 45(3) and 58(4) GPa, respectively.…”