Solid-State Phase Transition Induced by Pressure in LiOH·H₂O

Abstract

When the free energy surface of the lithium hydroxide monohydrate crystal was explored, the high-pressure solid-state phase transition was determined. The high-pressure phase has been obtained through ab initio Car-Parrinello molecular dynamics simulation in the isothermic-isobaric ensemble. The recent metadynamics method has been applied to overcome the high activation energy barriers typical of rare events, like solid-state phase transition at high pressures. In the LiOH x H2O system, there are two kinds of H bonds: water-water and hydroxyl-water. The effect of the pressure has been investigated, to give further insight into the high-pressure phase. The strengthening of the H bonds of the system produces modifications in the water and the hydroxyl ion dipole electronic environment. The infrared spectra of both phases have been calculated and compared with experiments, and the assignment of the external modes has been discussed.

Assignment of the vibrational spectra of lithium hydroxide monohydrate, LiOH·H2O

“…The isotopic spectra 7, 9 also show a marked dependence on both 6 Li/ 7 Li and H/D substitution indicating that coupling is present, in contradiction to the conclusions of Di Petro et al 8 This is also demonstrated by setting the cross section of all the atoms in the unit cell to zero except for the species of interest, thus only motions involving these entities will contribute to the INS spectrum so generated. This is indicated by the continuous intensity between 90 and 500 cm −1 in the experimental INS spectrum, indicating that the modes are dispersed.…”

“…17,18 Table II compares geometric parameters for the structure optimized at the experimental lattice parameters and also for where both geometry and lattice optimization have been carried out. 8 The results clearly show that the expectation of rock > wag > twist is incorrect; the twist and wag are at similar frequencies with a small factor group splitting. The intramolecular bond distances and angles are the same in the two calculations and agree well with experiment (<0.02 Å for distances and 0.5 • for the angle).…”

“…Two assignments of the hydroxyl librational modes have been proposed: that they are the lowest energy (near 650 cm −1 ) modes in this group 6,8 and that they are the highest energy modes 7 (near 1000 cm −1 ). We note that the librational modes in LiOH are at low energy (419, 620 cm −1 in 7 LiOH), [28][29][30] favoring the first assignment.…”

“…1 The presence of water in the compounds markedly alters their properties and behavior. 4 The possibility of isotopic substitution on all three elements makes LiOH·H 2 O a particularly attractive subject for vibrational spectroscopy consequently, it has been studied by both infrared [5][6][7][8] and Raman spectroscopy. This is particularly the case for the water librational modes and these have been extensively studied in a large number of cases.…”

“…The strength of the hydrogen bonding present is reflected in the vibrational spectrum as changes in position, width, and intensity of the bands. [8][9][10] However, despite efforts spanning half-acentury the interpretation of the spectra is controversial with no clear consensus as to the assignments and not all of the modes have been observed. 2,3 Lithium hydroxide monohydrate, LiOH·H 2 O, provides a useful test case for the assignment of water librational modes.…”