Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known struc...

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Bibliographic Details
Published in:The Journal of chemical physics 2024-03, Vol.160 (9)
Main Authors: Paschoal, Vitor H., Ribeiro, Mauro C. C.
Format: Article
Language:English
Subjects:
Ambient temperature
Anions
Choline
Density functional theory
Frequency ranges
Hydrogen bonding
Ion currents
Lattice vibration
Low temperature
Melting
Phase transitions
Raman spectra
Spectrum analysis
Temperature dependence
Thermodynamic properties
Vibrational spectra
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Summary:Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid–solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4].
ISSN:0021-9606
1089-7690
DOI:10.1063/5.0189049