Density functional theory studies on the electronic, structural, phonon dynamical and thermo-stability properties of bicarbonates MHCO3, M = Li, Na, K

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO3 (M = Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO3 has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO3 using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy (F-PH) calculations predict that LiHCO3 will be stable under suitable conditions of temperature and partial pressures of CO2 and H2O. Our calculations indicate that the HCO3- groups in LiHCO3 and NaHCO3 form an infinite chain structure through O center dot center dot center dot H center dot center dot center dot O hydrogen bonds. In contrast, the HCO3 anions form dimers, (HCO3-)(2), connected through double hydrogen bonds in all phases of KHCO3. Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical-transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0-900 K, the F-PH and the entropies (S) of MHCO3 (M = Li, Na, K) systems vary as F-PH(LiHCO3) > F-PH(NaHCO3) > F-PH(KHCO3) and S(KHCO3) > S(NaHCO3) > S(LiHCO3), respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO2 capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO2 capture technology, in agreement with experiments.