Density functional theory study of the structural, electronic, lattice dynamical, and thermodynamic properties of Li4SiO4and its capability for CO

Abstract

The structural, electronic, lattice dynamical, optical, thermodynamic, and CO 2 capture properties of monoclinic and triclinic phases of Li 4 SiO 4 are investigated by combining density functional theory with phonon lattice dynamics calculations. We found that these two phases have some similarities in their bulk and thermodynamic properties. The calculated bulk modulus and the cohesive energies of these two phases are close to each other. Although both of them are insulators, the monoclinic phase of Li 4 SiO 4 has a direct band gap of 5.24 eV while the triclinic Li 4 SiO 4 phase has an indirect band gap of 4.98 eV. In both phases of Li 4 SiO 4 , the s orbital of O mainly contributes to the lower-energy second valence band (VB 2 ) and the p orbitals contribute to the fist valence band (VB 1 ) and the conduction bands (CBs). The s orbital of Si mainly contributes to the lower portions of the VB 1 and VB 2 , and Si p orbitals mainly contribute to the higher portions of the VB 1 and VB 2 . The s and p orbitals of Li contribute to both VBs and to CBs, and Li p orbitals have a higher contribution than the Li s orbital. There is possibly a phonon soft mode existing in triclinic γ -Li 4 SiO 4 ; in the monoclinic Li 4 SiO 4 , there are three phonon soft modes, which correspond to the one type of Li disordered over a few sites. Their LO-TO splitting indicates that both phases of Li 4 SiO 4 are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li 4 SiO 4 . The calculated relationships of the chemical potential versus temperature and CO 2 pressure for reaction of Li 4 SiO 4 with CO 2 shows that Li 4 SiO 4 could be a good candidate for a high-temperature CO 2 sorbent while used for postcombustion capture technology.

Density functional theory calculations of point defects and hydrogen isotopes in Li4SiO4

“…By this means, the value of n for the present compound was determined at 1. This means that the optical transitions for the Li 4 SiO 4 are directly allowed, which is in good agreement with the band structure which were calculated by Duan et al 36 …”

“…5(a). Our calculated band gap of the perfect Li 4 SiO 4 crystal is 5.42 eV, which is very close to the value of 5.53 eV and 5.24 eV with GGA, 35,36 while it is lower than the experimental value 5.92 eV. When analyzing the thermodynamic transition model induced by the vacancy, we introduce a relative energy modification (∼0.5 eV) for each gap of defect models, which is the difference between the calculated gap of the perfect Li 4 SiO 4 model and the experimental value.…”

“…Tang et al 35 studied the crystalline and electronic structures of lithium silicates using the density functional theory. Duan et al 36 also studied the structural, electronic, lattice dynamical, and thermodynamic properties of Li 4 SiO 4 and its capability for CO 2 capture using density functional theory. Although some efforts [37][38][39] had been made to study the hydrogen isotopes or water molecular adsorption on and desorption from lithium silicates based on cluster model using quantum chemistry method, few such studies have been done on the radiation defects or the tritium behavior in Li 2 SiO 3 and Li 4 SiO 4 bulks.…”

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

“…The Born effective charge tensor of each atom type in the MHCO 3 structures and the macroscopic static dielectric tensor including local field effects of the MHCO 3 crystals were evaluated based on density functional perturbation theory (DFPT) [46][47][48] and were used in phonon calculations to obtain the infrared absorption spectrum and the longitudinal optical-transverse optical (LO-TO) splitting features of the solids [49].…”

First-principles study of the lattice dynamics of Sb2S3

“…One of the biggest challenges in phonon calculations is to determine whether the soft modes are artificial (an artefact of the numerical methods, etc) or genuine that are associated with unstable lattice structures and phase transitions 40,41 . The occurrence of artificial phonon soft modes found in this work should serve as a caution that convergence with respect to supercell size must be checked carefully when using the supercell force-constant method.…”