Density functional theory study of the structural, electronic, lattice dynamical, and thermodynamic properties of Li_4SiO_4 and its capability for CO_2 capture

摘要

The structural, electronic, lattice dynamical, optical, thermodynamic, and CO_2 capture properties of monoclinic and triclinic phases of Li_4SiO_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_4SiO_4 has a direct band gap of 5.24 eV while the triclinic Li_4SiO_4 phase has an indirect band gap of 4.98 eV. In both phases of Li_4SiO_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 .5 orbital of Si mainly contributes to the lower portions of the VB_1 and VB2, 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 r-Li_4SiO_4; in the monoclinic Li_4SiO_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_4SiO_4 are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li_4SiO_4. The calculated relationships of the chemical potential versus temperature and CO_2 pressure for reaction of Li_4SiO_4 with CO2 shows that Li_4SiO_4 could be a good candidate for a high-temperature CO_2 sorbent while used for postcombustion capture technology.