Optical Response, Lithium Doping, and Charge Transfer in Sn-Based 312 MAX Phases

The optical response, lithium doping, and charge transfer in three Sn-based existing M₃SnC₂ MAX phases with electron localization function (ELF) were investigated using density functional theory (DFT). Optical calculations show a slight optical anisotropy in the spectra of different optical parameters in some energy ranges of the incident photons. The peak height is mostly slightly higher for the polarization ⟨001⟩. The highest peak shifts toward higher energy when the M-element Ti is replaced by Zr and then by Hf. Optical conductivity, refractive index, extinction coefficient, and dielectric functions reveal the metallic nature of Ti₃SnC₂, Zr₃SnC₂, and Hf₃SnC₂. The plasma frequencies of these materials are very similar for two different polarizations and are 12.97, 13.56, and 14.46 eV, respectively. The formation energies of Li-doped Zr₃SnC₂ and Hf₃SnC₂ are considerably lower than those of their Li-doped 211 MAX phase counterparts Zr₂SnC and Hf₂SnC. Consistently, the formation energy of Li-doped Ti₃SnC₂ is lower than that of the corresponding 2D MXene Ti₃C₂, which is a promising photothermal material. The Bader charge is higher in magnitude than the Mulliken and Hirschfeld charges. The highest charge transfer occurs in Zr₃SnC₂ and the lowest charge transfer occurs in Ti₃SnC₂. ELF reveals that the bonds between carbon and metal ions are strongly localized, whereas in the case of Sn and metal ions, there is less localization which is interpreted as a weak bond.