title: Stabilities and defect-mediated lithium-ion conduction in a ground state cubic Li3N structure.
abstract: A stable ground state structure with cubic symmetry of Li3N (c-Li3N) is found by an ab initio initially symmetric random-generated crystal structure search method. Gibbs free energy, calculated within quasi-harmonic approximation, shows that c-Li3N is the ground state structure for a wide range of temperatures. The c-Li3N structure has a negative thermal expansion coefficient at temperatures lower than room temperature, mainly due to two transverse acoustic phonon modes. This c-Li3N phase is a semiconductor with an indirect band gap of 1.90 eV within hybrid density functional calculations. We also investigate the migration and energetics of native point defects in c-Li3N, including lithium and nitrogen vacancies, interstitials, and anti-site defects. Lithium interstitials are found to have a very low migration barrier (∼ 0.12 eV) and the lowest formation energy among all possible defects. The ionic conduction in c-Li3N is thus expected to occur via an interstitial mechanism, in contrast to that in the well-known α-Li3N phase which occurs via a vacancy mechanism.
authors.affiliation: Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA. mcnguyen@ameslab.gov.
authors.forename: Manh Cuong
authors.initials: MC
authors.lastname: Nguyen
authors.affiliation: Center for Computationally Assisted Science and Technology, North Dakota State University, Fargo, ND 58108, USA.
authors.forename: Khang
authors.initials: K
authors.lastname: Hoang
authors.affiliation: Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA. mcnguyen@ameslab.gov.
authors.forename: Cai-Zhuang
authors.initials: CZ
authors.lastname: Wang
authors.affiliation: Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA. mcnguyen@ameslab.gov.
authors.forename: Kai-Ming
authors.initials: KM
authors.lastname: Ho
mesh_terms: D007477:Ions / Q000737:chemistry; D008094:Lithium / Q000737:chemistry*; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D007477:Ions; D008094:Lithium; D009584:Nitrogen