Chemical and magnetic ordering derived from ab initio simulations: the case of β'-LiFeO2

First-principles periodic calculations (with the B3LYP (Becke, three-parameter, Lee-Yang-Parr) hybrid functional, all-electron localized basis functions, and the CRYSTAL code) were coupled to a cluster expansion scheme in order to investigate the monoclinic β' phase of LiFeO2, where a partially disordered Fe-Li distribution is observed within a rocksalt-type superstructure. By least-energy optimizing a limited number of ordered configurations, and employing a two-body truncated cluster expansion, the values J1 = - 0.06(2) and J2 = - 0.125(8) eV were obtained for the excess interaction energies Ji = JLiFe,i - (JLiLi,i + JFeFe,i)/2 corresponding to the first and second coordination spheres, respectively; negligible values were computed for third and further coordinations. The ordering phase transformation α→β'→γ was then addressed. Antiferromagnetic versus ferromagnetic ordering was taken into account too, and proved to lower the energy by - 0.0577 eV/f.u. The corresponding cluster expansion coefficients Ji = JAFM,i - JFM,i are J1 = 0.007(2) and J2 = - 0.044(5) eV.