Tetragonal distortion of the multiferroic compound α-LiFe5O8 from first-principles calculations.

The cubic space group P4332 has been previously reported for the multiferroic lithium ferrite (α-LiFe5O8, denoted LFO). Instead, results from the present study using PBEsol+U calculations with SSSP pseudopotentials suggest that the tetragonal space group P43212 is true for the ground state of LFO. Spin-resolved projected density of states analysis shows strong 3d electron localization of Fe3+ in FeO4 tetrahedra, mostly contributing to the high Hubbard energy U in the calculated tetragonal model. Its direct band gap of 2.050 eV is slightly lower than that of the cubic model (2.107 eV) in density of states calculations. The calculated ferrimagnetic ground state exhibits a collinear spin order along the crystallographic c axis in the tetragonal magnetic space group P4321'2', consistent with the group theoretical prediction. Decreasing ferromagnetic interactions and increasing antiferromagnetic interactions in the c direction, simultaneously, explain the origin of the tetragonality a/c > 1 with a = 8.3528 (2) Å and c = 8.3511 (1) Å. Strong Fe-O-Fe superexchange interactions contribute to the tetragonal lattice distortion of LFO as well.