Coexistence of polar displacements and conduction in doped ferroelectrics: an ab initio comparative study

Degenerately doped ferroelectrics may create an approximate polar metallic phase. We use first-principle calculations to investigate n-doped LiNbO₃-type oxides (LiNbO₃ as the prototype) and compare to perovskite oxides (BaTiO₃ as the prototype). In rigid-band approximation, substantial polar displacements in n-doped LiNbO₃ persist even at 0.3e/f.u., while that in n-doped BaTiO₃ quickly get suppressed and completely vanish at 0.1e/f.u. Supercell calculations which use oxygen vacancies as electron donors support results from rigid-band approximation and provide more detailed charge distribution. We find that in n-doped LiNbO₃, conduction electrons are not as uniformly distributed as in n-doped BaTiO₃. Insulating and conducting Nb sites coexist but substantial cation displacements are observed throughout n-doped LiNbO₃. Our work shows that polar distortions and conduction can coexist in a wide range of electron concentration in n-doped LiNbO₃, which is a practical approach to create an approximate polar metallic phase. Our results also show that Li displacements are not solely induced by Nb-O displacements, which may shed light on the origin of ferroelectricity in LiNbO₃.