Origin of polar distortion in LiNbO3-type ferroelectric metals: Role of A-site instability and short-range interactions

Since conduction electrons of a metal effectively screen the local electric dipole moments, it was widely believed that the ferroelectric-(FE-) like distortion cannot occur in metals. Recently, metallic LiOsO3 was discovered to be the first clear-cut example of an Anderson-Blount ferroelectric metal, which at 140 K undergoes a ferroelectriclike structural transition similar to insulating LiNbO3. This is very surprising because the mechanisms for structural phase transitions are usually quite distinct in metals and insulators. Through performing first-principles calculations, here we reveal that the local polar distortion in LiOsO3 is solely due to the instability of the A-site Li atom, in contrast to the LiNbO3 case where the second-order Jahn-Teller effect of the B-site Nb ion also plays an additional role. More importantly, the ferroelectriclike long-range order of the local polar distortion is found to be due to the predominantly ferroelectric short-range pair interactions between the local polar modes, which are not screened by conduction electrons. Furthermore, we predict that LiNbO3-type MgReO3 is also a ferroelectric metal but with a much higher structural transition temperature by 391 K than LiOsO3. Our paper not only unravels the origin of FE-like distortion in LiNbO3-type ferroelectric metals, but also provides a clue for designing other multifunctional ferroelectric metals.