Continuous crossover between insulating ferroelectrics and polar metals: Ab initio calculation of structural phase transitions of LiBO3 ( B=Ta , W, Re, …

Inspired by the recent discovery of a new polar metal LiReO₃ by K. Murayama et al, we calculate the temperature (T)-dependent crystal structures of LiBO₃ with B=Ta, W, Re, Os, using the self-consistent phonon (SCPH) theory. We have reproduced the experimentally observed polar-nonpolar structural phase transitions and the transition temperatures (Tc) of LiTaO₃, LiReO₃, and LiOsO₃. From the calculation, we predict that LiWO₃ is a polar metal, which is yet to be tested experimentally. Upon doping electrons to the insulating LiTaO₃, the predicted Tc is quickly suppressed and approaches those of the polar metals. Thus, there is a continuous crossover between ferroelectric insulators and polar metals if we dope electrons to the ferroelectric insulators. Investigating the detailed material dependence of the interatomic force constants (IFCs), we explicitly show that the suppression of Tc in polar metals can be ascribed to the screening of the long-range Li-O interaction, which is caused by the presence of the itinerant electrons. The quantitative finite-temperature calculations do not show signs of unscreened long-range interactions by the weak electron-phonon coupling or enhancement of polar instabilities by carrier doping, as expected in some previous works.