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

Abstract

Polar metals are rare because free carriers in metals screen electrostatic potential and eliminate internal dipoles. Degenerate doped ferroelectrics may create an approximate polar metallic phase.We use first-principles calculations to investigate n-doped LiNbO 3 -type oxides (LiNbO 3 as the prototype) and compare to widely studied perovskite oxides (BaTiO 3 as the prototype). In the rigid-band approximation, substantial polar displacements in n-doped LiNbO 3 persist even at 0.3 e/f.u. ( 10 21 cm −3 ), while polar displacements in n-doped BaTiO 3 quickly get suppressed and completely vanish at 0.1 e/f.u. Furthermore, in n-doped LiNbO 3 , Li-O displacements decay more slowly than Nb-O displacements, while in n-doped BaTiO 3 , Ba-O and Ti-O displacements decay approximately at the same rate. Supercell calculations that use oxygen vacancies as electron donors support the main results from the rigid-band approximation and provide more detailed charge distributions. Substantial cation displacements are observed throughout LiNbO 3−δ (δ = 4.2%), while cation displacements in BaTiO 3−δ (δ = 4.2%) are almost completely suppressed. We find that conduction electrons in LiNbO 3−δ are not as uniformly distributed as in BaTiO 3−δ , implying that the rigid-band approximation should be used with caution in simulating electron doped LiNbO 3type oxides. Our work shows that polar distortions and conduction can coexist in a wide range of electron concentration in n-doped LiNbO 3 , which is a practical approach to generating an approximate polar metallic phase. Combining doped ferroelectrics and doped semiconductors may create new functions for devices.

A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics

“…(about 1.6 × 10 21 cm −3 ), at which the polar displacement δ is just completely suppressed and the c / a ratio is reduced to unity. This result is consistent with the previous theoretical studies 36 , 37 . Experimentally, in metallic oxygen-deficient BaTiO 3− δ , the low-symmetry polar structure can be retained up to an electron concentration of 1.9 × 10 21 cm −3 (close to the theoretical result) 44 , 45 .…”

“…Motivated by the above experiments and theories, we use first-principle methods with no adjustable parameters to demonstrate a large modulation of electron-phonon coupling in doped strong ferroelectrics by utilizing soft polar phonons. We study BaTiO 3 as a prototype, because (1) previous studies found that in n -doped BaTiO 3 , increasing the carrier density gradually reduces its polar distortions and induces a continuous polar-to-centrosymmetric phase transition 36 , 37 ; and (2) the critical concentration for the phase transition is about 10 21 /cm 3 , which is high enough so that the electron-phonon coupling can be directly calculated within the Migdal’s approximation (in contrast, in doped SrTiO 3 , superconductivity emerges at a much lower carrier concentration 10 17 –10 20 /cm 3 so that its Debye frequency is comparable to or even higher than the Fermi energy ℏ ω D / ϵ F ~ 1 − 10 2 38 , which invalidates the Migdal’s approximation and Eliashberg equation) 29 . The key result from our calculation is that, contrary to Anderson/Blount’s argument for "ferroelectric-like metals” 32 – 34 , we find that the phonon bands associated with the soft polar optical phonons are strongly coupled to itinerant electrons across the polar-to-centrosymmetric phase transition in doped BaTiO 3 .…”

Pressure-induced metal-insulator transition in oxygen-deficient LiNbO$_3$-type ferroelectrics

“…3e and f show the electronic band structures of conducting and insulating oxygen-deficient LiNbO 3−δ , respectively. In conducting LiNbO 3−δ , the band width of the defect state is reduced by hydrostatic pressure, which is in contrast to pristine LiNbO of doped electrons is highly inhomogeneous [26]. As we showed previously in Fig.…”

“…However, different from pristine LiNbO 3 , there is a sudden "jump" in the polar displacements between 7 and 8 GPa, which is evident in the inset of panel d. For a given hydrostatic pressure, the average polar displacements of conducting oxygendeficient LiNbO 3−δ are slightly smaller than those of pristine LiNbO 3 . This is because mobile electrons in conducting LiNbO 3−δ can screen internal electric fields and suppress the polar displacements [26]. However, in insulating LiNbO 3−δ , the average polar displacements are almost the same as those of pristine LiNbO 3 because screening from mobile electrons is absent.…”

Design of a multifunctional polar metal via first-principles high-throughput structure screening

“…À1 ), implying that the structural transition temperature of BPTO is higher than that of LiOsO 3 40 . We note that the above second-order structural phase transition is a key property to distinguish instrinsic polar metals from degenerately doped ferroelectrics [32][33][34][35]41,42 , because realistic dopants (cation substitution or oxygen vacancies) make the crystal symmetry of doped ferroelectrics ill-defined and correspondingly there is no well-defined continuous structural phase transition at finite temperatures.…”