Persisting of Polar Distortion with Electron Doping in Lone-Pair Driven Ferroelectrics

TL;DR

This study demonstrates that polar distortion in lone-pair driven ferroelectrics like PbTiO3 can persist and even increase with electron doping, revealing new insights into designing non-centrosymmetric metals.

Contribution

First-principles calculations showing polar distortion persistence and enhancement in PbTiO3 with electron doping, highlighting the role of lone-pair mechanisms.

Findings

Polar distortion persists and increases with electron doping.

Pb-site polar instability remains due to lone-pair mechanism.

Enhancement linked to increased Ti ion size.

Abstract

Free electrons can screen out long-range Coulomb interaction and destroy the polar distortion in some ferroelectric materials, whereas the coexistence of polar distortion and metallicity were found in several non-central-symmetric metals (NCSMs). Therefore, the mechanisms and designing of NCSMs have attracted great interests. In this work, by first-principles calculation, we found the polar distortion in the lone-pair driven ferroelectric material PbTiO$_3$ can not only persist, but also increase with electron doping. We further analyzed the mechanisms of the persisting of the polar distortion. We found that the Ti site polar instability is suppressed but the Pb site polar instability is intact with the electron doping. The Pb-site instability is due to the lone-pair mechanism which can be viewed as a pseudo-Jahn-Teller effect, a mix of the ground state and the excited state by ion displacement from the central symmetric position. The lone-pair mechanism is not strongly affected by the electron doping because neither the ground state or the excited state involved is at the Fermi energy. The enhancement of the polar distortion is related to the increasing of the Ti ion size by doping. These results show the lone-pair stereoactive ions can be used in designing NCSMs.