Design of a new polar metal via 6s lone pair electrons*

Polar metals are extremely rare because mobile electrons
within a metal eliminate internal dipoles that may arise owing to
asymmetric charge distributions. In this work we combine
first-principles calculations and crystal structural search (CALYPSO) method
to design a new polar metal. We show that an ordered double perovskite
oxide BaBiTi₂O₆ undergoes a continuous transition from a
high-temperature centrosymmetric P4/mmm structure to a low-temperature
non-centrosymmetric Pmm2 structure. The material is conducting
throughout the structural phase transition. The underlying mechanism
is that the 6s lone-pair electrons in Bi³⁺ leads to polar
instability and simultaneously donotes electrons to empty Ti-d states,
which results in conduction. Our work demonstrates that the lone-pair
electrons are effective to induce polar instability in metals as they
induce ferroelectric instabilities in insulators. This provides a robust
design principle for new polar metals.