Parity-breaking in single-element phases: Ferroelectric-like elemental polar metals

TL;DR

This paper proposes a design principle for creating ferroelectric-like polar metals from single elements by breaking inversion symmetry through specific atomic arrangements, supported by first-principles calculations.

Contribution

It introduces a novel design rule for elemental polar metals based on Wyckoff positions and predicts new candidate materials with ferroelectric-like properties.

Findings

Identifies a class of potential ferroelectric-like elemental polar metals.

Shows these phases result from lone pair driven polar distortions.

Predicts a transition from metallic to semimetallic states.

Abstract

Polar metals based on binary and ternary compounds have been demonstrated in literature. Here, we propose a design principle for ferroelectric-like elemental polar metals and relate it to real materials. The design principle is that, to be an elemental polar metal, atoms should occupy at least two inequivalent Wyckoff positions in a crystal with a polar space group, where inversion symmetry is spontaneously broken. According to this rule, we propose the first class of potential ferroelectric-like elemental polar metals in a distorted {\alpha}-La-like structure with a polar space group P63mc in which two inequivalent Wyckoff positions 2a (0, 0, z) and 2b (1/3, 2/3, z) are occupied by group-V elements (phosphorus, arsenic, antimony, and bismuth). Analyses based on first-principles calculations indicate that the dynamically stable polar phase results from a lone pair driven polar distortion of the nonploar phase in P63/mmc symmetry where two inequivalent Wyckoff positions 2a (0, 0, 0) and 2c (1/3, 2/3, 1/4) are occupied. This ferroelectric-like transition involves a transition from a metallic state to a semimetallic state. These predicted polar phases are metastable with respect to their corresponding ground phases. Moreover, ionic bonding characters are found due to the inequivalence in Wyckoff positions between group-V atoms. Our work opens a route to single-element parity-breaking phases.