Charge Symmetry Beyond Wyckoff Equivalence

TL;DR

This paper explores how pressure can induce charge transfer in crystals, causing deviations from traditional symmetry-based expectations of electronic equivalence, with implications for understanding material properties.

Contribution

It introduces a minimal Landau theory explaining pressure-induced charge transfer and reveals cases where symmetry expectations are violated or restored.

Findings

Pressure can cause charge transfer, breaking or restoring symmetry expectations.

In Na, compression induces charge transfer and symmetry breaking.

Emergent hidden symmetries can keep inequivalent sites charge-equivalent at low pressure.

Abstract

Crystallographic symmetry is usually taken as a guide to electronic equivalence in crystals: atoms on the same Wyckoff position are expected to have the same charge, whereas atoms on different Wyckoff positions are expected to be electronically distinct. Here we show that both expectations can fail in oppo-site ways: crystallographically equivalent sites can become charge-inequivalent under compression, whereas crystallographically inequivalent sites can remain charge-equivalent at low pressure because of an emergent hidden symmetry. We develop a minimal Landau theory of pressure-induced charge transfer, in which compression enhances the intersite Coulomb energy gained by charge redistribution until it overcomes the onsite charging cost and destabilizes the charge-equivalent state. In BCC Na, all sites are charge-equivalent at low pressure, but compression drives charge transfer between neighboring sites, pro-ducing an electronically symmetry-broken CsCl-type state on an unchanged BCC ionic framework. In hP4 Na, the opposite anomaly occurs: two Na sites occupy distinct Wyckoff positions, yet remain charge-equivalent at low pressure because of an emergent gauge equivalence in the low-energy manifold, giving rise to near-Fermi doublets that appear accidental in conventional space-group analysis. Upon compres-sion, pressure-induced charge transfer breaks this hidden equivalence, splits the near-Fermi doublets, and drives a metal-insulator transition. These two complementary cases establish pressure-induced charge transfer as a mechanism by which electronic equivalence can either fall below or rise above what Wyckoff positions alone would suggest, showing that lattice symmetry constrains but does not uniquely determine the equivalence structure of the electronic state.