Charge Symmetry Beyond Space-Group Equivalence
Crystallographic space-group symmetry $G_{\rm lat}$, determined by atomic species and their spatial arrangement, is one of the most important descriptors in solid-state physics, underlying the classification of electronic states, spectral degeneracies, order parameters, and phase transitions. Yet the symmetry $G$ of a crystal also depends on the electronic coupling network between atomic sites, including electron hopping, Coulomb interactions, and orbital hybridization. This raises a fundamental question: must $G$ reproduce every equivalence relation imposed by $G_{\rm lat}$? Equivalently, must symmetry-related atoms at the same Wyckoff position be electronically identical, while atoms at inequivalent Wyckoff positions are electronically distinct? We develop a systematic theory of interaction-controlled electronic equivalence, with site charge imbalance as an order parameter whose stability is governed by the competition between onsite charging cost and intersite Coulomb gain. Group-theoretical analysis identifies the site-exchange operations lost from or added to $G_{\rm lat}$. Sites identified as equivalent by $G_{\rm lat}$ can spontaneously develop charge imbalance, lowering the realized symmetry to $G\subset G_{\rm lat}$. Conversely, sites identified as inequivalent by $G_{\rm lat}$ can remain equivalent through a hidden low-energy gauge symmetry. Within the low-energy $(s,p_z)$ manifold, this realizes $G\supset G_{\rm lat}$ and protects near-Fermi degeneracies that appear accidental in a $G_{\rm lat}$-based analysis. First-principles calculations verify both scenarios and establish pressure as a control parameter: it destabilizes the charge-equivalent state in Type I, whereas in Type II it destroys the hidden equivalence, splits the near-Fermi doublets, and can drive a metal-insulator transition.