Key Role of Charge Disproportionation in Monoclinic Semiconducting Fe$_2$PO$_5$, a Room-Temperature d-Wave Altermagnet Candidate

TL;DR

This study combines experimental and theoretical methods to reveal that charge disproportionation stabilizes the monoclinic structure and semiconducting gap in Fe$_2$PO$_5$, a promising room-temperature altermagnet for spintronics.

Contribution

It demonstrates that charge disproportionation and structural distortion are essential to accurately model Fe$_2$PO$_5$'s electronic properties using DFT+U, revealing its semiconducting nature.

Findings

Charge disproportionation stabilizes the monoclinic structure.

DFT+U captures the narrow band gap only when symmetry-breaking channels are included.

Fe$_2$PO$_5$ is a rare room-temperature semiconducting d-wave altermagnet.

Abstract

$\beta$-Fe$_2$PO$_5$ is an emerging room-temperature d-wave altermagnet featuring quasi-one-dimensional crystal and magnetic structures, orthogonal transport channels for opposite spins, and large band spin splitting, which is a promising material for next-generation spintronics and magnonics. However, its crystal and electronic structures remain inconclusive. Here, joint experimental and theoretical studies confirm and explain the appearance of its monoclinic structure and semiconducting band gap. We discover that an electronic instability appears in the tetragonal metallic state as the joint effect of density functional theory and Hubbard U correction (DFT+U) and results in a charge disproportionation, which in turn stabilizes the monoclinic distortion with narrow gap formation. The successful capture of this effect within DFT+U requires accounting for the relevant symmetry-breaking energy-lowering channels -- charge disproportionation and structural distortion; otherwise, tetragonal-symmetry-constrained calculations yield only a metallic state. Fe$_2$PO$_5$ is thus best described as a correlation- and hybridization-assisted, distortion-coupled, charge-disproportionated semiconductor. It represents a rare room-temperature semiconducting d-wave altermagnet. It also provides a rare platform for studying the coexistence of altermagnetism and charge density wave in quasi-one-dimensional systems.