Potential ring of Dirac nodes in a new polymorph of Ca$_3$P$_2$

TL;DR

This paper reports the discovery of a new polymorph of Ca3P2 with a unique electronic structure featuring a ring of Dirac nodes, indicating potential as a three-dimensional Dirac semimetal protected by symmetry.

Contribution

The study identifies a new Ca3P2 polymorph, determines its crystal structure, and predicts it hosts a novel ring of Dirac nodes at the Fermi level, expanding the class of topological semimetals.

Findings

Ca3P2 has a crystal structure similar to Mn5Si3 with Ca deficiency.

Electronic structure calculations reveal a ring of Dirac nodes at the Fermi level.

The Dirac states are protected by a mirror plane, akin to graphene.

Abstract

We report the crystal structure of a new polymorph of Ca$_3$P$_2$, and an analysis of its electronic structure. The crystal structure was determined through Rietveld refinements of powder synchrotron x-ray diffraction data. Ca$_3$P$_2$ is found to be a variant of the Mn$_5$Si$_3$ structure type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry to yield a charge-balanced compound. We also report the observation of a secondary phase, Ca$_5$P$_3$H, in which the Ca and P sites are fully occupied and the presence of interstitial hydride ions creates a closed-shell electron-precise compound. We show via electronic structure calculations of Ca$_3$P$_2$ that the compound is stabilized by a gap in the density of states compared to the hypothetical compound Ca$_5$P$_3$. Moreover, the calculated band structure of Ca$_3$P$_2$ indicates that it should be a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to graphene. The results suggest that further study of the electronic properties of Ca$_3$P$_2$ will be of interest.