We report the crystal structure of a new polymorph of Ca3P2, and an analysis of its electronic structure. The crystal structure was determined through Rietveld refinements of powder synchrotron x-ray diffraction data. Ca3P2 is found to be a variant of the Mn5Si3 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, Ca5P3H, 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 Ca3P2 that the compound is stabilized by a gap in the density of states compared to the hypothetical compound Ca5P3. Moreover, the calculated band structure of Ca3P2 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 Ca3P2 will be of interest.
@article{arxiv.1504.01731,
title = {Potential ring of Dirac nodes in a new polymorph of Ca$_3$P$_2$},
author = {Lilia S. Xie and Leslie M. Schoop and Elizabeth M. Seibel and Quinn D. Gibson and Weiwei Xie and Robert J. Cava},
journal= {arXiv preprint arXiv:1504.01731},
year = {2015}
}