A new form of Ca₃P₂ with a ring of Dirac nodes

We report the synthesis and crystal structure of a new high-temperature form of Ca₃P₂. The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca₃P₂ has a crystal structure of the hexagonal Mn₅Si₃ type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca²⁺ and P^3-. We also report the observation of a secondary hydride phase, Ca₅P₃H, which again is a charge-balanced compound. The calculated band structure of Ca₃P₂ indicates that it is 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 what is seen for graphene.