The crystal structures of Hf₃N₂ and Hf₄N₃

Abstract

The crystal structures of ε-Hf₃N₂ and ζ-Hf₄N₃ were determined from X-ray powder photographs. The structure of both phases is trigonal, space group D ^3d₅ -R−3m, with unit cells ofa _R = 7.972Å, α= 23 deg 12 min (hexagonal axes:a = 3.206Å,c = 23.26Å) for ε-H₃H₂, and a_R= 10.54Å, α = 17 deg 32 min (hexagonal axes:a = 3.214Å,c = 31.12Å) for ζ-Hf₄N₃. The nine close packed metal layers in ε-Hf₃N₂ are stacked according to (hhc)₃, or ABABCBCAC. The structure of ζ-Hf₄sN₃, isomorphous with ζ-V₄C₃,¹ consists of twelve close-packed metal layers in a stacking sequence (hhcc)₃. The nitrogen atoms occupy octahedral interstices in the metal lattice. The experimentally observed compositions, Hf₃N₁.₆₉ and Hf₄N₂.₅₆, shows both phases to be substantially deficient in nitrogen. ε-Hf₃N₂ is unstable above 2000°C, and ζ-Hf₄N₃ above 2300°C.