Defect rocksalt structures in the La-Na-N system

Sodium azide (NaN ₃ ) is a versatile nitrogen source that can be used for the synthesis of new nitrides under high-pressure and temperature conditions. Reactions between lanthanum nitride (LaN) and sodium azide (NaN ₃ ) at 800°C under 8 GPa pressure have led to the discovery of two defect rocksalt phases which are the first reported ternaries in the La-Na-N system. Preliminary structure assignments have been made based on fits to powder X-ray diffraction profiles. One phase is La _{1− x} Na _{3 x} N with vacancies at octahedral La sites and interstitial tetrahedral Na cations. This phase has a tetragonally distorted rocksalt structure (space group I 4 / mmm , a  = 3.8704(2) and c  = 5.2098(3) Å for nominal x  = 0.10) and the distortion decreases with increasing Na content (space group I 4 / mmm , a  = 3.8060(2) Å, c  = 5.2470(3) Å for nominal x  = 0.14), further giving a cubic phase ( a  = 5.3055(2) Å) for nominal x  = 0.25. This coexists with another cubic F m 3 ¯ m phase ( a  = 5.1561 (5) Å), tentatively identified as rocksalt ‘NaN _1/3 ’ stabilized by a small amount of La; NaLa _y N _{(1+3 y )/3} with y  ≈ 1%. These initial investigations reveal that the high-pressure La-Na-N phase diagram may be rich in defect rocksalt-type materials although further work using neutron diffraction will be needed to confirm the structures.

This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.