Defect rocksalt structures in the La-Na-N system

Abstract

Sodium azide (NaN₃) 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 (NaN₃) 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-xNa_3xN with vacancies at octahedral La sites and interstitial tetrahedral Na cations. This phase has a tetragonally distorted rocksalt structure (space group I4[Formula: see text]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 I4[Formula: see text]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 [Formula: see text] phase (a = 5.1561 (5) Å), tentatively identified as rocksalt 'NaN_1/3' stabilized by a small amount of La; NaLa_yN_(1+3y)/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)'.

Keywords: high-pressure chemistry; nitrides; structure elucidation.