Structural characterization of sodium azide and sodium bifluoride at high pressures

Abstract

This paper reports the structural characterization of sodium azide and sodium bifluoride (hydrogenfluoride) at elevated pressures using neutron powder diffraction. Compression of sodium azide at 294 K induces a transition from the β-phase to the α-phase. This structure responds to increasing pressure by progressive tilting of the azide groups. Compression of the α-form to 3.33 GPa at 393 K results in a phase transition to the tetragonal γ-phase (space group I4/mcm), which is isostructural with the azides of the heavier Group 1 elements and features square-antiprism co-ordination of the cations. On decompression at ambient temperature, the γ-phase reverts to the α-phase, but with substantial peak broadening indicative of significant strain within the recovered sample. Compression of sodium bifluoride (NaDF2-I to 0.66 GPa at ambient temperature resulted in the formation of NaDF2-II, which adopts an orthorhombic, marcasite-like (space group Pnnm) structure. Further compression to 4.58 GPa resulted in a transition to NaDF2-III, which adopts the archetypal I4/mcm structure shared by the heavier alkali metal bifluorides and γ-NaN3.