Structural changes in the antifluorite crystal (NH₄)₂SnBr₆

Abstract

A study of structural phase transitions in the antifluorite crystal (NH 4 ) 2 SnBr 6 by nuclear quadrupole resonance and neutron powder diffraction techniques is presented. Interpretation of the bromine nuclear quadrupole resonance spectrum leads to a prediction concerning the nature of the distortions that occur as the temperature is lowered. Profile analysis of neutron diffraction data substantiates the prediction by revealing that the 157 K transition involves the antiferro-rotation of halide octahedra accompanied by a relatively small distortion of the unit cell whereas the 144 K transition involves a further ferro-rotation of the octahedra and a relatively large distortion of the unit cell.