Structural changes in the antifluorite crystal (NH4)2SnBr6

Abstract

A study of structural phase transitions in the antifluorite crystal (NH4)2SnBr6 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.