Disordered crystal structure of SrAl₂O₄ at high temperatures

Abstract

The crystal structures of SrAl₂O₄ at 1073 K and 1273 K were determined from conventional X-ray powder diffraction data using direct methods, and they were further refined by the combined use of Rietveld method and maximum-entropy method (MEM). The structure at 1273 K is hexagonal (space group P6322, Z = 2) with a = 0.51615(5) nm, c = 0.85408(3) nm and V = 0.19705(1) nm3. Final reliability indices were Rwp = 7.50% (S = Rwp /Re = 1.25), RB = 1.21% and RF = 1.58%. The three-dimensional electron density distribution image showed that the oxygen ions were split to occupy the lower symmetry sites. The orientational disorder of the SiO4 tetrahedra is consistent with that determined in Sr0.864Eu0.136Al2O4 and Ba0.6Sr0.4Al2O4 at ambient temperature.