Transitions between P21, P63(3A), and P6322 modifications of SrAl2O4 by in situ high-temperature X-ray and neutron diffraction

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl2O4. The material undergoes two reversible phase transitions P21↔P63(3A) and P63(3A)↔P6322 at ∼680 and ∼860 °C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P63(3A)↔P6322 transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al–O–Al angle to the ideal value 180°. The analysis of the literature suggests that it is a common feature of the P6322 phases of stuffed tridymites.

SrAl2O4 crystal structure evolution through the phase transitions between P21, P63(3A), and P6322 modifications. Insets show displacements of apical oxygen atoms linking tridymite-type layers.Figure optionsDownload as PowerPoint slide