Transitions between P2 1, [formula omitted], and P6 322 modifications of SrAl 2O 4 by in situ high-temperature X-ray and neutron diffraction

Abstract

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 SrAl 2O 4. The material undergoes two reversible phase transitions P 2 1 ↔ P 6 3 ( 3 A ) and P 6 3 ( 3 A ) ↔ P 6 3 22 at ∼680 and ∼860 °C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P 6 3 ( 3 A ) ↔ P 6 3 22 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 P6 322 phases of stuffed tridymites.