An  in situ  study was made, by means of high-temperature polarized optical microscopy, of twin formation during the annealing of single crystals. The latter were grown under a 21%O partial pressure atmosphere by using a top-seeded solution-growth method. The cut samples were heated to 700C in a pure Ar gas flow, followed by cooling (10C-steps) from 700C (in an O gas flow) until a twin formed. Its formation, which involved a tetragonal-orthorhombic phase transition, was observed at 660 and 620C in a 100 and a 21%O partial-pressure gas flow, respectively.

A.Oka, S.Koyama, H.Kutami, Y.Shiohara: Applied Superconductivity, 1997, 5[1-6], 79-86