Precise measurements of the temperature dependence of birefringence Δn, lattice strain X12 and of X-ray profiles from 4 K (in an optical study) or from 20 K (in an X-ray study) to 550 K were made, special attention being paid to the low temperature phase transition at 110 K (TL). Δn and X12 manifested clear temperature hysteresis at TL, but did not vanish below it. However when the specimen was kept at 77 K during 48 hours, in decreased to zero. These results, together with TEM observation of the appearance of microdomains, revealed that the low temperature phase is tetragonal as has been reported 1), but is obtained only after a quite long relaxation time.
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