Reentrant phase transition of Ba₂NaNb₅O₁₅

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.