STUDY OF TWIN FORMATION IN NDBA2CU3O7-DELTA SINGLE-CRYSTAL BY USING HIGH-TEMPERATURE POLARIZED OPTICAL MICROSCOPY

In order to determine an accurate retragonal-orthorhombic structural t ransition temperature for NdBa2Cu3O7-delta (Nd123) Single crystal easi ly, in-situ observation of twin formation during annealing of the Nd12 3 single crystal was performed by using high temperature polarized opt ical microscopy. The Nd123 single crystal was grown under 21% oxygen p artial pressure atmosphere by a top-seeded solution-growth (TSSG) meth od. The cut sample was heated to 700 degrees C in a pure argon gas flo w, followed by a step-wise cooling treatment every 10 degrees C from 7 00 degrees C, in a 100 and a 21% oxygen partial pressure gas flow unti l the twin was formed. The twin formation, which takes place with a te tragonal-orthorhombic structural phase transition, was observed at 660 and 620 degrees C in a 100 and a 21% oxygen partial pressure gas flow , respectively. Furthermore, the oxygen contents (7-delta) in the Nd12 3 single crystal, which are changed by oxygen absorption and desorptio n in the crystal, were examined with a thermogravimetry (TG) on powder samples and an iodometric titration technique. The relations between the annealing temperatures and the oxygen contents (7-delta) were dete rmined under 100 and 21% oxygen partial pressure atmospheres. These re lations suggested that the:critical oxygen content with which the tetr agonal-orthorhombic structural phase transition took place a as around 6.80 for the Nd123 single crystal. (C) 1998 Published by Elsevier Sci ence Ltd. All rights reserved.