Study of twin formation in NdBa2Cu3O7−δ single crystal by using high temperature polarized optical microscopy

In order to determine an accurate tetragonal–orthorhombic structural transition temperature for NdBa2Cu3O7−δ (Nd123) single crystal easily, in-situ observation of twin formation during annealing of the Nd123 single crystal was performed by using high temperature polarized optical microscopy. The Nd123 single crystal was grown under 21% oxygen partial pressure atmosphere by a top-seeded solution-growth (TSSG) method. The cut sample was heated to 700°C in a pure argon gas flow, followed by a step-wise cooling treatment every 10°C from 700°C, in a 100 and a 21% oxygen partial pressure gas flow until the twin was formed. The twin formation, which takes place with a tetragonal–orthorhombic structural phase transition, was observed at 660 and 620°C in a 100 and a 21% oxygen partial pressure gas flow, respectively. Furthermore, the oxygen contents (7−δ) in the Nd123 single crystal, which are changed by oxygen absorption and desorption 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−δ) were determined under 100 and 21% oxygen partial pressure atmospheres. These relations suggested that the critical oxygen content with which the tetragonal–orthorhombic structural phase transition took place was around 6.80 for the Nd123 single crystal.