Phase control and growth of Y123 and Y124 crystals below 600 °C by molten KOH flux

• KOH flux can grow Y123 and Y124 crystals below 600 °C.
• Growth phases of Y123 and Y124 are controlled by changing growth atmosphere.
• p–T Phase relation diagram involving KOH and Y–Ba–Cu–O is determined.
• Liquid phase epitaxial films of Y123 and Y124 is obtained at low temperatures.

Phase formation temperatures of YBa2Cu3O7−δ (Y123), YBa2Cu4O8 (Y124) and Y2Ba4Cu7O15 (Y247) in a system containing molten KOH have been studied with respect to partial oxygen pressure. At the low oxygen partial pressure of approximately 10−5–10−4 atm, crystalline Y123 phase was generated at as low as 550 °C with the superconducting transition temperature above 90 K. In the air atmosphere, Y124 phase was formed below 600 °C and Y123/Y124 phase boundary was observed approximately 775 °C. At the oxygen pressure of 1 atm, Y247 was formed between the regions of Y123 and Y124 formation. Phase formation relation among Y123, Y124 and Y247 is similar to that of the conventional system without KOH, however, phase boundary sifts toward lower temperature by 50–150 °C in the KOH contained system. The morphology of the obtained crystals indicates that they grew by the liquid phase growth mode. The process with the molten KOH has a potential to fabricate superconducting liquid phase epitaxial films at very low temperatures below 600 °C.