Oxygen stoichiometry and expansion of yttrium barium copper oxide (YBa2Cu4O8)

The effects of temperature and oxygen partial pressure on the oxygen stoichiometry, expansion, and electrical resistivity of YBa2Cu4O8 have been determined. Samples prepared by two low-pressure synthesis methods-prolonged reaction between YBa2Cu3O7 and Cu(NO3)2 and reaction between BaCO3, Y2O3, and CuO at 4 Torr of oxygen-behave similarly. Heating to 800-degrees-C in oxygen produces a loss of 0.08 atom of oxygen, which is completely recovered during cooling. At 500-degrees-C a change of the ambient gas from pure oxygen to 0.25% oxygen causes a release of 0.055 atom of oxygen and an expansion of approximately 200 ppm. Two diamagnetic transitions, a weak one at 81 K and a strong one at 70 K, are observed. The average thermal expansion between 25 and 800-degrees-C is 13.2 ppm/degrees-C in oxygen.