Abstract: A method for producing an oxide superconductor formed according to a chemical composition formula (R.sub.1-x Ca.sub.x)(Ba.sub.1-y Sr.sub.y).sub.2 Cu.sub.4 O.sub.8 and having a transition temperature of 77K or above, comprising the step of causing a reaction between (R.sub.1-x Ca.sub.x)(Ba.sub.1-y Sr.sub.y).sub.2 Cu.sub.3 O.sub.7-z and a Cu material such as CuO, wherein R stands for one or more rare earth elements selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb, and wherein 0.01.ltoreq.x.ltoreq.0.25, 0.ltoreq.y.ltoreq.0.5, and 0.ltoreq.z.ltoreq.1. The advantage of this production method is that this production method does not require a hot static pressing process which takes a long time.

Abstract: Zirconium oxide sinter for use in forming a thin film thereof by vapor deposition etc., having a purity not less than 99.0% by weight with unavoidable hafnium oxide and a density more than 4.9 g/cm.sup.3. Desirably the sinter further contains calcium oxide in an amount from 50 to 2000 ppm. The sinter can produce a uniform zirconium oxide thin film which can be used in optical applications with superior characteristics.