It was found that a stable shell of Cu4Ba2YO8 formed in powdered and bulk material during O out-diffusion. The new oxide then acted as a barrier to further O out-diffusion. A superior thermal stability of the derived oxide was attributed to a stronger bonding configuration of O(1) atoms in a double-chain structure.

L.T.Shi, K.N.Tu: Applied Physics Letters, 1991, 59[16], 2040-2