In the high temperature superconducting material yttrium barium copper oxide (YBCO) the basal plane oxygen plays a dominant role in determining the superconducting properties of the material. We have carried outin situ high temperature measurements of (a) electrical resistivity and (b) X-ray diffraction in vacuum as well as in helium atmospheres to probe the nature of ordering of oxygen atoms. While published phase diagram of temperature vs oxygen concentration in different partial pressures allow only a tetragonal phase for the annealing temperatures we have investigated, the experiments point to the coexistence of orthorhombic (OII) and tetragonal phases.

The effect of doping iron at the yttrium site in hexagonal KCaY(PO₄)₂ is studied for various concentrations ofx (0≤x≤1), of iron using Mössbauer and X-ray diffraction methods. For low iron concentrations, very little changes in structure are seen but atx≈0·1, onset of new peaks in the XRD pattern is observed. The Mössbauer study of the doped samples reveals that iron has a solubility of up to 2·5% in the parent phase with any excess iron precipitating out to form a new and unknown phase. From a detailed analysis of the X-ray diffraction pattern (corresponding tox=1) of the latter phase, it is found that this phase is rhombohedral with the possible space groupR3.