Structural and superconducting properties of oxygen-deficient GdBaSrCu3Ox (6.5<x<7.0)

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O_x with both orthorhombic and tetragonal structure were synthesized using different heat treatments. From each type, a series of oxygen-depleted samples, denoted as “O” and “T”, with oxygen contents 6.5<x<7.0 were prepared by annealing in argon at different temperatures in the range 300–550 °C. For O- and T-samples, superconductivity vanishes below x≈6.8 and x≈6.6, respectively. While T-samples show a continuous variation of both T_c and the unit-cell volume with x, O-samples show coinciding anomalies in these dependences. In all cases, the oxygen loss with respect to fully oxygenated samples was determined by both weight measurements (δ_w) and iodometric titration (δ_t) With T-samples, δ_w and δ_t are in good agreement, whereas with O-samples considerable discrepancies between these values were found. These disparities in the properties of O- and T-samples may be related to differences in the oxygen sublattice.