Anomalies of the Y1 − xCaxBa2Cu3Oy crystal structure in the temperature range 80–300 K

The effect of doping on the temperature evolution of the crystal structure of the YBa2Cu3O y high-temperature superconductor has been investigated. The crystal structure of Y1 − x Ca x Ba2Cu3Oy (x = 0, 0.1; y = 6.6, 6.8, 6.95) has been studied using the full-profile analysis with X-ray powder diffraction data in the temperature range 100–300K. The unit cell parameters, the bond lengths, and the calculated linear and volume coefficients of thermal expansion are characterized by a nonmonotonic behavior in the temperature range ∼160–225 K. Samples with the oxygen content y = 6.6–6.8 in both systems (with x = 0 and 0.1) have negative linear coefficients of thermal expansion at low temperatures. A possible reason for the anomalous behavior can be the change in the state of the electronic subsystem. The data analysis confirms that the doping mechanisms upon oxygen introduction, and when Y is substituted for Ca are different.