Superconductivity of the HTS compounds Y1–<i>x</i>Ca<i>x</i>Ba2Cu4O8 ∓<i>y</i>

Crystal lattice parameters of 1–2–4 compounds are measured as functions of calcium concentration in the interval 0&amp;lt;x &amp;lt;0.3. The degree of rhombic distortion of the crystal lattice is found to be nonlinear. The effect of hydrostatic pressure on the superconducting transition temperature of 1–2–4 compounds with a substitution of yttrium by calcium is studied. The dependence of Tc and the pressure derivative of Tc on calcium concentration (0&amp;lt;x &amp;lt;0.3) is found to be nonlinear. Possible mechanisms of the observed nonlinear dependences of Tc and dTc/dP on x are discussed.

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