The fixed triangle chemical bond and its effect in the Y1-xCaxBa2-yLayCu3Oz system from underdoped to overdoped

Samples of Y1-xCaxBa2-yLayCu3Oz (YCBLCO) were synthesized by the standard solid-state reaction with both x and y taking values from 0 to 0.5. Structural and superconducting properties have been investigated by x-ray diffraction, Rietveld refinement and dc magnetization measurement. The three-dimensional phase diagram in the YCBLCO system is obtained, describing the dependence of Tc on the content of two dopants simultaneously. By careful study of the chemical bond lengths related to the Cu(2)O2 plane and the Cu(1)-O chain, it is found that Tc has opposite dependences on the bond lengths of Cu(1)-O(4) or Cu(2)-O(4) in the underdoped (xy) regions. In addition, calculation of the bond angles for the optimal-doped samples indicates that Tc is closely related to four pairs of bond angles, all of which characterize the curvature of the CuO2 plane. It is demonstrated that the interaction between the perovskite block and the rock salt block in a unit cell has a close relationship with the superconductivity. The calculation of bond lengths and angles indicates that Cu(2), O(2) and O(3) on the CuO2 plane form a 'fixed triangle', independent of the doping level in the system. It is suggested that the collective behavior of the fixed angle might result in a particular phonon mode, which may induce a special kind of electron-phonon coupling, different from that appearing in conventional superconductors. This may be of great importance for uncovering the elusive mechanism of high-temperature superconductivity.