Manifestation of anisotropy in melting systematics of RBa₂Cu₃O_7-δ(R=lanthanides)

The conventional isotropic Debye temperature fails to account for the trend of melting temperatures for the high Tc superconductors, R Ba2 Cu3 O7-δ (R-123), as a function of the ionic radius of R3+. We overcame this problem by calculating Debye temperatures using mean sound velocity along the c axis that features an anisotropic layered structure. Using the "improved" Debye temperature, the trend of derived melting temperatures based on the "Lindemann law" matches well with experimental data. This trend is also confirmed by comparing theoretical and experimental Raman active modes corresponding to the Cu-O (plane copper and apical oxygen) and Ba-O (in-plane) bonds in R-123 series.