Substitution of Ga for Cu in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ systems (R=Yb, Er, Y, Dy, Gd, Eu, and Nd)

We have investigated the effect of Ga doping on the structure, oxygen content, and superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ (R=Yb, Er, Y, Dy, Gd, Eu, and Nd, and x=0, 0.05, 0.1, 0.15, 0.2, and 0.3). We observed that in these systems, the superconducting ${\mathit{T}}_{\mathit{c}}$, the melting point, and the critical Ga concentration ${\mathit{x}}_{\mathit{O}\mathrm{-}\mathit{T}}$ at which the samples undergo an orthorhombic-tetragonal transition, are all rare-earth-ion size dependent. The results suggest that the decrease of density of states N(${\mathit{E}}_{\mathit{F}}$) and/or the localization of the carriers, which is likely related to a Mott transition due to Ga substitution, are the possible origins for the suppression of superconductivity and the observed metal-semiconductor transition.