T<SUB>c</SUB> suppression and conduction mechanisms in Bi<SUB>2.1</SUB>Sr<SUB>1.93</SUB>Ca<SUB>0.97-x</SUB>R<SUB>x</SUB>Cu<SUB>2</SUB>O<SUB>8+y</SUB> (R= Pr, Gd, and Er) systems

Systematic substitutional studies in the Bi2.1Sr1.93Ca0.97-xRxCu2 O8+y (R=Pr,Gd,Er; 0&#8804;x&#8804;0.3 in steps of 0.05 and 0.4&#8804;x&#8804;0.97 in steps of 0.1) system were carried out in order to determine the effect of the magnetic moment and ionic radius of the rare-earth ion on the Tc suppression rate. X-ray-diffraction studies indicate that the solid solubility of Gd and Er exists up to x=0.97 whereas that of Pr is limited to x=0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconductivity persists up to x=0.4 and a metal-semiconductor transition occurs for x&gt;0.4. The most interesting observation is that the rate of Tc suppression for the superconducting phases is found to be identical for all rare earths. We have explained that hole filling rather than Abrikosov-Gor'kov pair breaking is responsible for the decrease in Tc. The insulating phases with 0.5&#8804;x&#8804;0.97 exhibit the phenomenon of Mott's variable-range-hopping mechanism (VRH). The physical parameters related to VRH such as localization length (&#945;&#8722;1), hopping range (R), and activation energy (W) for conduction, have been evaluated and discussed in detail