Effect of ion substitution in the) system: Implications of ion dependence and disorder

Systematic studies on the (Formula presented) ((Formula presented) system were carried out in order to determine the effect of the rare-earth ionic size and magnetic moment on the (Formula presented) suppression rate. The phases were characterized by powder x-ray diffraction (XRD), resistivity, and ac susceptibility measurements. XRD studies indicate a higher solubility limit of (Formula presented) ions in the (Formula presented) system as compared to the (Formula presented) system. Resistivity and ac susceptibility studies indicate that the (Formula presented) suppression rate for a given (Formula presented) ion depends on the ionic radius of the rare earth ((Formula presented)) and is higher for larger rare earths. The trend in (Formula presented) suppression as a function of concentration ((Formula presented)) shows deviation from Abrikosov-Gorșkov behavior. A metal-insulator transition is observed at higher dopant concentrations, and the semiconducting phases are found to obey the Mottșs variable range hopping mechanism of conduction. The parameters related to hopping conduction; viz., the characteristic temperature ((Formula presented)), localization length ((Formula presented)), hopping range ((Formula presented)), and hop energy ((Formula presented)) have been calculated, and a comparative study of the variation of these parameters in the two systems has been made. © 1996 The American Physical Society.