Ion size effect on the charge transfer and Raman spectrum of the (Pb 0.65Sr 0.35)Sr 2(R 0.5Ca 0.5)Cu 2O y compound

Abstract

The effect of rare earth ion size on the charge transfer and Raman spectra of the rare-earth-substituted Pb-based 1212 compounds (Pb 0.65Sr 0.35)Sr 2(R 0.5Ca 0.5)Cu 2O y (R = Y, La, Ce, Pr, Nd, Yb, Lu) has been studied systematically. X-ray diffraction showed that all the rare-earth-substituted samples were single phase of the Pb-1212 except R = Ce. The compound with R = Ce did not form under the same synthesis conditions. Both the a and c parameters increased monotonically with the substitution of larger size rare earth. AC susceptibility measurement revealed that all the samples showed superconducting transitions between 43 and 68 K except the largest ion La-substituted sample. T c showed a tendency to decrease with rare earth radius. Raman spectra of these samples illustrated that only one peak at about 540 cm −1 appeared for all the rare-earth-substituted samples in the range of 100 to 800 cm −1. This peak was attributed to the vibration of the apical-O(2) in this system. Its frequency showed a tendency to increase with rare earth ion. The relationship between lattice parameter, superconductivity, apical-O(2) vibrational characteristic and ionic radius were discussed in this paper. Based on several experimental facts, our present Pb-1212 samples are considered to lie in the underdoped region of the T c- n diagram.