Ion size effect on the charge transfer and Raman spectrum of the (Pb0.65Sr0.35)Sr2(R0.5Ca0.5)Cu2Oy compound

The effect of rare earth ion size on the charge transfer and Raman spectra of the rare-earth-substituted Pb-based 1212 compounds (Pb0.65Sr0.35)Sr-2(R0.5Ca0.5)Cu2Oy (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. (C) 1997 Elsevier Science B.V.