Field-modulated microwave absorption study of Pb-modified BiCaSrCuO

High-resolution transmission-electron-microscopy (HRTEM) has established that samples of nominal composition Bi2Sr2Ca2Cu4Oy prepared by solid-state reaction are of mixed phase. These samples are mostly composed of the 115 K polytypoid structure with the 80 K polytypoid structure found at grain-boundaries and at planar defects within grains. The partial replacement of Bi by Pb leads to a single phase of the 115 K polytypoid as determined from HRTEM. Low-magnetic-field modulated microwave absorption in the absence of Pb yields absorption onsets associated with transitions around 115 K and around 80 K. The addition of Pb eliminates the lower temperature transition. This observation is consistent with the HRTEM observations and supports the general view that microwave absorption arises primarily from grain boundaries and planar defects. © 1989.