The existence of two phases within one and the same hexagonal lattice of MgB2 compound, differing in Mg and B (in the homogeneity region) and especially in impurity oxygen content, as well as in microstructure, is demonstrated by various techniques.
The regions corresponding to these two phases of MgB2 have the sizes of 100-500 {mu}m, and they fill the whole bulk of specimens, alternating with each other. It is suggested that the two-phase state of MgB2 compound is caused by specific features of its formation mechanism (as a result of synthesis at 800-1000{deg}C), including the stages of Mg melting, dissolution of solid boron in it up to the composition of MgB2 and further crystallization of the MgB2 compound from the melt with the formation of dendrite-like structure with corresponding redistribution of main components and impurities.
Annealing of Bi,Pb-2223/Ag composites in (O2+N2) atmosphere at 820-780C is believed to reduce the number of the accompanying phases, to make contacts between crystallites closer and to increase the critical current. The goals of this study were to re
veal the changes in the 2223 lattice at annealing in the reduced oxygen atmosphere, to elucidate the reasons of these changes and to discuss their effect on the ceramics superconductivity. After the annealing the transversely-polarized displacement waves of oxygen atoms in [010]2223 direction have been found in the 2223 phase by electron diffraction analysis. These waves could appear due to the lack of oxygen in the 2223 lattice or to the nitrogen penetration in it. As demonstrated by the X-ray photo-electron spectroscopy and nuclear microanalysis, nitrogen does not interact with the 2223 lattice, and the oxygen index decreases to 9.67, which is lower than the stoichiometric. Thus, the atomic displacement waves result from the lack of oxygen in Bi-O bilayers.
