Inter- and intragranular nanostructure and possible spinodal decomposition in low-resistivity bulk MgB2 with varying critical fields

Three electromagnetically well-characterized bulk samples with nominal resistivities at 40 K [rho(40 K)], varying from 1 to 18 muOmegacm, were investigated by conventional and hiah-resolution transmission electron microscopy. Clean, coherent, or semi-coherent grain boundaries and dirty-grain boundaries wetted by amorphous phases were found in all three samples, even though the starting sample A had the very low resistivity of 1 muOmegacm at 40 K, characteristic of clean-limit samples. Taking into account its porosity and wetted-grain boundary area, the true resistivity value is about 0.5 muOmegacm. Additional samples B and C, prepared by exposing sample A to Mg vapor, showed enhanced rho(40 K) values of 14 and 18 muOmegacm, without noticeable change in either inter- or intra-granular microstructure. Intragranular nanoprecipitates with characteristics of a spinodal of MgB7, with a size of 1-5 nm, were observed in a few areas of samples A and B at high local density; however at too low an overall density to explain the increased resistivities and upper critical fields.