Reaction mechanism in fast combustion synthesis of superconducting FeSe and FeSe0.7Te0.3

Abstract The reaction mechanism in fast combustion synthesis of FeSe and FeSe0.7Te0.3 is investigated by a special quenching experiment and thermal analysis. The reaction temperature exceeds the melting point of Se but is below that of Fe, and the reaction takes place at the interface between solid Fe and liquid Se. By thermal analysis, the reaction almost immediately starts after Se melts and persists in a broad temperature range. By linear fitting from DSC data, the activation energies are calculated to be 102 ± 8 kJ/mol for the reaction of Fe + Se = FeSe and 138 ± 22 kJ/mol for Fe+0.7Se + 0.3Te = FeSe0.7Te0.3. The reaction zone consists of continuous liquid Se and separately distributed Fe solid particles, where each Fe particle together with the surrounding Se liquid layer can be regarded as a reaction cell. In a planar reaction front, the mass transfer and heat exchange between two reaction cells can be ignored, and the reaction happens individually but simultaneously in every cell. From microstructure characterization, it is suggested that the infiltration of Se liquid also plays an important role in mass transfer besides the conventional diffusion process. An evident element segregation is observed in the Fe(Se,Te) product, and the distributions of Se and Te are complementary. To explain the segregation phenomenon, an oscillatory reaction mechanism is proposed, where Se and Te alternatively reacts with Fe accompanying the oscillation of concentrations of Se and Te in the liquid. The reaction mechanism discussed here is likely to also operate in other solid-state synthesis approaches, and may help to optimize the processing parameters to improve the quality of synthesized Fe(Se,Te) samples.