Tin Starved Pseudo-Phase in the Growth of Superconducting Thin-Film Nb3Sn: a DFT and Monte Carlo Study

While the art of making Nb₃Sn wires was perfected long ago, vapor diffusion grown Nb₃Sn layers have so far fallen well short of their theoretical potential, leaving applications like high-T_c SRF cavities tantalizingly out of reach. Recently, high-resolution microscopy has revealed sharply defined regions of tin depleted Nb_0.83Sn_0.17 stoichiometry just beneath the surface of these layers. Because the superconducting properties of Nb₃Sn are known to be sensitive to even slight changes in stoichiometry, these regions could be limiting the performance of the layers.

Using density-functional theory, we reproduce the known stoichiometry range of Nb₃Sn and determine whether tin-depleted regions represent a new phase in the Nb/Sn system or result from kinetic limitations during growth. Specifically, we develop and validate a cluster-expansion model for the relevant defects, whose thermodynamics we explore using Monte Carlo methods. Based on this model and the latest experimental data, we propose a kinetic mechanism explaining the presence of the tin depleted pseudo-phase in Nb₃Sn layers, and propose modifications to present growth procedures to mitigate the appearance of this pseudo-phase in the future.