Formation of ε-Co nanocrystals in borosilicate and aluminosilicate glasses

Abstract Formation of cobalt nanocrystals in durable borosilicate and aluminosilicate glass matrices is studied. Using a hydrogen gas flow treatment, e-Co nanocrystals precipitate primarily in these glasses. This rare Co allotrope, which so far has mainly been synthetized by organometallic routes is found for the first time in a glass. XRD and TEM results indicate that e-Co is present in glasses ≥ 500 °C and that it successively transforms into the more stable α- and β-Co allotropes. Primary formation of the e-Co in glasses is in agreement with the Ostwald-Volmer rule of stage as it yields the lowest density change with respect to the parent glass among the three Co allotropes.