  We address the hypothesis that in intermetallic compounds, contrary to a
long-standing view that considers electron count and crystal structure type as
the only significant chemical criteria for the occurrence of superconductivity,
consideration of the actual elements present is a third equally important
factor. The importance of chemical identity may seem obvious, especially to
chemists, but it has not previously been explicitly tested for intermetallic
superconductors. Here we test the hypothesis by searching for and finding a new
superconductor in the tetragonal symmetry Hf5Sb3-xMx solid solution. This phase
is ideal for this study, because M can be many 3d, 4d and 5d transition metals
in an M-Sb chain and is a minor elemental constituent. We find
superconductivity for M = Ru only. This is the case even when the electron
count can be adjusted to the same value with a different transition element
within the same structure type. This leads us to propose that, like Cu-O
(cuprate superconductors) and Fe-As (iron-pnictide superconductors) in
different classes of compounds, Ru and Sb may be a critical element pair for
superconductivity in intermetallic phases.
