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Zr$_5$Sb$_3$$_-$$_x$Ru$_x$, a new superconductor in the W$_5$Si$_3$ structure type

We report that at low Ru contents, up to x = 0.2, the Zr$_5$Sb$_3$$_-$$_x$Ru$_x$ solid solution forms in the hexagonal Mn$_5$Si$_3$ structure type of the host (x = 0), but that at higher Ru contents (x = 0.4 - 0.6) the solid solution transforms into the tetragonal W$_5$Si$_3$ structure type. We find that tetragonal Zr$_5$Sb$_2$$_.$$_4$Ru$_0$$_.$$_6$ is superconducting at 5 K, significantly higher than the transition temperature of hexagonal Zr$_5$Sb$_3$ (x = 0), which has a T$_c$ of 2.3 K. In support of a hypothesis that certain structure types are favorable for superconductivity, we describe how the W$_5$Si$_3$ and Tl$_5$Te$_3$ structure types, both of which support superconductivity, are derived from the parent Al$_2$Cu type structure, in which superconductors are also found. Electronic structure calculations show that in Zr$_1$$_0$Sb$_5$Ru, a model for the new superconducting compound, the Fermi level is located on a peak in the electronic density of states.