Crystal Structures, Superconducting Properties, and the Coloring Problem in ReAlSi and ReGaSi.

The only nonsuperconducting rhenium-silicon binary compound, ReSi1.75, was heavily p-doped with Ga and Al into ReGaSi and ReAlSi in an attempt to evoke superconductivity. They were synthesized and their crystal structures were studied by both X-ray and neutron diffraction. Si and Ga/Al atoms are ordered into alternating layers, which was rationalized with the "coloring problem" study via first-principles calculations. ReGaSi cannot be further p-doped with more Ga, but ReAlSi can be doped with more Al to ReAl1.2Si0.8, in which Si and Al atoms are not ordered but randomly distributed on the same sites. The superconductivity measurements over these compounds demonstrate that the ordered ReAlSi and ReGaSi are not bulk superconductors. However, ReAl1.2Si0.8 becomes bulk superconductor with Tc = ∼3.5 K, which has been confirmed by magnetism, resistivity, and specific heat measurements.