Effects of pressure on the structural and superconducting properties of Nb 3 As, Nb 3 Si, Nb 3 Ge, and Nb 0.82 Ge 0.18

The effects of pressure on both the structure and superconducting transition temperatures (T/sub c/) of several Nb-based compounds are examined. Comparisons are made between the Ti/sub 3/P-structure type, in which Nb/sub 3/As and Nb/sub 3/Si normally crystallize, and the A-15 structure in which Nb/sub 3/Ge and many other high-T/sub c/ materials form. A comparison is also made between stoichiometric and nonstoichiometric forms of the A-15 structure. The compressibilities are measured under hydrostatic pressures to about 6 GPa; the pressure dependences of T/sub c/ are measured to about 10 GPa for the As and Si compounds and to about 1 GPa for the Ge compounds. The structural and electrical measurements are combined and the volume dependences of T/sub c/ are reported and compared. Considerations are given to the possibility of a pressure-induced transition from the Ti/sub 3/P type to the A-15 structure for the As and Si compounds.