A survey of the known highT_c ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highT_c superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highT_c materials.

A series of ternary equiatiomic transition metal silicidesMM′Si (M=Zr, Nb, Ta;M′=Ru, Re) and Zr_1/2M_1/2 RuSi (M=Nb, Ta) have been synthesized and characterized. Guinier x-ray data clearly indicate that the compoundsMM′Si (M=Nb, Ta;M′=Ru, Re) crystallize with the orthorhombic TiFeSi-structure. This is contrary to one of the literature reports ascribing the hexagonal ZrRuSi structure toMReSi,M=Nb, Ta. ZrRuSi and Zr_1/2M_1/2RuSi (M=Nb, Ta), however, crystallize with the ZrRuSi structure. All the compounds are metallic and exhibit small and negative Seebeck coefficients. Superconductivity has been discovered inMM′Si,M=Nb, Ta;M′=Ru, Re and the transition temperatures, as measured by the low frequency a.c. susceptibility technique, range from 2·7 to 5·1 K. Zr_1/2Nb_1/2RuSi is not superconducting above 2·2 K. The observed properties are explained in terms of the electronic charge transfer effects and crystal structures adopted by these metal excess ternary silicides.