Formation of transition metal boride and carbide perovskites related to superconducting MgCNi₃

A general study of the formation of intermetallic perovskite borides and carbides in the ternary systems AXM₃ (A=Mg, Ca, Sc, Y, Lu, Zr, Nb; X=B, C; M=Ni, Ru, Rh, Pd, Pt) is reported. MgB_xPd₃, MgB_xPt₃, CaB_xPd₃, MgC _xRh₃, LuC_xRh₃, and ZrC _xRh₃ represent new intermetallic perovskites that form at a composition that is nominally stoichiometric (x=1). ScB_xPd ₃, YB_xPd₃, and NbB_xRh₃ are new single-phase perovskites that form only at a substoichiometric (x<1) nominal composition. The variable boron content of AB_xPd₃ (A=Mg, Ca, Sc, Y) was studied using lattice parameter data from X-ray diffraction measurements. For A=Ca and Mg, the unit cell volume increases with increasing boron content for 0.25≤x≤1.0, while boron uptake is limited to 0≤x≤0.5 for A=Sc and Y. Neutron diffraction studies indicate that CaB _xPd₃ is not actually stoichiometric at a nominal composition of x=1. Rather, saturation of the boron content occurs at CaB _0.76Pd₃. Evidence for superconductivity was found in samples of CaB_xPd₃ (x≥1) and NbB_xRh ₃, but bulk analysis suggests that superconductivity may be attributed to minority phases.