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

Abstract

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. MgBxPd3, MgBxPt3, CaBxPd3, MgCxRh3, LuCxRh3, and ZrCxRh3 represent new intermetallic perovskites that form at a composition that is nominally stoichiometric (x=1). ScBxPd3, YBxPd3, and NbBxRh3 are new single-phase perovskites that form only at a substoichiometric (x<1) nominal composition. The variable boron content of ABxPd3 (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 CaBxPd3 is not actually stoichiometric at a nominal composition of x=1. Rather, saturation of the boron content occurs at CaB0.76Pd3. Evidence for superconductivity was found in samples of CaBxPd3 (x⩾1) and NbBxRh3, but bulk analysis suggests that superconductivity may be attributed to minority phases. © 2003 Elsevier Inc.