Hardness and oxidation resistance of the perovskite-type RRh ₃B_xC_1-x (R = Y, Sc)

Perovskite-type RRh₃B and RRh₃C (R = Y, Sc) form a continuous solid solution, RRh₃B_xC_1-x, in the range of 0 ≦ x ≦ 1 with cubic structure (space group: Pm3m, Z = 1). The values of the microhardness of YRh₃B_xC_1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are investigated as 4.4 ± 0.1, 4.9 ± 0.1, 5.5 ± 0.2, 6.4 ± 0.2 and 7.5 ± 0.15 GPa, respectively. On the other hand, the values of the microhardness of ScRh ₃B_xC_1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 4.5 ± 0.2, 6.1 ± 0.2, 7.4 ± 0.2, 8.9 ± 0.2 and 9.6 ± 0.1 GPa, respectively. Thus, the microhardness of RRh ₃B_xC_1-x continuously becomes larger with increasing boron content. The oxidation onset temperatures of YRh ₃B_xC_1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 604, 631, 655, 687 and 978 K, respectively. On the other hand, the oxidation onset temperatures of ScRh₃B_xC_1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 674, 675, 695, 725 and 753 K, respectively. Thermogravimetric analysis of the phase indicates that the oxidation onset temperature also increases with boron content. Thus, it appears that both mechanical strength and chemical stability of the RRh ₃B_xC_1-x phase essentially depend on its boron content. Ab initio calculations have been performed to obtain the equilibrium lattice constants and the bulk moduli. The calculated lattice constants are in excellent agreement with experimental results.