Isothermal section of the Ho-Co-Bi system at 800 °C and new ternary Mo5B2Si-type bismuthides R5Co2Bi (R = Gd, Ho, Dy)

The isothermal section of the ternary Ho-Co-Bi phase diagram has been investigated at 800 °C. The existence of three previously known compounds was confirmed: Ho12Co5Bi (own type, Immm, a = 9.3730(5) Å, b = 9.3873(5) Å, c = 9.8544(5) Å), Ho6CoBi2 (Zr6CoAl2-type, P6¯2m, a = 8.2520(3) Å, c = 4.0974(2) Å), and Ho5CoBi2 (β-Yb5Sb3-type, Pnma, a = 12.1526(9) Å, b = 8.9270(7) Å, c = 7.9247(6) Å). Ho5CoBi2 was demonstrated to be the end-member of a solid solution, Ho5CoxBi3−x, with a homogeneity range of 0.65 < x < 1.0. A new ternary compound, Ho5Co2Bi, has been discovered, adopting the Mo5B2Si-type structure (I4/mcm, a = 7.4816(4) Å, c = 13.8838(9) Å). Rare-earth substitution can be extended to yield R5Co2Bi (R = Gd, Dy).