The Binary Bi-Rh Phase Diagram: Stable and Metastable Phases

The binary bismuth-rhodium (Bi-Rh) phase diagram was reinvestigated from 23 to 60 at.% Rh with focus on the BiRh phase, applying powder-x-ray diffraction (XRD), high temperature powder-XRD, differential thermal analyses and scanning electron microscopy. The phase boundaries of the BiRh phase at 750 °C and the temperature of its peritectic decomposition were refined. In addition, the existence of the two phases Bi₄Rh and Bi₂Rh (in two modifications depending on temperature) could be confirmed. Most of the reaction temperatures reported in the literature could be verified within a range of about ± 10 °C. Nevertheless, a few temperatures had to be revised, such as those of the peritectic reactions L + Rh  $$  BiRh at 979 °C and L + BiRh  $$  β-Bi₂Rh at 785 °C. No evidence could be found for the presence of a stable Bi₃Rh phase in well annealed samples; from the present results it must be concluded that Bi₃Rh is actually metastable. On the other hand, a new orthorhombic phase BiRh_0.81 was discovered which crystallizes in the MnP structure type (Pmna). It was found that the temperatures of the transition between the low-temperature modification α-Bi₂Rh and its high-temperature form β-Bi₂Rh depend considerably on the presence or absence of metastable Bi₃Rh and stable BiRh_0.81, respectively.