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

Abstract

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 BiRh0.81 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 BiRh0.81, respectively.