Effect of heat treatment of pure and carbon-polluted rhodium samples on the low-temperature resistivity

We present a systematic investigation of conditions for heat treatment of Rh with the aim of increasing the residual resistivity ratio (RRR). The maximal value of RRR for a 25 μm thick foil was found to be 1050 and the optimal treatment conditions were high temperatures, above 1400°C, and a low pressure of pure oxygen, around 1 μbar. Another batch of foils, containing less magnetic impurities, showed an RRR of only 600. A 0.4 mm thick single crystal was heat treated to an RRR value of 740. Our findings are discussed in the light of a model with magnetic and non-magnetic impurities in Rh, where the latter is found to have an important contribution for this unusual metal. Especially carbon impurities were found to be quite detrimental for the resistivity, and the recovery of the RRR after a carbon contamination is extremely slow in subsequent heat treatments.