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The reaction of elemental rhodium and concentrated sulfuric acid (97 %) at 400 °C leads to red plate‐shaped single crystals of Rh2(SO4)3 [trigonal, R3, Z = 6, a = 806.8(1) pm, c = 2204.8(4) pm, V = 1242.8(4) × 106 pm3]. The same reaction carried out at a temperature of 475 °C leads to orange crystals of Rh2(SO4)3·2H2O [orthorhombic, Pnma, Z = 4, a = 920.46(2) pm, b = 1244.47(3) pm, c = 833.37(2) pm, V = 954.61(4) × 106 pm3]. Thermal analysis and X‐ray powder diffraction measurements were performed to investigate the decomposition and resulting residues of the compounds. For both compounds the decomposition occurs through two steps and is finished at around 1000 °C at the stage of elemental rhodium. Additionally both sulfates were characterized by means of IR spectroscopy.
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Schwarzer et al. (2016) studied this question.
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