Theoretical study of the high-pressure effect in the cristalline structure and properties of the rhodium (III) sesquioxide

Abstract

Rhodium sesquioxide is the inorganic compound with the formula Rh2O3. It is a gray solid that is insoluble in ordinary solvents. It can be produced via several routes [1]: The most common route is treating RhCl3 with oxygen at high temperatures. Other option is fusing Rh metal powder with potassium hydrogen sulfate and adding sodium hydroxide, that results in hydrated rhodium oxide, which upon heating converts to Rh2O3. It is possible by exposing Rh layer to oxygen plasma too and finally, nanoparticles can be produced by the hydrothermal synthesis. Rhodium oxide films behave as a fast two-color electrochromic system: Reversible yellow to dark green or yellow to brown-purple color changes are obtained in KOH solutions by applying a voltage of 1 V. Rhodium oxide films are transparent and conductive, like indium tin oxide (ITO) - the common transparent electrode, but Rh2O3 has 0.2 eV lower work function than ITO. Consequently, deposition of rhodium oxide on ITO improves the carrier injection from ITO thereby improving the electrical properties of organic lightemitting diodes. Rhodium oxides are catalysts for hydroformylation of alkenes, N2O production from NO, and the hydrogenation of CO. Rhodium as a metal has been used both in catalysts and electrochemical application. Therefore, knowledge of the formation of Rh (III) oxide polymorphs, their structures and relative thermodynamic stability is important for understanding the nature of catalytic and electronic systems. Several published experimental studies provide evidence for the existence of three polymorphs of Rh2O3. These are: the corundum form Rh2O3 I (space group R-3c) described as the low-temperature, low-pressure form and two other corundum-related orthorhombic structures; the high-temperature, high-pressure form Rh2O3 II (space group Pbna) and the high-temperature, low-pressure form Rh2O3 III (space group Pbca). So far, however, beyond these broad categorizations, the relative stabilities of the three phases are still unclear, and the P-T phase diagram for the three rhodium sesquioxide phases is basically unknown because the treatment of ionic solids is rare. [...]