Alkaline-Earth Rhodium Hydroxides: Synthesis, Structures, and Thermal Decomposition to Complex Oxides

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Materials Science

Background:

• Rhodium(III) compounds are of interest due to their diverse applications.
• Hydrothermal synthesis offers a route to novel inorganic materials.
• Characterization of complex hydroxides is crucial for understanding their properties.

Purpose of the Study:

• To synthesize and determine the crystal structures of new rhodium(III) hydrogarnets.
• To discover and characterize novel complex rhodium hydroxides.
• To investigate the thermal decomposition pathways of these rhodium compounds.

Main Methods:

• Hydrothermal synthesis at 200 °C.
• Synchrotron X-ray diffraction (XRD) for Rietveld refinements.
• Single-crystal XRD for structure solution.
• Magic-angle spinning Nuclear Magnetic Resonance (NMR) spectroscopy.
• X-ray thermodiffractometry.
• X-ray absorption near-edge structure (XANES) spectroscopy.

Main Results:

• Crystal structures of Ca3Rh2(OH)12 and Sr3Rh2(OH)12 were determined.
• A new phase-pure complex rhodium hydroxide, BaNaRh(OH)6, was synthesized and its structure solved.
• NMR confirmed distinct sodium sites and diamagnetism in BaNaRh(OH)6.
• Thermal decomposition studies revealed different pathways for each material, yielding CaRh2O4 and Sr6Rh5O15.

Conclusions:

• Hydrothermal conditions are effective for synthesizing novel rhodium(III) hydrogarnets and complex hydroxides.
• The determined crystal structures provide insights into bonding and coordination in these rhodium compounds.
• Thermal decomposition leads to new oxide phases, including CaRh2O4 synthesized under ambient pressure conditions.