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

Abstract

The rhodium(III) hydrogarnets CaRh(OH) and SrRh(OH) crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl·3HO and either Ca(OH) or Sr(OH) in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH), are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. CaRh(OH) yields CaRhO and CaO above 650 °C, from which phase-pure CaRhO is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. SrRh(OH) decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite SrRhO, which contains mixed-valent Rh, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH), a complex set of decomposition events takes place via transient phases.

Syntheses, Crystal Structures and Physical Properties of Chromium and Rhodium Hydrogarnets Ca3[Cr(OH)6]2, Sr3[Cr(OH)6]2 and Sr3[Rh(OH)6]2

“…The remaining strontium atoms are located between the columns. A very similar structure is realized in Sr 6 Rh 5 O 15 , which was recently stated as a possible decomposition product of Sr 3 [Rh(OH) 6 ] 2 . In Sr 6 Rh 5 O 15 , one‐dimensional columns of face‐sharing RhO 6 polyhedra are arranged in groups of three RhO 6 octahedra linked via one RhO 6 trigonal prism; the strontium atoms reside between the columns, too.…”

“…Syntheses and X‐ray powder diffraction data have been reported for Ca 3 [Cr(OH) 6 ] 2 , Sr 3 [Cr(OH) 6 ] 2 and Sr 3 [Rh(OH) 6 ] 2 as well as some thermal data on the strontium hexahydroxochromate(III) , . Recently the mixed transition metal hydrogarnets Ca 3 (Ru 2–x Fe x )(FeO 4 ) 2–y (O 4 H 4 ) 1+y and Ca 3 (Ir 2–x Fe x )(FeO 4 ) 2–y (O 4 H 4 ) 1+y have been reported, and on finalizing this manuscript we became aware of a manuscript describing synthesis, powder diffraction data and thermal decomposition of the calcium and strontium hexahydroxorhodates A 3 [Rh(OH) 6 ] 2 …”

CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

“…Upon further heating to 1000°C and 1200°C, the intermediate decomposition products thermal treatment. [6,7] For example, the decomposition of the hydrogarnet Sr 3 [Al(OH) 6 ] 2 to Sr 3 Al 2 O 6 takes place at about 600°C, whereas the solid-state synthesis with Al 2 O 3 and SrCO 3 as starting materials necessitates at least 1200°C and several grinding steps. [8,9] Furthermore, thermal decomposition of hydroxometalates could also be interesting for the formation of catalysts, absorbents or pigments.…”

The Hydrogarnets Sr3[RE(OH)6]2 (RE = Sc, Y, Ho – Lu): Syntheses, Crystal Structures, and their Thermal Decomposition to Ternary Rare‐Earth Metal Oxides

“…At 655 °C the formation of Sr 4 Fe 3 O 10–δ starts in a second reaction step . Hydrogarnets with main group and transition metal ions were thoroughly investigated . However, only little is known about rare‐earth metal containing hydrogarnets.…”