Hydrothermal synthesis and structure determination of a new calcium iron ruthenium hydrogarnet

A new calcium iron ruthenium hydrogarnet with the approximate composition Ca₃(Ru_2-xFe_x)(FeO₄)_2-y(H₄O₄)_1+y (x=1, y∼0.35) has been obtained by hydrothermal synthesis under oxidizing alkaline conditions. The compound crystallizes in the cubic space group Ia anti 3d (No. 230) with a lattice parameter of a=12.4804(4) Aa (T=100 K) and Z=8. The octahedral site of the garnet structure is equally occupied by Ru and Fe, whereas the tetrahedral site is partially occupied by Fe only. A partial substitution of the oxide anions by hydroxide ions is necessary for charge balancing, corresponding to the so-called hydrogarnet defects. The presence of hydroxide groups is proven by infrared spectroscopy. ⁵⁷Fe Moessbauer spectroscopic data provide evidence for two different Fe³⁺ coordination environments as well as a magnetic ordering of two iron substructures with the respective ordering temperature above room temperature. The crystal composition was verified by energy-dispersive X-ray spectroscopy and the thermal behavior of the calcium iron ruthenate was studied by difference thermal analysis.