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

Seven rare-earth metal hydrogarnets Sr₃[RE(OH)₆]₂ (RE = Sc, Y, Ho-Lu) were synthesized at about 200 °C starting from the respective RE₂O₃ and Sr(NO₃)₂ in a KOH hydroflux with a water-to-base ratio of 1.6. All seven hydrogarnets crystallize in the acentric variant (space group I4 3d) of the cubic garnet structure. The crystal structures of the hydrogarnets Sr₃[RE(OH)₆]₂, the differences between both structural variants including a simple method to distinguish between them, and the crystal-chemical classification with respect to other known strontium hydrogarnets are discussed. The rare-earth hydrogarnets can be used as carbon-free precursors for magnetic oxides. Starting at about 300 °C, the hydrogarnets decompose in a two- or three-step dehydration to SrRE₂O₄ and SrO. The decomposition of Sr₃[Sc(OH)₆]₂ follows a different mechanism and was studied by in-situ temperature-dependent powder X-ray diffraction up to 1000 °C. The final decomposition products were SrO and an unknown strontium scandium oxide, with an X-ray pattern similar to BaSc₂O₄. Magnetic measurements of the erbium and ytterbium hydrogarnets revealed paramagnetic behavior down to 1.8 K.     

Hydrothermal Synthesis,Crystal Structure Determination,and Magnetic Properties of a New Calcium Iron Iridium Hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(O4H4)1+x with 0 ≤ x ≤ 1

The new calcium iron iridium hydrogarnet Ca₃(Ir_2–xFe_x)(FeO₄)_2–x(H₄O₄)_1+x (0 ≤ x ≤ 1) was obtained by hydrothermal synthesis under strongly oxidizing alkaline conditions. The compound adopts a garnet‐like crystal structure and crystallizes in the acentric cubic space group I4 3d (no. 220) with a = 12.5396(6) Å determined at T = 100 K for a crystal with a refined composition Ca₃(Ir_1.4Fe_0.6)(FeO₄)_1.4(O₄H₄)_1.6. Iridium and iron statistically occupy the octahedrally coordinated metal position, the two crystallographically independent tetrahedral sites are partially occupied by iron. Hydroxide groups are found to cluster as hydrogarnet defects, i.e. partially substituting oxide anions around the empty tetrahedral metal sites. The presence of hydroxide ions was confirmed by infrared spectroscopy and the hydrogen content was quantified by carrier gas hot extraction; the overall composition was verified by energy dispersive X‐ray spectroscopy. The structure model is supported by ⁵⁷Fe‐Mössbauer spectroscopic data evidencing different Fe sites and a magnetic ordering of the octahedral iron sublattice at room temperature. The thermal decomposition proceeds via three steps of water loss and results in Ca₂Fe₂O₅, Fe₂O₃ and Ir. Mössbauer and magnetization data suggest magnetic order at ambient temperature with complex magnetic interactions.     

水合石榴石Sr3Fe2(OH)12的温和水热合成与M(o)ssbauer谱研究

在温和水热体系中合成出单相水合石榴石Ｓｒ３Ｆｅ２（ＯＨ）１２．产物在５７３Ｋ左右经历快速的与Ｓｒ最近邻的ＯＨ－断裂与分解过程，在空气或氧气中焙烧出现从Ｆｅ（Ⅲ）到Ｆｅ（Ⅳ）的氧化．骨架Ｆｅ离子价态变化直接决定分解过程及产物的结构类型．采用Ｍｏｓｓｂａｕｅｒ谱联用ＸＰＳ法测出Ｓｒ－Ｆｅ水合石榴石晶格中存在２种八面体Ｆｅ（ＯＨ）６．     

Controllable hydrothermal synthesis of star-shaped Sr3Fe2(OH)12 assemblies and their thermal decomposition and magnetic properties

Pure phase star-shaped hydrogarnet Sr₃Fe₂(OH)₁₂ assemblies were synthesized by a mild hydrothermal method (210 °C, 12 h), and the effects of the preparation conditions on the phase composition of the product were investigated. It was found that the impurity phases could be decreased or eliminated by increasing the molar ratio of Sr²⁺ to Fe³⁺, and that high temperatures favored the formation of Sr₃Fe₂(OH)₁₂ and reduced the concentration of CO₃^2–-containing byproducts. The thermal decomposition of the star-shaped Sr₃Fe₂(OH)₁₂ assemblies was examined, and the results showed that the dehydration process at higher temperatures is accompanied by the formation of SrFeO_3–δ. Above 655 °C, a solid state reaction between the SrFeO_3–δ and Sr(OH)₂ or SrCO₃ results in the formation of Sr₄Fe₃O_10–δ.The magnetic properties of the as-synthesized Sr₃Fe₂(OH)₁₂ and of samples calcined at different temperatures were assessed. A sample calcined at 575 °C exhibited greatly enhanced ferromagnetic properties, with a remanent magnetization of 1.28 emu/g and a coercivity of 4522.1 Oe at room temperature.