From Coordination Polymers to Doped Rare-Earth Oxides

A series of bimetallic coordination polymers were synthesised from lanthanide salts (chlorides or nitrates of Ce, Nd, Gd) and 2,5‐dihydroxy‐1,4‐benzoquinone (DHBQ) under aqueous and anhydrous conditions, to be used as preceramics for mixed oxide synthesis. Syntheses in water or tetrahydrofuran (THF) gave rise to two different types of metal–organic frameworks. A new polymeric species, [Gd(DHBQ)Cl(THF)2]n, was characterised by single‐crystal X‐ray diffraction {[Gd(DHBQ)Cl(THF)2]n, a = 8.643(2) Å, b = 9.917(5) Å, c = 10.422(3) Å, α = 104.611(3)°, β = 109.623(3)°, γ = 96.097(3)°, V = 796.1(5) Å3, triclinic, P$\bar {1}$, Z = 2}. Characterisation by IR and Raman spectroscopy, powder X‐ray diffraction and scanning electron microscopy showed that the mixed‐metal polymers are solid solutions and highlighted the fact that their shape and morphology are strongly influenced by the presence of water in the reaction mixture. Micrometric, well‐defined cylindrical objects were obtained from the aqueous synthesis, while the reaction in anhydrous THF led to nanometric spheres. Calcination at 850 °C of the coordination polymer preceramics easily yielded mixed NdxGd(1–x) and NdxCe(1–x) oxides as solid solutions, in which x varies between 0 and 1. The morphology of the preceramic powders is retained upon calcination. This allows easy control of some of the oxide properties from the coordination compound synthesis step. Indeed, BET measurements pointed out that the choice of anhydrous reaction conditions for the preceramic synthesis leads to enhancement of the specific surface areas of the oxides by one order of magnitude. Oxides based on f‐elements are of great interest. Control of the macroscopic properties of metal oxide materials is crucial and related to the microscopic structure of the ceramic. We show that thermal conversion of bimetallic coordination polymers with well‐defined structure and morphology is a promising way towards mixed rare‐earth oxides of controlled composition and flexible properties.