Crystalline orthorhombic Ln[CO₃][OH] (Ln=La, Pr, Nd, Sm, Eu, Gd) compounds hydrothermally synthesised with CO₂ from air as carbonate source

Crystalline orthorhombic rare earth carbonate hydroxides Ln[CO₃][OH] (Ln=La, Pr, Nd, Sm, Eu, Gd) were synthesised as phase pure powders via a simple hydrothermal reaction. CO₂ from air acted as natural carbonate source and cetyltrimethylammonium bromide was added as templating agent to an aqueous rare earth nitrate solution. Single-crystal X-ray structure determination was performed on La[CO₃][OH] (Pnma, a=7.4106(5), b=5.0502(3), c=8.5901(6) Aa, 563 independent reflections, 38 refined parameters, wR2=0.037), Pr[CO₃][OH] (Pnma, a=7.2755(4), b=4.9918(3), c=8.5207(5) Aa, 744 independent reflections, 38 refined parameters, wR2=0.04), Eu[CO₃][OH] (Pnma, a=7.1040(4), b=4.8940(3), c=8.4577(5) Aa, 1649 independent reflections, 38 refined parameters, wR2=0.05) and Gd[CO₃][OH] (Pnma, a=7.069(7), b=4.874(5), c=8.464(9) Aa, 431 independent reflections, 38 refined parameters, wR2=0.051). These findings are supported by powder XRD, infrared spectroscopy, UV/Vis spectroscopy and, for Pr[CO₃][OH] and Eu[CO₃][OH], by measurements of the non-linear optical properties. Thermal analysis could demonstrate the possible use of the Ln[CO₃][OH] phases as precursors for rare earth carbonate dioxides Ln₂[CO₃]O₂ and rare earth oxides Ln₂O₃. The decomposition products inherit the precursor's morphology. The lattice parameters of Pr₂[CO₃]O₂ were refined from high-temperature powder XRD data.