Novel precursors for the deposition of rare earth oxides

During this work rare earth solvates with nitrate and perchlorate anions have been investigated. All compounds have been structurally characterized and analyzed using thermal gravimetric analysis. The decomposition residues were analyzed using powder diffraction methods. Almost all compounds showed a characteristically intense exothermic decomposition step during the thermal decomposition, most likely caused by an intramolecular redox reaction between the nitrate or perchlorate anion respectively and the organic solvent molecules. The nitrates RE(NO₃)₃(CH(OCH₃)₃)₂ (RE = Sm, Eu) were isolated and characterized for the first time as the intermediate of the dehydration reaction with trimethyl orthoformate. The known compound group of dimethoxyethane solvates was then expanded with RE(NO₃)₃(O₂C₄H₁₀) (RE = La, Sm, Eu). Considering the possible use as precursor material the already described neodymium compound is also discussed. The thermal decomposition of these compounds yields the respective cubic rare earth oxide and shows the typical intense exothermic decomposition reaction. A variety of different precursor system based on nitrate solvates for the deposition of rare earth oxide layers on a silicon surface was developed and investigated in collaboration with the group of Prof. Dr. Al-Shamery (Univ. Oldenburg). Ultra thin films on a H-Si(111) surface were obtained via the deposition of the precursor, which was dissolved in organic solvents. An oxide layer was detected after the heating of the sample. The film thickness was measured as < 10 nm, whereas the thickness of the film was controlled by the concentration of the precursor solution. Sm(ClO₄)₃(CH(OCH₃)₃)₃ was isolated and characterized for the first time as the intermediate of the dehydration reaction with trimethyl orthoformate. Eu(ClO₄)₃(CH(OCH₃)₃)₂(MeOH)₂ was obtained without recrystallization. The methanol molecules, formed during the hydrolysis of the trimethyl orthoformate, were not fully replaced by the trimethyl orthoformate molecules. The thermal decomposition yield the respective rare earth oxide chloride REOCl (RE = Sm, Eu) and is divided into three endothermic steps followed by an exothermic step. Presumably, the compounds first lose the solvent molecules in multiple steps followed by a decomposition of the solvent free RE(ClO₄)₃. Three different structure types have been determined for the dimethoxyethane solvates of the rare earth perchlorates, corresponding to the lanthanide ion radii. For the lighter rare earth elements the compound [RE(ClO₄)₂(O₂C₄H₁₀)₃](ClO₄) (RE = La, Eu) was synthesized and characterized. Non coordinating perchlorate anions exist alongside the complex [RE(ClO₄)₂(O₂C₄H₁₀)₃]⁺ cation. In addition, RE(ClO₄)₂(O₂C₄H₁₀)₂ (RE = Pr, Nd) was obtained, analogue to the respective nitrates. [RE(ClO₄)₂(O₂C₄H₁₀)₃](ClO₄)(O₂C₄H₁₀)_0.5 (RE = Er, Lu) was found for the heavier rare earth elements, which contains further non coordinated solvent molecules in contrast to the first structure type. The thermal decomposition is dominated by the typical exothermic step as well. The residue is either the respective rare earth oxide chloride REOCl (RE = La, Pr, Eu), or cubic rare earth oxide RE₂O₃ (RE = Nd, Er, Lu). The known structure type of the perchlorate tetrahydrofuran was expanded with the compound RE(ClO₄)₄(OC₄H₈)₄ (RE = Ce, Sm, Er, Lu). The orthorhombic modification for the heavier rare earth elements erbium and lutetium were also obtained. [Sc(ClO₄)(OMe)(OC₄H₈)₄](ClO₄) was structurally characterized using a completely new ionic structure type and analyzed thermally. The thermal decomposition is again dominated by an intense exothermic step. Contrary to the dimethoxyethane solvates, the respective rare earth oxide RE₂O₃ (RE = Er, Lu) and CeO₂ or the oxide chloride SmOCl is the remaining residue.