Thermal behaviour, infrared spectra and x-ray structures of some new rare earth chromates(VI)

Abstract

Five kinds of rare earth chromates(VI) and one rare earth dichromate(VI), altogether sixteen compounds, were synthesized. The crystal structures were determined by single crystal X-ray diffraction for eleven of the compounds: [Ln₂(CrO₄)₃(H₂O)₅]‧2H₂O (Ln=Nd, Eu, Tb), [Yb₂(OH)₂-(CrO₄)₂(H₂O)₂.₅]‧0.5H₂O, Ln₂(OH)₂(CrO₄)₂ (Ln= Nd, Gd, Tb, Yb), Ln₃(OH)(CrO₄)₄(H₂O)₃.₅ (Ln= Pr, Nd), and Pr(OH)CrO₄. The rare earth dichromate(VI), Gd₂(Cr₂O₇)₃‧9H₂O, formed poor quality crystals and was studied only by thermal and infrared analysis. All the rare earth chromate(VI) compounds with the same chemical formula crystallized isostructurally. Different phases were not found for any of the compounds. As the main object of the work was to synthesise new rare earth chromate(VI) crystals good enough for structural analyses, three different preparation methods were investigated: (1) crystallization by evaporation in air at standard temperature and pressure, (2) crystallization in gel and (3) crystallization in an autoclave. The last way proved to be the best. Exceptionally, gadolinium dichromate was prepared by evaporation. Study of the thermal behaviour of the compounds revealed six decomposition mechanisms. Comparison of the thermal decomposition of the rare earth chromates(VI) from the crystal structure point of view showed the decomposition to depend on (1) how the CrO4 tetrahedra are coordinated to the structure (is there any bidentate coordination) and (2) the degree and nature of the distortion of the CrO₄ tetrahedra. Infrared spectra were recorded for eleven of the compounds, and powder diffraction data were obtained for [Ln₂(CrO₄)₃(H₂O)₅]‧2H₂O Ln= La, Pr, Nd, Sm, Eu. A short review of rare earth chromate chemistry is included as an introduction.