Synthesis and characterisation of Y2O3 co-doped with Co (III) or Eu (III) and Ba (II) or Ca (II)

Abstract

Two years ago, the solid state chemistry research group at Jaume I University studied the possibility of developing new lanthanide-based inorganic pigments. The synthesis of M (II) (Ca or Ba) and Tb (III) co-doped Y2O3 ceramics by a modified Pechini method was reported to obtain new environmental friendly yellow pigments as an alternative to the toxic existing ones Different solid solutions were prepared to accomplish the best composition. The general structure was Y1.86−xMxTb0.14O3−x/2 where M represents calcium or barium ions and x ranges from 0.06 to 0.64. Spectroscopic measurements and thermal treatments showed that all samples incorporated terbium (IV) and terbium (III) ions into the Y2O3 structure. However, samples with calcium showed more intense yellow colouration due to the higher amount of terbium (IV) ions, whereas samples with barium were pale beige. It was concluded that the presence of calcium (rCa (II) (IC=6) = 114 pm) or barium (rBa (II) (IC=6)= 149 pm) alters the structure of the Y2O3 lattice varying the distance terbium-oxygen. A possible explanation to those results was that calcium, with a similar radius to yttrium (rY(III) (IC=6)= 104 pm) allows the charge transfer between terbium and oxygen ions and thus the oxidation of terbium ions, but barium is bigger than yttrium and the charge transfer transitions are hindered. Additional studies on the same structure were performed substituting terbium (IV) ions for other chromophores such as chromium (III) or iron (III). Results also showed a major deviation in colour for the samples containing calcium in the matrix. The present project deepens into the reasons behind the change in colour between samples doped with barium and calcium.