Synthesis and characterization of praseodymium-containing ZrSiO4 solid solutions from gels

The preparation and characterization of a series of praseodymium–zircon solid solution (Prx–ZrSiO4) materials with increasing nominal amounts of Pr is reported. Pr-doped zircon gels were prepared by gelling mixtures of zirconium n-propoxide, praseodymium acetylacetonate and tetraethylorthosilicate, and annealed over the range of temperature up to the formation of Pr–zircon solid solutions. The reaction sequence was followed by X-ray powder diffraction (XRD), ultraviolet–visible diffuse reflectance (DR) and infrared spectroscopy (IR). The first crystalline phase detected on annealing gels was a tetragonal praseodymium-containing ZrO2 phase (t-Pr–ZrO2). On further annealing, the subsequent transformation to the monoclinic form (m-Pr–ZrO2) took place. The formation of final Pr–ZrSiO4 solid solutions occurred by the reaction between m-Pr–ZrO2 and amorphous silica phase. The mechanism of solid solution formation inferred from energy dispersive X-ray microanalysis (SEM/EDX) data, variation of lattice parameters and DR of final Pr–ZrSiO4 solid solutions involved the replacement of Zr4+ by Pr4+ in dodecahedral sites of the zircon structure. DR revealed that a relatively small amount of Pr3+ was still present in final Pr-containing ZrSiO4 products. The estimated solubility of praseodymium in the zircon was around 0.067 mol of praseodymium per mol of zircon (∼11.5 wt% as Pr2O3). This study opens new perspectives to the development of more ecological zircon-based ceramic pigmenting systems by using mineralizer-free sol–gel synthetic techniques.