Sol-gel synthesis, characterization and application of lanthanide-doped cobalt chromites (CoCr2-xLnxO4; Ln = Tm3+ and Yb3+)

Cobalt chromite based pigments CoCr2-xLnxO4 (Ln = Tm3+ and Yb3+) with different substitutional level of lanthanide (x = 0-0.5) have been synthesized using aqueous sol-gel synthetic approach. The XRD analysis revealed that single phase spinel was obtained only with low content of lanthanide ions (x = 0.01-0.04). The sol-gel derived powders with higher concentration of lanthanide (x = 0.05-0.2) contained minor amount of orthochromite phase. At the highest substitutional level (x ≥ 0.2) the perovskite phase became the main crystalline phase. The colour of obtained pigments and corresponding ceramic glazes were analogous. Depending on the dominant phase, the colour varied from bluish-green (prevailing spinel phase) to dark brownish green (the main perovskite phase). This study proved that the replacement of chromium ions by thulium and ytterbium was successful at low content of lanthanides influencing the shade of pigment and corresponding glazes.