|dc.description.abstract||Several ceramic pigments based on Cr-doped Mg0.5FeTi1.5O5 (“ferrian armalcolite”) solid solutions were prepared by the solid-state (ceramic) route, with the aim to investigate the effect of Cr-doping on the resulting reddish or dark-brown color, as well as its coloring performance as ceramic pigments or dyes in conventional low-temperature ceramic glazes.
For this purpose, two different doping schemes were formulated (Arm1 series: Mg0.5-x(Fe1Cr2x)Ti1.5-xO5, with x=0-0.25, in which Cr3+ is expected to replace both Ti4+ and Mg2+ ions; and Arm2 series: Mg0.5(Fe1-xCrx)Ti1.5O5, with x=0-0.5, in which Cr3+ is replacing exclusively Fe3+ ions).
The samples were calcined in industrial-like firing conditions up to 1000°C and 1200 °C (3 h soaking), and the resulting powders were characterized by XRD (formation of solid solution and cell parameters measurement) and UV-vis-NIR spectroscopy (optical spectra and CIE-L*a*b* parameters of fired powders and of samples enameled within two different ceramic glazes).
XRD results showed that at 1000 °C is observed that the increase of Cr doping is accompanied by a larger amount of secondary phases. This indicates that with a quickly thermal treatment (3 h/ 1000 °C) an armalcolite phase is not still stabilized by entropy. Remarkably, the increase of firing temperature from 1000 °C to 1200 °C leads to an important increase of reactivity in both series (Arm 1 and Arm2). In effect, at 1200 °C in the case of Arm1 series, the armalcolite phase appears almost as single phase from x=0 to x=0.1, with only a very small and almost negligible quantity of residual rutile. A monophasic system of armalcolite is not formed in ARM1Cr0.25 at 1200 °C. In contrast, Arm2 series, in which Cr3+ is replacing exclusively by Fe3+ ions, gets stabilize the solid solution of armalcolite.
On the other hand, UV-vis-NIR results showed that is very difficult to distinguish the presence of Cr3+ or Cr4+ in an octahedral sites in samples containing Fe3+ ions in these positions simultaneously, because the bands of Cr3+ and Cr4+ are overlapped to a high extent with the absorption bands of iron (III).||ca_CA