Investigation of inorganic materials for the application of inorganic pigments

A range of inorganic materials, with potential applications as inorganic pigments have been synthesised, characterised using powder X-ray and neutron diffraction (PXD, PND) UV-visible spectroscopy, electron microscopy, colour measurements and placed in context with commercially available inorganic pigments. Lanthanide chromates(V), LnCrO4 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), have been prepared, as green powders, by the reaction of the component metal oxides in air. The materials adopt a zircon-type structure with the space group 141/amd, with the exception of lanthanum chromate(V) which adopts a related, but more complex, monazite-type structure in the space group P21/n. The zircon type structure consists of LnO8 polyhedra linked via discrete Cr(V)O4 tetrahedra, the chromophore in these materials. The colour of the compounds in the series Pr to Yb is seen to become increasingly more yellow and lighter as the series is traversed. Analysis of PXD data using Rietveld profile fitting methods shows that the structural changes that are a result of the decreasing size of the lanthanide ions are responsible for the colour change. Contraction of the lattice parameters increases the distortion to the Cr(V)O4 tetrahedra and at the same time a small decrease in the Cr-O bond length. UV-visible spectroscopy identified that the combination of the 2T2 ← 2E transition and charge transfer bands of the Cr(V)O4 unit imparts the green colour. Colour values for the series are comparable to the inorganic pigment chromium(III) oxide. The green alkaline earth bischromates(V), M3(CrO4)2 (M=Ba and Sr) space group R - 3 m, have been prepared via conventional solid state synthesis from the component metal oxides. As with the lanthanide chromates(V), UV-visible spectroscopy identified the green colour as being derived from a combination of the 2T2 ← 2E transition coupled with charge transfer bands; however for these materials the colours, in terms of use as inorganic pigments, were found to be inferior to chromium(III) oxide. The space group of calcium analogue, Ca3(CrO4)2, was found to be R3c. Initial analysis of PXD data indicated that the material was calcium deficient but the collection and analysis of PND data indicated defect levels were low. The presence of three distinct chromium(V) sites in this material, and the possible mixed chromium valence resulting from small levels of calcium deficiency, may be the origin of the black colour of this material.