Oxynitride systems as potential inorganic pigments

• Main Author: Rooke, Joanne Claire
• Published: University of Southampton 2004
• Subjects: 546.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.494463

Inorganic oxynitride materials, with potential applications as pigments, have been synthesised and characterised in terms of their crystal structure, band gap energies and physical properties using powder X-ray and powder neutron diffraction (PXD,PND), solid state UV-Visible spectroscopy, L*a*b* colour measurements, scanning electron microscopy (SEM), energy dispersive analysis of X-rays and thermal analysis. Oxynitride perovskites, AII,IIIBV(O,N)3, where A is a group 2 or lanthanide metal and B an early transition metal have been synthesised by the direct ammonolysis of stoichiometric metal oxides and carbonates at high temperatures, typically 850°C, with the inclusion of group 1 and 2 metal halide salts acting as mineralisers. Powder X-ray diffraction studies show the synthesised perovskites to have primarily an orthorhombic distortion, crystallising in the space group Pnma, similar to CaTaO2N. Exceptions include compounds crystallising in a rhombohedral system, Ca0.77Eu0.23TaO1.77N1.23 or in a monoclinic system, LaTaON2. PXD profiles have highlighted the presence of a competing pyrochlore phase, which becomes more prevalent as the radius of the divalent and trivalent cations decreases such that perovskite phases for smaller lanthanides do not form. Neutron diffraction studies show that the oxygen to nitrogen ratio in the orthorhombic perovskite is Ca0.5Nd0.5TaO1.5N1.5 and there is a slight ordering of O/N over the two anionic sites. In the octahedra forming the framework, the axial positions have greater nitrogen occupancy, whereas the equatorial positions favour oxygen. In contrast to this, the group 2 substitutions onto the lanthanide position in LaTaON2 yielding A(1-x)LaxTaO(2-x)N(1+x), have shown a random distribution of oxygen and nitrogen over both positions. UV-Visible spectroscopy of these oxynitrides reveal broad reflectance bands and a steep absorption edge, which is directly related to the and gap energy of the compound. By varying the doping levels on the B site with transition elements other than Ta such as Nb, Ti, W and Zr, the colour of the compound can be seen to visibly alter. Solution methods, such as co-precipitation techniques, have been employed to deposit oxide precursors onto mica flakes, which are then fired under flowing ammonia to target thin films of oxynitride pigment on mica to produce interference effects. SEM has shown the presence of two separate morphologies, mica and pigment, thus resulting in unsuccessful depositions. Although the pearlescent nature of mica remains after heat treatment with ammonia, solution methods do not yield the same intense colours seen by the solid state route. This necessitates the use of halide salts to target pigments with high quality hues but this destroys the pearlescent effects seen in mica.