The Solid-State Chemistry of Some Titania-And Tin Pigments

Abstract

Natural inorganic pigments have been known since prehistoric times. Initially they were obtained from natural sources, but later synthetic pigments were produced empirically. The modern ceramic pigment industry began early in the18th century and since then, ceramic pigments have been the subject of scientific studies to the point where today research makes use of the latest techniques in solid state chemistry and physics to try to understand their properties. Over and above the all-important colour properties of any colouring material, ceramic pigments require additional properties including exceptional thermal and chemical stability. It is through a better understanding of the nature of ceramic pigments and their extraordinary properties that new and improved pigments, and related materials will be synthesised. In the current investigation, a number of Titania- and tin materials suitable for colouring ceramic articles were investigated using a twofold approach. Firstly, their synthesis was investigated and optimised, and secondly, they were investigated structurally. The synthesis focused on their physical and aesthetic properties, and the economic implications of the synthesis techniques. The structural investigation was two dimensional. Firstly, the macroscopic details of the host materials were investigated using scanning electron microscopy (SEM) and x-ray techniques. This is important since it is the host lattice which provides the extraordinary properties required of a ceramic pigment. Secondly, the microscopic structural details of the interaction of the host lattice with the small quantities of dopant(s) were investigated using SEM, vibrational and electronic spectroscopy. This interaction is of paramount importance since it is the interaction between the host lattice and guest chromophore that results in the all-important colour of the pigment. Pigments investigated include chromium doped malayaite, Cr4+ :CaSnOSiO4 , and its titanium analogue titanite, Cr4+:CaTiOSiO4. The chemical state of the chromium in both materials was investigated. In both cases, the chromium was found to be in its fourth oxidation state. Other properties investigated include the chromium coordination states, bond lengths, bond orders and force constants. InCr4+:CaTiOSiO4, the chromium was used as a probe to investigate the paraelectric-antiferroelectric temperature phase transition which titanite undergoes at ~500 K using resonance Raman spectroscopy. This technique was also used to investigate the high-pressure phase transition of titanite. This transition was shown to occur at ~27 kbar. Other pigments investigated include two minerals with the pyrochlore structure 2 Ti2O7 and Y 2Sn2O7, co-doped with calcium and vanadium. The coordination states and chemical states of the calcium and vanadium in these host lattices were investigated. The vanadium was found to be in its fifth oxidation state. The bond lengths and bond orders of the vanadium were determined for these compounds .Following the investigation of these doped mixed metal oxides, various other compounds were synthesised which were found to have potential for use as pigments