This proposal is a collaboration between Professor A.M. Glazer (Crystallography Group, Clarendon Laboratory, Oxford) and Professors P.A. Thomas and M.E.Smith (Physics Dept., University of Warwick). The overall aim is to make for the first time, the crucial link between the absolute crystal structure of solid solutions in the lithium niobate and lithium tantalate series (hereafter LNT) and some of their highly unusual physical properties. A particular focus and point of key interest in this proposal is the existence of a composition in the series where the birefringence is zero at room temperature, so that the crystals become optically isotropic and yet remain electrically polar, which is an unique and extremely odd combination of properties in a nonlinear-optical, functional ferroelectric material. As part of our research programme, we intend to investigate this fascinating composition closely with a view to establishing whether such a material has potential as an unusually sensitive component in optically-based sensing applications, which are of high technical importance and timely relevance. For other LNT compositions, the point of optical isotropy can be obtained by raising the temperature, so that a locus line of zero birefringence points exists in the two-dimensional composition-temperature map. It is our goal to understand the occurrence of this behaviour across the LNT series from a fundamental point of view, whilst keeping in mind the potential for devices based on a combination of compositional and temperature tuning. In an entirely new and innovative twist, we will investigate for the first time the effect of the additional parameter pressure on the structure and properties of LNT in general, and particularly in the vicinity of the points of zero birefringence . Using birefringent imaging microsocopy, x-ray diffraction and solid state NMR at elevated pressures in a powerful combination of methodologies, we will map out the occurrence of the contours of zero birefringence in a three-dimensional parameter space to construct a composition-temperature-pressure (x-T-P) diagram. Since LN itself has large photoelastic and piezoelectric coefficients, we expect the pressure-dependence of the zero-birefringence points to be extremely high, thereby opening up the potential for a highly-sensitive and tunable pressure sensor. Our research will concentrate on expert x-ray structural analysis including absolute polarity determination (that is determination of the relationship between the direction of off-centre ions in the structures and the sense of electrical polarization) using anomalous x-ray scattering. These studies will be extended to non-ambient temperatures and pressures in order to fill out the parameter map and give the necessary data for interpretation of the zero birefringence contours. Alongside this, birefringent imaging microscopy will be used to map out the optical properties and thus, the zero birefringence contours of LNT compositions as a function of temperature, pressure and optical wavelength. Multinuclear solid state NMR will include 7Li, 17O and 93Nb, particularly to understand the role that octahedral distortions and cation displacements play in structure-property relations for compositionally-disordered crystals such as the LNT family. These will be extended to high temperatures using a dedicated probe constructed for 93Nb NMR and ultimately, to pressures of up to 5 GPa for sensitive zero-birefringence compositions as high-pressure NMR comes on-line. In summary, this research programme combines state-of-the-art methodologies to undertake novel science of a fundamental nature on the LNT series. It will both reveal new materials physics and answer some long-standing questions in the x-T-P space for LNT. Ultimately, and most speculatively, it may provide a new impetus for the development of devices based on this most unusual combination of physical properties in future years.