Structure and properties of MTiOXO4 crystals

This thesis is a study of the structural, optical and electrical properties of MTiOXO4 crystals, where M is a monovalent cation such as K, Rb etc and X is P or As. Low and high-temperature single-crystal and powder x-ray diffraction experiments have been performed of K0.6Rb0.4TiOAsO4 and RbTiOAsO4 using synchrotron radiation and sealed tube sources. This has allowed a detailed comparative structural study of the arsenate series KTiOAsO4, K0.6Rb0.4TiOAsO4 and RbTiOAsO4 to be completed. The mixed cation site crystal, K0.6Rb0.4TiOAsO4, has been seen to follow the same trends as its two end members, exhibiting structural features that are generally intermediary. Thermo-optic measurements of the three arsenate materials have been collected to determine the behaviour of the refractive indices and expansion of the samples as a function of temperature. The variation of Ps fits well to a Landau relationship, confirming that the crystals exhibit typical ferroelectric behaviour and undergo a second-order phase transition. The Tc of each crystal is seen to be depressed with the inclusion of a heavier cation. Modelling of the structural and optical data has confirmed that a relationship exists between the polarisability of certain atoms and the refractive indices of the arsenate crystals. The anisotropy of the optical properties is shown to be linked to chains of particular atoms along the three crystallographic axes. Dielectric measurements of a series of arsenate crystals and various doped phosphate crystals demonstrate that MTiOXO4 isomorphs exhibit dielectric relaxation of a non-Debye type and appear to conform to the hopping charge-carrier and low frequency dispersion response models. A reduction in the ionic conductivity is observed in the arsenate crystals and phosphate crystals doped with trivalent ions. Arrhenius plots indicate that the activation energies of the mixed cation arsenate crystals are significantly higher than the other KTiOPO4 isomorphs. This observation suggests that the modified oxygen framework in these mixed arsenate crystals contributes intrinsically to the large activation energies required for ionic conduction. (author)