This thesis is an investigation of the elastic and an-elastic properties of LiYF4 and materials of related structure (such as CaWO4-Scheelite).These materials have a space group symmetry I41/a, the symmetry of the elastic constant tensors is therefore that of the TII Laue group. However when the basis of reference is transformed to that of the "acoustic symmetry" axes (Katkevich 1962 and Farley and Saundees 1972) the symmetry of the second order elastic constants of materials belonging to the TII Laue group exhibit the higher symmetry of the TI Laue group. This has been investigated experimentally for CaWO4 and some other oxide-"scheelites" (eg Farley and Saunders 1972.), and is here investigated for the first time in a flouride scheelite LiYF4. While the increased symmetry of the second order elastic constants with use of the "acoustic symmetry" axes as reference basis has been well established the validity of applying similar simplifactions, when investigating the third order elastic constants, has here been demonstrated for the first time. In establishing the position of the "acoustic symmetry" axes in LiYF4 it was first necessary to determine the conventional sense of +Z axis within the specimens used, extensive X-ray analysis was carried out to achieve this. The second order elastic stiffness constants of LiYF4 were measured (derived from the velocity of certainn ultrasound modes), as were the temperature dependences of these constants. The hydrostatic-pressure dependence of the second order elastic constants of LiYF4, Li(Y0.5Tb0.5)F4, and CaWO4 have been measured. In the last section results of stress dependence measurements of certain ultrasound nodes, measured in CaWO4 samples cut on the basis of the "acoustic symmetry" axes for this material (made by Dr A J Miller), have been used to determine the pertinent transformed set of third order elastic constants.
|Date of Award||1981|