Simulations of cubic-tetragonal ferroelastics

We study domain patterns in cubic-tetragonal ferroelastics by solving numerically equations of motion derived from a Landau model of the phase transition, including dissipative stresses. Our system sizes, of up to 256^3 points, are large enough to reveal many structures observed experimentally. Most patterns found at late stages in the relaxation are multiply banded; all three tetragonal variants appear, but inequivalently. Two of the variants form broad primary bands; the third intrudes into the others to form narrow secondary bands with the hosts. On colliding with walls between the primary variants, the third either terminates or forms a chevron. The multipy banded patterns, with the two domain sizes, the chevrons and the terminations, are seen in the microscopy of zirconia and other cubic-tetragonal ferroelastics. We examine also transient structures obtained much earlier in the relaxation; these show the above features and others also observed in experiment.Comment: 7 pages, 6 colour figures not embedded in text. Major revisions in conten

Images of absolute retardance L.Deltan, using the rotating polariser method

Modulation techniques for measuring changes in optical birefringence, such as the rotating-polariser method (Wood and Glazer, 1980, J. Appl. Crystallogr. 13, 217), allow one to determine |sin delta|, delta = 2piLDeltan/lambda, Deltan = double refraction, L = light path and lambda = wavelength. However, they generally suffer from not providing absolute values of the optical retardance or are limited to relatively low retardance values. In addition, knowledge of the absolute phase is required when establishing the correct values of optical orientation information. In this paper, it is shown how the phase delta, and thus optical retardance, can be extracted from combining measurements of |sin delta| at different wavelengths. The new approach works on each single point of a 2-D picture without the need to correlate with neighbouring points. There is virtually no limit to the retardance, and the computational efforts are small compared with other methods (e.g. Ajovalasit et al. 1998, J. Strain Analysis 33, 75). When used with imaging techniques, such as the rotating polariser method of Glazer, Lewis and Kaminsky 1996 (Proc. R. Soc. London Series A452, 2751) this process has the potential to identify automatically optically anisotropic substances under the microscope. The algorithm derived in this paper is valid not only for birefringence studies, but can be applied to all studies of interfering light waves

Investigation of a peculiar relaxor ferroelectric: Na0.5Bi0.5TiO3.

The relaxor ferroelectric, Sodium Bismuth Titanate, (Na0.5Bi0.5TiO3), NBT is an A-site substituted distorted perovskite. An extensive temperature-, pressure- and substitution-dependent study of the crystal structure and phase transitions in this compound has been carried out using neutron/x-ray diffraction techniques. This has been complemented with optical, Raman and computational modelling studies.The temperature-dependent investigation shows that NBT undergoes a first phase transition from the high-temperature cubic prototype (Pm (3) over bar m) Phase I to Phase II of tetragonal symmetry (P4bm) and then a second one with strong diffuse character to Phase III of rhombohedral symmetry (R3c). The high-pressure study of NBT has revealed an interesting and unusual relaxor-to-antiferroelectric (R3c --> Pnma) phase transition. Structural variations as a function of temperature and doping across the (Na1-xKx)(0.5)Bi0.5TiO3 (NKBT) series have also been established