X-ray and neutron scattering

Abstract:

The (Na1-xKx)0.5Bi0.5TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5Bi0.5TiO3 (rhombohedral at 300 K) and K0.5Bi0.5TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+TiO3 and (Na1-2xK2x)+TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral ↔ tetragonal phase transition goes through an intermediate phase, located at 0.5 ≤ x ≤ 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations. © 2000 IOP Publishing Ltd.

Abstract:

Modulation techniques for measuring changes in optical birefringence, such as the rotating-polariser method (Wood & 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 & 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.

Abstract:

Measurements of the low-energy magnetic excitations in single-crystal PrBa2Cu3O6.2, and in YBa2Cu3O6.2 for comparison, have been performed using inelastic neutron scattering. An excitation with weak dispersion is seen, which is compared to a spin-wave model based on the lowest lying crystal field levels of the Pr ion, and including the effect of an anisotropic coupling to the Cu magnetic moments, as well as a Pr-Pr exchange interaction. (C) 2000 Elsevier Science B.V. All rights reserved.