Birefringence crystal structure

Refractive indices. The relations between the refractive indices of crystals and their atomic structures were first pointed out by W. L. Bragg (1924 6), who succeeded not only in correlating birefringence with structure in a general way but even in calculating, from the known... 

Knowles and Turan (2000) and Knowles (2005) have used the approach of Parsegian and Weiss (1972) to examine the effect of anisotropy on Hamaker constants. Such calculations are relevant for materials such as 7 -BN and rutile, Ti02, which exhibit strong anisotropy in their refractive indices because of their crystal structure. They make little difference to predicted values of Hamaker constants as a function of grain orientation for materials such as Si3N4 and SiC which, by comparison, exhibit modest values of birefringence. 

Diperoxotriamminechromium(IV) crystallizes in rhombic birefringent prisms of density 1.96 g./ml. (15°), which appear as deep brownish-red to black needles. Although insoluble in most solvents, it may be recrj stallized from aqueous ammonia. It is soluble in water with decomposition. The absorption spectra, magnetic susceptibility, and crystal structure of the compound have been measured. A crystallographic study reports the structure as a deformed pentagonal bipyramid. A recent investigation of the infrared spectrum" shows the presence of peroxide bands. 

Characterization of the crystal structure uses the birefringence of ice (/ e 1.3105, Uo = 1.3091). Between crossed polarizers, only a crystal oriented with the c-axis along the light... 

Ha)nvard et al. (2005) performed a comprehensive investigation of LaAlOs in the temperature range of 10-750 K and determined the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes via Raman spectroscopy. While all these experiments show that the behaviour at the critical point around Tc = 813 K is consistent with a second-order transition, some evidence for an additional anomalous behaviour below 730 K have been shown. This anomaly was explained by a biquadratic coupling between the primary order parameter of the transition and the hopping of intrinsic oxygen vacancies. 

Keywords crystal structure, process variables, molecular weight, molecular weight distribution (MWD), polydispersity, copoljoners, nucleating agents, melt spinning, spinline, effects of molecular variables, effects of processing variables, tensile properties of filaments, birefringence, take-up velocity, conformationally disordered structure, amorphous orientation, crystalline orientation. 

Some minerals exhibit birefringence with refractive indices different in differing directions, due to an asymmetric crystal structure. Plastics filled with these are dichroic and exhibit unusual visual appearances. 

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