Large, high-quality single-crystal seeds are required for the orientationcontrolled growth of iteratively larger high-quality bulk crystals, but seeds matching the desired growth material, such as UO 2 , are not always available. Refractory UO 2 is hydrothermally grown on CaF 2 , ThO 2 , UO 2 , and YSZ seeds, and the results are analyzed visually, with X-ray fluorescence, scanning electron microscopy-energy dispersive X-ray spectroscopy, μ-Raman spectroscopy, and scanning electron microscopy-electron backscattering diffraction. The interfaces of the heteroepitaxial growths are complex as the CaF 2 and YSZ partially back-dissolve to produce intermediate phases up to 100 μm thick that are chemically distinct from both the seed and the final UO 2 growth. The CaF 2 intermediate phase is a reasonable match to the high-pressure CaU 3 O 8+x phase, and the YSZ intermediate phase seems to be highly defective UO 2 . Despite the intermediate phases on the non-native substrates, the final UO 2 growth is of high quality and of the same orientation as the original seed, except when YSZ is used. The seeds with the highest quantity of growth have the poorest quality, likely a result of non-optimal growth conditions, which are also discussed. The final UO 2 layer in all cases is a viable native seed for future growths, dramatically expediting seed production when compared to the traditional iterative regrowth procedure.
Crystallite orientation identification is invaluable, but is often limited to small area identification or requires a large area sample. Nondestructive optical methods such as polarized Raman spectroscopy, in contrast, can be used to completely map a variety of sample sizes, but their potential is not yet fully realized. Here, we report a systematic study of polarized Raman scattering of high-quality, hydrothermally grown, single crystals of urania and thoria. The peak intensity variations for as-grown major crystal planes, post-growth polished crystal planes, and a post-growth polished non-crystallographic plane are directly linked to crystallographic orientation and crystal rotation, and agree with computed models. In particular, the parallel polarized peak intensity results are directly correlated with metal-oxygen-metal chains in the fluorite structure and can be used to determine both orientation and rotational alignment of a given crystal face if sufficiently small rotational steps are applied. These results are structure based, being applicable to the larger fluorite phase space, which is useful for optical, semiconductor, nuclear, and solid oxide fuel cell industries. Further, these results suggest that Raman spectroscopy can identify non-crystallographic orientations that are not discernable by traditional means.
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