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The preparation, determination of optical properties, and approximate crystal structure of the ferroelectric manganese (III) trioxides of holmium, erbium, thulium, ytterbium, lutetium, and yttrium are reported. The compounds crystallize in the hexagonal system, probable space group P6acm , and their structure is characterized by unusual five-and sevenfold coordination polyhedra about manganese and rare-earth atoms respectively. The unacceptable values assumed by the temperature factors of two oxygen ions throw doubt on the exact determination of atom positions by leastsquares refinement. Possible causes of this difficulty are reviewed.

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Binary mixtures of protonated and perdeuterated 1,4-polybutadiene have been examined by small-angle neutron scattering and are shown to exhibit an upper critical solution temperature. The scattering results above the critical temperature are described by the mean-field theory of de Gennes for homogeneous binary polymer mixtures. These results contradict the universally held assumption that mixtures of deuterated and protonated polymers of otherwise identical chemical structure form ideal solutions.