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-507Orthorhombic Polymorph of ErPSO,,, Crystal Structure and Characterization Erbium pentaphosphate crystals have been characterized and the structure of the orthorhombic polymorph (type 111) determined by X-rays. It has been found that the orthorhombic samples of ErP,O,,, space group Pnma, crystallize mainly at higher temperatures and usually are much larger than monoclinic crystals, space group C2/c, obtained from lower-temperature crystallizations. Differences in fluorescence and upconversion properties between orthorhombic and monoclinic polymorphs have been detected.