The crystal structure of the low-temperature A-type modification of Pr 2O 3 from X-ray powder and electron single crystal diffraction

Abstract

The crystal structure of A-Pr 2O 3 has been studied by X-ray powder diffraction and high-energy electron diffraction (HEED) from single crystals. The space group has been unambiguously established as P 3 m1 (No. 164) by HEED, though the intensities of hhl reflections with l=2 n+1 are very weak. These reflections are difficult to observe in X-ray diffractometry and could mislead one to consider the existence of a c-glide plane and, therefore, to a wrong space group. The structure is in agreement with the usually accepted so-called Pauling model for the A-type of rare-earth sesquioxides. The final value of R conv=2.1% indicates an excellent refinement. Slightly distorted OPr 4 tetrahedra are the most important polyhedra in this structure. They share edges and form (PrO) 2 2+ layers, which are embedded in O 2− layers. The OPr 4 tetrahedron has four OPr distances of 2.344 Å on average. Three of them are very short with distances of 2.305 Å. These partially covalent bonds should be emphasized with respect to similar short distances in ordered PrOF structures.