THE CRYSTAL-STRUCTURE OF THE LOW-TEMPERATURE A-TYPE MODIFICATION OF PR2O3 FROM X-RAY-POWDER AND ELECTRON SINGLE-CRYSTAL DIFFRACTION

The crystal structure of A-Pr2O3 has been studied by X-ray powder diff raction and high-energy electron diffraction (HEED) from single crysta ls. The space group has been unambiguously established as P (3) over b ar ml (No. 164) by HEED, though the intensities of hhl reflections wit h l = 2n+1 are very weak. These reflections are difficult to observe i n X-ray diffractometry and could mislead one to consider the existence of a c-glide plane and, therefore, to a wrong space group. The struct ure 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 OPr4 tetr ahedra are the most important polyhedra in this structure. They share edges and form (PrO)(2)(2+) layers, which are embedded in O2- layers. The OPr4 tetrahedron has four O-Pr distances of 2.344 Angstrom on aver age. Three of them are very short with distances of 2.305 Angstrom. Th ese partially covalent bonds should be emphasized with respect to simi lar short distances in ordered PrOF structures.