ON THE PRNO2N-2 SERIES OF OXIDES AND THE STRUCTURE OF PR24O44 - AN INVESTIGATION BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

The ordered intermediate rare earth oxides of praseodymium in the composition range between PrO1.714 and PrO1.833 (PrnO2n-2, n = 7, 9, 10, 11, 12) are easily transformed into each other by the variation of temperature and/or oxygen pressure. X-ray powder diffraction patterns of all the intermediate phases are fluorite-related and it is supposed that there are structural relationships between members of the homologous series. Group-subgroup relationships are discussed within this framework, and the probable structure of Pr24O44 is proposed. The unit cell, reported as monoclinic from electron diffraction studies, is assumed to contain two clusters, each with contents Pr7O30, but no definitive structure analysis has been made. From the contrast variation observed in a high-resolution electron microscope (HREM) image of Pr24O44 the relative positions and orientation of these clusters can be inferred, which allows selection of a unique model from those possible. This same image further suggests that the unit cell is actually triclinic. The atomic coordinates have been calculated for the two defect clusters within the unit cell, and have been used for image calculations and simulation comparisons. © 1991.