A contribution to the structural chemistry of A-type rare earth sesquioxides

La₂O₃ and Nd₂O₃ have been annealed at temperatures between 1000 and 1600°C for decontamination. Only products annealed at least at these temperatures appeared to be monophasic. Both X-ray powder and electron single-crystal diffraction revealed the A-type sesquioxide structure. No difference could be found in samples either quenched or slowly cooled to room temperature. Two space groups, $\text{P}\text{3}\text{m1}$ and $\text{P6}{}_3\text{mmc}$, are reported for the A-type structure. X-Ray powder studies seemed to support the latter on the evidence of extinctions. Electron diffraction from single crystals, however, indicated the space group $\text{P}\text{3}\text{m1}$, confirming the so-called Pauling structure, while $\text{P6}{}_3\text{mmc}$ can now be excluded unequivocally.