[en] Full text of publication follows. Recently, the UO_2 lattice parameter has been re-evaluated by Leinders et al. using X-ray diffraction [Ref.1]. The updated unit cell parameter (547.127 ± 0.008 pm) is larger and has been determined with a higher accuracy than the previously accepted value (547.04 ± 0.08 pm) (Ref.2). Today's accepted value for the Cu Kα_1 X-ray wavelength partly explains the difference in cell parameter, but the largest contribution is due to the avoidance of hyper-stoichiometry. Controlling and measuring sample stoichiometry is not straight-forward, as illustrated by the rather large scatter on published lattice parameter values for UO_2 [Ref.1]. The present work discusses important experimental aspects for the production of stoichiometric UO_2, and the accurate measurement of sample stoichiometry. Uranium dioxide powder is known to be sensitive to oxidation, with oxygen taken up at the surface rapidly already at room temperature [Refs.3, 4]. At room temperature, the solubility of oxygen in UO_2 is quite low (O/U ∼2.01), with the precipitation of ordered structures such as U_4O_9_-_y occurring with increasing oxygen content [Ref.5]. At elevated temperatures, a broad range of hyper-stoichiometry exists, and here numerous studies have shown a linear decrease in lattice parameter with increasing oxygen content (Δa = -10*(O/U-2) pm, approximately. Evidently, in order to perform any lattice parameter study on UO_2 precise measurement of the stoichiometry is desirable. The American Society for Testing and Materials (ASTM) has provided guidelines for measuring stoichiometry of uranium dioxide powders and pellets (ASTM C-1453-00). The method is based on the conversion of a sample with unknown stoichiometry UO_2_±_x to U_3O_8. The mass increase corresponding to the oxidation reaction: 3UO_2_±_x + 1/2(2 ± 3x)O_2 → U_3O_8, determines the initial oxygen content x. Comparison of the mass increase measured by in-situ thermogravimetric analysis (TGA) and by ex-situ weighing on an analytical balance revealed an important effect: adsorption of atmospheric species on the sample material and/or the sample crucible. The mass increase as measured ex-situ was consistently larger as compared to the actual in-situ analysis. As a result, sample stoichiometry is underestimated when performing ex-situ weighing. We propose to strictly perform sample retrieval and weighing in an inert atmosphere, or alternatively, to use in-situ TGA when maximum accuracy is required. Where some of the early investigators used the ex-situ weighing approach, samples may have falsely been identified as being stoichiometric, whereas in reality they were hyper stoichiometric. Hence the difference in reported lattice parameter values. References: [1) G. Leinders,T. Cardinaels, K. Binnemans, M. Verwerft, J. Nucl. Mater. 2015, 459, 135. [2) F. Gronvold, J. Inorg. Nucl. Chem. 1955, 1, 357. [3) L. E. J. Roberts, J. Chem. Soc. 1954, 3332. [4) J. S. Anderson, L. E. J. Roberts, E. A. Harper, J. Chem. Soc. 1955, 3946. [5) B. E. Schaner, J. Nucl. Mater. 1960, 2, 110. (authors)