Being non-equilibrium metastable systems, mechanoactivated materials are interesting for studying their stability. On the other hand, the main difficulties in producing compact nanomaterials from mechanochemically obtained nanopowders arise from their pressing and following annealing, which may cause an increase of grain size and lack of important features for nanomaterials. Therefore, the constancy of different mechanoactivated oxides (LaMnO_3±δ, Zr_0.835Y_0.165O_2-γ, Mn₂O₃, Mn₃O₄, Bi₄V_2-xFe_xO_11-x) produced with the help of a high-energy planetary ball mill was examined in a wide temperature range (300-1000K) in air under different partial oxygen pressures (1-10^-18atm). For this purpose the X-ray diffraction measurements were carried out in situ under non-ambient conditions with the help of an automated measuring complex based on a high temperature attachment with a microprocessor regulator of partial oxygen pressure. It brought out the phase composition, structural parameters and microstructural characteristics (volume averages of sizes and strains) of the samples depending on the temperature and partial oxygen pressure, as well as on the crystallite size and structural features of the oxide. Specific features of the structural phase transitions of the first order in mechanoactivated oxides with Jahn-Teller ions were also investigated. To interpret the results obtained, data on kinetics of the isotope exchange between the oxygen gas (enriched with the ¹⁸О isotope) and the mechanoactivated ceramics in the temperature range of 550 – 850K gathered by using nuclear microanalysis technique, together with X-ray photoelectron spectroscopy data have been used. In particular it has been revealed, starting from some values nanosized crystallites are able to resist to heating under ambient oxygen pressure without dramatic growth. It is also true even when coming through structural phase transition. The work was supported by the Russian Foundation for Fundamental Research, grants No.06-03-32943, 06-03-32378.

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