Stability of the nanocrystalline LaMnO_3±δunder non-ambient conditions

Abstract

Keywords: nanocrystalline materials, redox, X-ray high-temperature powder diffractionProduction of the solid compact nanomaterials is one of the most actual topics in chemistry, physics of condensed matter, and material science nowadays. The main difficulties in producing compact nanomaterials from nanopowder arise from their pressing and following annealing, which may cause an increase of grain size and lack of important features for nanomaterials. Therefore, compact nanocrystalline oxides for the present investigations were produced by a high-pressure torsion method [1,2] well-developed for metals. Being non-equilibrium metastable systems, nanomaterials are interesting for studying their stability. 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 allows us to maintain in the closed volume the given values of partial oxygen pressure in the range of 1-10 -22 atm and also to check the processes of oxygen exchange taking place in the sample during heating or cooling. The control is provided by a circulating electrochemical ''pump-sensor'' system assembled on the base of oxygen-conducting solid electrolytes. The constancy of the nanocrystalline LaMnO 3 was examined in a wide temperature range (300-1000K) and under different partial oxygen pressures (1-10 -22 atm). It brought out phase composition of the samples, cation distribution among sublattices, volumetric crystallite size distribution and microstress analysis at different temperatures and partial oxygen pressures.