The reaction of Ta metal with potassium nitrate melts at 600 o C has been studied. It was established that the composition of the reaction product could be varied by an appropriate choice of the base-acid properties of the melt, which were regulated by additions of potassium hydroxide. Pure potassium metatantalate KTaO 3 is formed when the KOH/KNO 3 molar ratio is about 1.2 or slightly higher. A mixture of the metatantalate with tetra-and ditantalate (K 2 Ta 4 O 11 and K 2 Ta 2 O 6) is formed at lower molar ratios. The content of tetratantalate increases as the concentration of KOH in the melt decreases and it becomes the main reaction product in pure nitrate melt. Pure metatantalate was obtained in the form of powders with grain sizes of about 80 nm. This product possessed ferroelectric properties with a dielectric constant of 250-300. Such nano-powders exhibit the properties of a semiconductor, which is not common for large-crystal materials. The conductivity is 10-5 S cm-1 at room temperature and increases exponentially with increasing temperature.

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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.