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Sr2FeMoO6–δ single-phase samples without Fe/Mo cations superstructural ordering (P) and with Curie temperature 407 K were obtained by the solid-phase technique. According to the XRD data, the growth dynamics of the parameter P is nonlinear. In this case, the process of reaching maximum values of P (Pmax) is long and its rate is several times lower than the change of the oxygen index 6–δ. It was found that with increasing temperature of isothermal annealing, P increases and reaches maximal values 88 % at T = 1320 K for 120 h, Pmax = 92 % at T = 1420 K for 100 h, while Pmax = 90 % at T = 1470 K for 45 h. One can assume that the lower values of Pmax at T = 1470 K than at T = 1420 K are due to the influence of thermal energy on the destruction of chain ordering of Fe and Mo cations placed in staggered order. Based on the analysis of P time dependences, two relaxation processes can be found and the dP/dt = ¦(t) can be divided into two regions – I and II. In the region I the relaxation time is shorter than that in the region II. The point is that the ordering of cations in the –O–Fe–O–Mo–O chains in the region I requires atomic displacements by approximately one interatomic distance, whereas in the region II cation displacements occur over long distances with the formation of long[1]chain long-range ordering.

Single-phase Sr2FeMoO6-d powders with various degrees of superstructural ordering of Fe/Mo cations and oxygen index d were obtained by solid-phase synthesis. With the help of X-ray phase analysis and neutron diffraction studies it has been established that the Fe-O1 and Mo-O2 values decrease with increasing oxygen content and the superstructural ordering of Fe/Mo cations increases, whereas the Fe-O2 and Mo-O1 bond lengths decrease, which leads to a decrease in the crystal lattice parameters and in the unit cell volume accordingly. This indicates an increase in the covalence of the bond along the axis (c), which stimulates the redistribution of the electron density from the (Fe/Mo-O2) layers to the (Fe/Mo-O1) layers and to an increase in the concentration of spin-polarized electrons with spins¯, which are located in the conduction band on Mo(t2g)¯ electron orbitals. When studying the dynamics of the change in oxygen non-stoichiometry and the degree of superstructural ordering it was found that the rate of desorption of oxygen is several times larger than the rate of Fe/Mo cations ordering.

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