Neutron diffraction on functional materials at extreme conditions

Abstract

In the work structure mechanisms of Mn segregation from solid solutions with perovskite-like and spinel structure were investigated. The complex Mn-content oxides are often used as catalysts. Spinels (Mn 3-x Al x O 4) are known as catalysts of deep oxidation. The La 1-x Ca x MnO 3 solid solutions with the perovskite-like structure are catalysts for the partial oxidation of methane. In both systems an active component of catalysts is manganese oxide which is formed under catalytic reaction conditions. Spinel catalysts are activated by heating under oxidizing atmosphere. Perovskite catalysts are used under reducing atmosphere. To simulate real conditions of catalytic processes we applied high-temperature XRD in the air (oxidizing atmosphere) and in the vacuum (reducing atmosphere). Mn-content spinel (Mn 1.5 Al 1.5 O 4) is stable under heating in the vacuum: spinel keeps structure and composition. On the contrary decomposition is observed in the air during heating in the air. Nanocrysralline phases of β-Mn 3 O 4 and spinel Mn 2.4 Al 0.4 O 4 are the products of decomposition. The lattice parameter of cubic phase decreases from 8.285 Å to 8.043Å when sample was heated from ambient temperature to 700 o C. This fact shows change of spinel composition from Mn 1.5 A 1.5 O 4 to Mn 0.4 Al 2.4 O 4 , i.e. manganese ions leave the structure. TEM data indicates that small particles of β-Mn 3 O 4 are appeared on the surface of the well crystallized cubic spinel when sample was calcined at the temperature of 300-400 o C. After further increase calcination temperature β-Mn 3 O 4 particles grow. As a result we can conclude that Mn cations segregation from initial spinel occurs with formation of new phase β-Mn 3 O 4. At the same time spinel Mn 1.5 A 1.5 O 4 transforms to aluminum-enriched spinel Mn 0.4 Al 2.4 O 4 with statistical distribution of cation vacancies. Temperature stability of the series of samples La 1-x Ca x MnO 3 (0≤x≤1) was investigated using in situ XRD (vacuum and air atmosphere). According to TEM and XRD analysis the microstructure and phase structure of samples keeps during heating in the air. To model a process of catalysis reduction in the reactor we heated samples in the vacuum. According to TEM data multiple planar defects in perovskite structure were formed starting from x=0.3, the amount of the defects grows with the increase of x value. In the sample with the x=0.9 areas of the planar defects segregation and superstructure formation is observed. According to EDX analysis in these areas the deficit of Mn cations (~20%) is detected. The superstructure is characterized by quadruple period of the crystal lattice in [001] direction. Thus Mn cations segregation was observed in the both systems. In the spinel oxide segregation is accompanied by partial oxidation of Mn 2+ to Mn 3+ due to oxygen embedding. In the perovskite oxides part of Mn cations transform to Mn 2+ due to oxygen loss in the vacuum conditions. Microstructure of systems changes by different ways during high-temperature treatment. Planar defects are...