Crystal Structure and Characterization of La1-xBixMnO3+δ

The La_1-xBi_xMnO_3+δ (x=0.0-0.5) solid solutions were synthesized using three different methods: solid-state synthesis, citrate-nitrate combustion synthesis and mechanochemical activation. All obtained compositions crystallize in a rhombohedral perovskite structure, independent of the synthesis method. Two ranges with linear temperature dependence of the unit cell parameters have been observed during in-situ heating X-ray powder diffraction experiments. The transition between the two ranges occurs at 730°C and is explained by chemical expansion at high temperatures. The chemical compatibility between the obtained solid solutions and bismuth-containing electrolytes (Bi₄V_1.7Fe_0.3O₁₁, Bi₁₃Mo₅O₃₄ and Bi₃Nb_0.8W_0.2O_7.1) was studied. Interaction between these materials has been observed at temperatures higher than 600°С.

X-ray diffraction data collected at room temperature confirm that the La_1-xBi_xMnO_3+δ solid solutions form in the whole range of the investigated bismuth concentrations independent of the synthesis method. All samples crystallize in a rhombohedral perovskite structure with a trigonal unit cell (S.G. R-3c).

For a detailed microstructural characterization by transmission electron microscopy, the La_0.8Bi_0.2MnO_3.15 composition obtained using the solid-state synthesis was chosen. All obtained diffraction patterns can be indexed in a trigonal unit cell with a=5.5300(1) Å, c=13.382(1) Å. An analysis of systematically absent reflections revealed the presence of two types of crystals. Extinction conditions for the fist type coincide with those obtained from XRD (hkl: -h+k+l=3n; h-hl: h+l=3n, l=2n; hhl: l=3n; 00l: l=6n), indicating two possible space groups: R-3c and R3c. A second type of crystals obeys the same extinction rules except for the 00l reflections for which l=3n and h-hl reflections for which l¹2n, pointing towards the following possible space groups: R3, R-3, R32, R3m, R-3m. The formation of two different types of crystallites with trigonal crystal symmetry has been observed in perovskite systems before and explained by the presence or absence of antiphase octahedral tilting about the [111] axis of the cubic perovskite sub-lattice. It is worth to mention that the X-ray powder diffraction pattern of this sample displays asymmetric line broadening of peaks, which can point towards the presence of an additional structurally related compound.

A loss of oxygen was observed in the TG curves of La_1-xBi_xMnO_3+δ (x=0.0-0.5; Δх=0.1) samples at temperatures >700°С, supporting the previously discussed results. TG/DSC curves of La_0.9Bi_0.1MnO_3+δ are shown in Figure 4. The loss of mass was found to be 0.6%.

The excess of oxygen in La_1-xBi_xMnO_3+δ (x=0.0-0.5; Δх=0.1) was determined using redox titration. The value of the oxygen nonstoichiometry (δ) was found to be positive and equal to 0.15 at all concentrations of dopant, which can be explained by the identical oxidation states of La³⁺ and Bi³⁺. To determine the concentration of metal atoms in the obtained solid solutions, surfaces and fractures of pellets have been studied using EDS and AAS. The elemental content of 15–20 crystallites of La_1-xBi_xMnO_3+δ (x=0.1, 0.2) compositions was studied by EDS analysis inside the scanning electron microscope. It was found to be La_0.9(1)Bi_0.07(4)Mn_0.9(7) and La_0.9(8)Bi_0.1(6)Mn_0.9(6) for x = 0.1 and 0.2, respectively. The obtained results showed a slightly inhomogeneous cation distribution and a lower manganese concentration compared to the nominal value for both the bulk and the surface of the pellets. AAS measurements gave similar results for all synthesized solid solutions, showing the manganese content to be 0.9, and thus indicating that all obtained solid solutions lie in the homogeneity region of lanthanum manganite with a small manganese deficiency. Taking into account the obtained values of manganese deficiency and oxygen nonstoichiometry, the chemical formula of the solid solutions can be rewritten as La_1-xBi_xMn_1-yO_{[(3+(1-y))/2]} (where V_Mn is the average oxidation state of manganese). For example, for the La_0.9Bi_0.1MnO_3+δ compound with δ=0.15 and y=0.1, the average oxidation state of manganese (V_Mn) will be equal to 3.67. Thus, the ratio of manganese in different oxidation states (Mn⁴⁺/Mn³⁺) in the studied solid solutions is equal to 2, which means that 67% of the manganese cations in these compounds have oxidation state +4. This fact is probably the reason why we observe the formation of solid solutions with a rhombohedrally distorted perovskite structure already at room temperature.

A study of the chemical compatibility between La_1-xBi_xMnO_3+δ (x=0.0-0.5; Δх=0.1) solid solutions and bismuth-containing electrolytes shows that depending on the chemical nature of the electrolyte material, the formation of additional phases begins at different temperatures: at T³700°С for the studied niobate, at T³600°С for the studied vanadate and at T³500°С for the studied molybdate.

The work was financially supported by Russian Fund of Fundamental Research, grant 14-03-92605.