Nanocomposites and nanomaterials
Sol gel synthesis and crystal structure of new mixed rare earth manganite-chromites 
V.M. Hreb, I.V. Lutsiuk, V.V. Kochubei, L.O. Vasylechko
Lviv Polytechnic National University, Lviv, Ukraine
E-mail: wasiahreb@meta.ua
Nanocrystalline powders of PrMn0.1Cr0.9O3, PrMn0.5Cr0.5O3, SmMn0.5Cr0.5O3 and EuMn0.5Cr0.5O3 were obtained by sol-gel citrate method from corresponding rare earth oxides (Pr6O11, Sm2O3, Eu2O3), Mn(NO3)24H2O and Cr(NO3)3·9H2O as initial reagents. Rare earth nitrate solutions were prepared by dissolving of corresponding oxides in HNO3, whereas manganese and chromium nitrates were dissolved in distilled water. As prepared solutions of metal nitrates were mixed together on magnetic stirring for 30 min, after that water solution of citric acid (CA) was added to the reaction mixture under continuous stirring. The molar ratio of  reagents was n(R3+) : n(Mn3+) : n(Cr3+) : n(CC) = 1 : (1-x) : x : 2. Prepared solutions were gelled at 353-363 K for 4 h after that heat treated sequentially at 573 K and 723 K. According to DTA results all endo- and exo-processes and mass loss occurs up to 983 K. Further, the obtained products depending on composition were sequentially calcined at the temperatures 1073 K - 1473 for 2-4 hours, in order to obtain single-phase materials.
X-ray powder diffraction revealed that even short-term annealing at 1073 K lead to formation of single-phase PrMn0.1Cr0.9O3 and PrMn0.5Cr0.5O3 materials with perovskite structure. Additional heat treatment of the samples at 1173, 1273 and 1473 K does not affect on the phase composition and crystal structure of the materials. However, detectable narrowing of the Bragg’s peaks indicates improving of crystallinity of the samples. In contrast, single phase SmMn0.5Cr0.5O3 material was obtained only after its annealing at 1473 K for 4h. In the case of EuMn0.5Cr0.5O3 the traces of impurity phases were still presented in the material.
The average grain size, derived from angular dependence of Bragg’s peaks profile, increase from 20-60 to 40-100 nm for the powders annealed at 1073 and 1473 K, respectively. Detectable decrease of microstrain values from 0.1-0.4 % to 0.05-0.2 % points on the reducing dispersion of interplanar distances in corresponding structures with increasing heat treatment temperature. Analysis of tobtained structural parameters of the materials studied revealed lattice parameters crossover in the mixed manganite-chromite systems RMn1-xCrxO3 (R = Pr, Sm, Eu).