STRUCTURAL CHARACTERIZATION OF THE YTTRIUM DOPED CaMnO3 NANOPOWDERS AND THEORETICAL MODELLING OF THE PEROVSKITE STRUCTURE STABILITY

Niš : [J. B. Zagorac]

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Umnoženo za odbranu. Univerzitet u Nišu, Prirodno-matematički fakultet, Departman za hemiju, 2014. Bibliografija: listovi 136-160. Rezime ; Summary.

Applied chemistry, Material science

In the first part of dissertation structural, microstructural and phase analysis of the nanopowders with the general formula Ca1-xYxMnO3 (0 ≤ x ≤ 1) were performed using XRPD diffraction technique and Rietveld refinement. The most abundant crystal phases in these nanopowders are the members of ABO3 solid solutions with the perovskite structure type. Seven nanopowders of nominal composition CaMnO3 (Ca100), Ca0.95Y0.05MnO3 (Ca95), Ca0.85Y0.15MnO3 (Ca85), Ca0.75Y0.25MnO3 (Ca75), Ca0.5Y0.5MnO3 (Ca50), Ca0.25Y0.75MnO3 (Ca25) and YMnO3 (Y100) were prepared using a modified glycine/nitrate process. Yttrium doped CaMnO3 crystallizes in the space group Pnma, and one of the goals of the research is to investigate the stability of the perovskite structure type depending on the dopant concentration. Two phase composition of all samples and amount of each phase was revealed using XRPD diffraction and Rietveld refinement. The most abundant phase in all samples has the perovskite structure type. As a result of doping with Y, the XRPD analysis showed that all the perovskite phases are deformed, with symmetry reduced from cubic to orthorhombic, and that they crystallize in the Pnma space group. Unit cell parameters analysis showed that the increase of the unit cell parameters, which is related to the higher amount of Y in the structure, is a consequence of reduction of Mn4+ to Mn3+. Besides the reduction of the Mn and the effect of doping with Y, presence of vacancies in the structure also affects the mechanism of this transformation. The chemical compositions, calculated from the refined occupancy values, are compared with nominal compositions. Effect of yttrium on the bond lengths and bond angles, tilting and deformation of octahedra caused by presence of Mn3+ and Jahn-Teller effect, was analyzed. In order to investigate the coordination of the A and B sites, bond valence analysis was performed. In addition, concentration of yttrium in the doped perovskite phases was investigated using X-ray photoelectron spectroscopy (XPS). In the second part of dissertation we have performed a crystal structure prediction study of CaMnO3 focusing on structures generated by octahedral tilting according to group-subgroup relations from the ideal perovskite type ( Pm3m), which 173 is the aristotype of the experimentally known CaMnO3 compound in the Pnma space group. Using software SPuDS we obtained initial structure parameters for most of the perovskite structure candidates. Furthermore, additional structure candidates have been obtained using data mining. For each of the structure candidates, a local optimization on the ab initio level using density functional theory (LDA and hybrid B3LYP) and the Hartree-Fock (HF) method was performed, and we find that several of the modifications may be experimentally accessible. In the high-pressure regime, we have identified a post-perovskite phase in the CaIrO3 type, not previously observed in CaMnO3. Similarly, calculations at negative pressure predicted a phase transition from the orthorhombic perovskite to an ilmenite-type (FeTiO3) modification of CaMnO3.

Nanoprahovi, CaMnO3, dopiranje, itrijum, strukturna karakterizacija, perovskit, teorijsko modelovanje

XRPD diffraction, Rietveld refinement, modified glycine/nitrate process, nanopowders CaMnO3, X-ray photoelectron spectroscopy, ab initio methods, post-perovskite

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