Strukturna karakterizacija CaMnOз nanoprahova dopiranih itrijumom i teorijsko modelovanje stabilnosti perovskitske strukture

Abstract

In the first part of dissertation structural, microstructural and phase analysis ofthe nanopowders with the general formula Ca1-xYxMnO3 (0 ≤ x ≤ 1) were performedusing XRPD diffraction technique and Rietveld refinement. The most abundantcrystal phases in these nanopowders are the members of ABO3 solid solutions with theperovskite 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 preparedusing a modified glycine/nitrate process. Yttrium doped CaMnO3 crystallizes in thespace group Pnma, and one of the goals of the research is to investigate the stability ofthe perovskite structure type depending on the dopant concentration. Two phasecomposition of all samples and amount of each phase was revealed using XRPDdiffraction and Rietveld refinement. The most abundant phase in all samples has theperovskite structure type. As a result of doping with Y, the XRPD analysis showedthat all the perovskite phases are deformed, with symmetry reduced from cubic toorthorhombic, and that they crystallize in the Pnma space group. Unit cell parametersanalysis showed that the increase of the unit cell parameters, which is related to thehigher 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 ofvacancies in the structure also affects the mechanism of this transformation. Thechemical compositions, calculated from the refined occupancy values, are comparedwith nominal compositions. Effect of yttrium on the bond lengths and bond angles,tilting and deformation of octahedra caused by presence of Mn3+ and Jahn-Tellereffect, 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 thedoped perovskite phases was investigated using X-ray photoelectron spectroscopy(XPS).In the second part of dissertation we have performed a crystal structureprediction study of CaMnO3 focusing on structures generated by octahedral tiltingaccording to group-subgroup relations from the ideal perovskite type ( Pm3m), which173is the aristotype of the experimentally known CaMnO3 compound in the Pnma spacegroup. Using software SPuDS we obtained initial structure parameters for most of theperovskite structure candidates. Furthermore, additional structure candidates havebeen obtained using data mining. For each of the structure candidates, a localoptimization on the ab initio level using density functional theory (LDA and hybridB3LYP) and the Hartree-Fock (HF) method was performed, and we find that severalof the modifications may be experimentally accessible. In the high-pressure regime,we have identified a post-perovskite phase in the CaIrO3 type, not previouslyobserved in CaMnO3. Similarly, calculations at negative pressure predicted a phasetransition from the orthorhombic perovskite to an ilmenite-type (FeTiO3) modificationof CaMnO3.