Abstract
Abstract The manganese-doped polymorphs of bismuth orthoniobate were studied using thermal analysis, ESR, XPS and NEXAFS spectroscopy. It was shown that the phase transition from the orthorhombic (α) to the triclinic (β) modification can be reversible and it can be achieved by prolonged calcination of samples at 750°C. The DSC curves revealed no thermal effects associated with the phase transformation from the triclinic to the orthorhombic modification near 750°C. In the ESR spectra, the absorption band with g = 3.8 characterized by sextet structure disappeared during the phase transformation from the - to the -form and only the broad component with average intensity was observed at g = 2.2-2.0. According to NEXAFS spectroscopy data, the content of oxidized Mn(III) atoms increased and the content of Mn(II) decreased in the orthorhombic polymorph obtained at 750°C from the tri-clinic modification.
Highlights
The manganese-doped polymorphs of bismuth orthoniobate were studied using thermal analysis, ESR, XPS and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy
In the ESR spectra, the absorption band with g = 3.8 characterized by sextet structure disappeared during the phase transformation from the β- to the α-form and only the broad component with average intensity was observed at g = 2.2-2.0
The phase transition from the triclinic to the orthorhombic modification triggered by prolonged calcination probably resulted in the following simultaneous processes that affected the shape of the ESR spectra of manganese in the solid solutions: the oxidation of a part of manganese atoms predominantly to Mn(III) and Mn(IV) and the rearrangement of the crystal structure of the solid solutions
Summary
Abstract: The manganese-doped polymorphs of bismuth orthoniobate were studied using thermal analysis, ESR, XPS and NEXAFS spectroscopy. The phase transition from the orthorhombic to triclinic modification is irreversible due to significant structural differences and it occurs at temperature near 1040∘C, as it was shown in [12,13,14]. Recent studies have shown the phase transition from the triclinic modification to the orthorhombic one in powder and compact state of BiNbO4 [18] and in its solid solutions [19] using the methods of X-ray phase analysis. NEXAFS, XPS and ESR spectroscopy techniques, as well as thermal analysis, were used to study the phase transformations of manganese doped solid solutions of bismuth orthoniobate in order to obtain new facts about the reversibility of the phase transformation from the orthorhombic modification to the triclinic one
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