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Martina Vrankić

Bio: Martina Vrankić is an academic researcher. The author has contributed to research in topics: Nanocrystalline material & X-ray crystallography. The author has an hindex of 7, co-authored 17 publications receiving 116 citations.

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Journal ArticleDOI
TL;DR: In this article, the lattice dynamics calculation of the ZnAl2O4 phonons were performed within the density functional theory for the 3D periodic crystal structures, using Gaussian-type wavefunctions as basis set and a local density approximation gave a good agreement between observed and predicted Raman bands.

24 citations

Journal ArticleDOI
TL;DR: In this paper, X-ray powder diffraction was used to characterize the structure of manganese-doped ZnAl2O4 (gahnite) samples, with doping levels 0, 4.3, 7.6 and 11.7 µm.

18 citations

Journal ArticleDOI
TL;DR: The magnetic behavior of CoMn2O4 was successfully tailored; depending on the heat treatment, it is possible to switch between the superparamagnetic and ferrimagnetic ordering and to tailor the magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior.
Abstract: The sizes of CoMn2O4 nanoparticles can easily be tuned, from 40 to 8 nm, depending on the temperature of decomposition of the single-source molecular precursor {[Co(bpy)3][Mn2(C2O4)3]·H2O}n. The structural features of the CoMn2O4 spinel are also affected by the heat treatment temperature, showing a pronounced expansion of unit cell parameters as a consequence of thermally induced cation redistribution between tetrahedral and octahedral sites. Moreover, the magnetic behavior of CoMn2O4 was successfully tailored as well; depending on the heat treatment, it is possible to switch between the superparamagnetic and ferrimagnetic ordering and to tailor the magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior.

18 citations

Journal ArticleDOI
TL;DR: Powder samples of pure BaAl2O4 and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route, and the observed emission spectrum indicated that Eu3+ ions occupied the Ba site with lower symmetry in the doped sample.
Abstract: Powder samples of pure BaAl2O4 and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route. The samples were characterized by X-ray diffraction, 151Eu Mossbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy at the Ba L3- and Eu L3-edges, and photoluminescence measurements. Diffraction lines were broadened, indicating that the samples were nanocrystallline. The samples possessed a hexagonal crystal structure, space group P63. 151Eu Mossbauer spectroscopy revealed the presence of Eu in the 3+ oxidation state. The same information on the Eu oxidation state was also obtained by the Eu L3-edge X-ray absorption near-edge structure of the doped sample. Extended X-ray absorption fine structure showed an Eu3+ ion substituted for Ba2+ on the Ba2 site in the BaAl2O4 host structure, with charge compensation by an interstitial O in the vicinity of the Ba2 site. That was confirmed by a Rietveld structure refinement for the Eu-doped BaAl2O4 sample. Analysis of the diffraction lin...

15 citations

Journal ArticleDOI
TL;DR: X-ray diffraction patterns indicated that samples were nanocrystalline with a hexagonal crystal structure, space group P63, and extended X-ray absorption fine structure suggested an unusual tetrahedral coordination of Cr(3+) ions within the BaAl2O4 host phase.
Abstract: Powder BaAl2O4 samples doped with 0 and 1.76 atom % Cr in relation to Al were hydrothermally prepared. Both samples were characterized by X-ray diffraction and synchrotron based X-ray absorption spectroscopy at the Cr K- and the Ba L3-edge. Diffraction patterns indicated that samples were nanocrystalline with a hexagonal crystal structure, space group P63. Chromium doping of barium aluminate caused an increase of the unit-cell volume and diffraction line broadening. The doped sample contained a small amount of an impurity phase, namely, BaCrO4. Analyzed Cr K-edge X-ray absorption near edge structure for the doped sample showed the presence of chromium in 6+ and 3+ oxidation states: Cr(6+) was characteristic for chromium in the impurity phase BaCrO4, while Cr(3+) participated in the formation of the doped phase BaAl2O4:Cr. Extended X-ray absorption fine structure suggested an unusual tetrahedral coordination of Cr(3+) ions within the BaAl2O4 host phase. The structure of samples was refined by the Rietveld method, simultaneously with the analysis of diffraction line broadening. Rietveld structure refinement showed that in doping the Cr(3+) ions likely substituted for Al(3+) ions on Al1 tetrahedral sites of barium aluminate crystal lattice. Crystallite sizes in the samples decreased with chromium doping, from 32 nm for pure BaAl2O4 to 24 nm for Cr-doped BaAl2O4. The dopant Cr(3+) cations acted as defects in the barium aluminate structure that increased lattice strain from 0.02% for pure BaAl2O4 to 0.14% for doped BaAl2O4 and disturbed the crystallites to grow.

14 citations


Cited by
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Journal Article
TL;DR: In this article, the free-running characteristics and the effective emission cross section of the Ti:Al2O3 were analyzed under argon-ion laser pumping and the temperature dependence of the 2E lifetime was measured from 4 to 650 K.
Abstract: Cw laser action of Ti:Al2O3 under argon-ion laser pumping is presented. We analyze the free-running characteristics and the effective emission cross section of the laser. The temperature dependence of the 2E lifetime was measured from 4 to 650 K and is explained in a single-phonon model. The quantum efficiency of Ti:Al2O3 at room temperature is 80%.

147 citations

Journal ArticleDOI
TL;DR: In this article, a combustion method using urea as fuel was used to improve the optical properties and crystallinity of zinc aluminate (ZnAl 2 O 4 ) nanocrystals.

88 citations

Journal ArticleDOI
TL;DR: A high sensitive, cost effective, robust electrochemical sensor for 4-nitrophenol (4-NP) in environmental water samples and holds reliable long-term durability, good reproducibility, and excellent working stability.

80 citations

Journal ArticleDOI
TL;DR: In this paper, X-ray diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements were performed on ZnAl2O4 spinel powders.
Abstract: ZnAl2O4 spinel powders were prepared using the Pechini or co-precipitation synthetic route and were then treated at different temperatures (600–1350 °C). These powders were characterised by X-ray diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements. SEM investigations and the X-ray patterns showed that the spinel crystallite size was dependent on the synthetic route and the treatment temperature. In addition, the structural evolution was investigated by Rietveld refinements. The inversion rate decrease was correlated with the temperature, leading to a direct spinel phase for the sample treated at high temperature. Furthermore, luminescence measurements showed various emissions linked to the presence of defects in the matrix structure. The two main emissions observed were attributed to oxygen vacancy and Zn in the interstitial positions (as revealed by differential Fourier maps). The luminescence spectra exhibited strong differences between 1200 °C and 1350 °C. At the higher temperature, the characteristic emission spectra can be attributed to the direct spinel phase. The indirect–direct spinel transformation can be monitored through the change in the optical properties and correlated to the thermal history of the sample.

76 citations

Journal ArticleDOI
TL;DR: The 3D transition-metal oxides are a class of promising catalysts for the oxygen evolution reaction (OER) in alkaline media as discussed by the authors, but they usually do not show impressive OER performance.
Abstract: The 3d transition-metal oxides are a class of promising catalysts for the oxygen evolution reaction (OER) in alkaline media. Manganese-based spinel oxides usually do not show impressive OER perform...

70 citations