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    Air-Exposure-Driven Color and Optical Variations in Hydroxyapatite Extracted from Fish Scales
    (Sakarya University, 2025) Okur, H. Esma
    The disposal of fish scales as waste presents an environmental challenge and an untapped opportunity for resource recovery. In this study, hydroxyapatite (HAp) was extracted from European seabass (Dicentrarchus labrax) scales to explore how air exposure during calcination affects its optical and surface properties. HAp powders were prepared under two distinct calcination conditions: fully exposed to air (producing white powder) and partially shielded from air (resulting in gray powder). Rietveld refinement of X-ray powder diffraction (XRPD) data confirms that both powders crystallize in the hexagonal HAp structure, with a minor Mg-whitlockite impurity. Despite these differences in air exposure, the bulk structure of the HAp remains unchanged. The color variations are linked to surface oxidation, as subsurface layers in the partially shielded scales retain a grayish tone while the exposed surfaces turn completely white. Scanning electron microscopy reveals subtle differences in particle morphology: the white powder had a smoother surface compared to the slightly rougher gray powder. Fourier transform infrared spectra confirm the presence of characteristic phosphate and hydroxyl groups in both powders, indicating that the core chemical structure of HAp is intact in both cases. The Ca/P ratios—1.504(7) for the white powder and 1.505(7) for the gray powder obtained from the Rietveld analysis—further support the stoichiometric integrity of the material. UV-Vis spectroscopy reveals direct bandgap values of 3.99 eV for the white powder and 3.87 eV for the gray powder. These bandgap values, which are lower than those typically reported for defect-free HAp (5–6 eV), suggest that the optical differences between the powders are driven by surface effects, such as oxygen vacancies or trace impurities. This study highlights how calcination conditions, particularly air exposure, influence surface properties and optical behavior, paving the way for potential applications of fish-scale-derived HAp in electronic and optical materials. © 2025, Sakarya University. All rights reserved.
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    Air-Exposure-Driven Color and Optical Variations in Hydroxyapatite Extracted from Fish Scales
    (2025) Okur, H. Esma
    The disposal of fish scales as waste presents an environmental challenge and an untapped opportunity for resource recovery. In this study, hydroxyapatite (HAp) was extracted from European seabass (Dicentrarchus labrax) scales to explore how air exposure during calcination affects its optical and surface properties. HAp powders were prepared under two distinct calcination conditions: fully exposed to air (producing white powder) and partially shielded from air (resulting in gray powder). Rietveld refinement of X-ray powder diffraction (XRPD) data confirms that both powders crystallize in the hexagonal HAp structure, with a minor Mg-whitlockite impurity. Despite these differences in air exposure, the bulk structure of the HAp remains unchanged. The color variations are linked to surface oxidation, as subsurface layers in the partially shielded scales retains a grayish tone while the exposed surfaces turn completely white. Scanning electron microscopy reveals subtle differences in particle morphology: the white powder had a smoother surface compared to the slightly rougher gray powder. Fourier transform infrared spectra confirms the presence of characteristic phosphate and hydroxyl groups in both powders, indicating that the core chemical structure of HAp is intact in both cases. The Ca/P ratios—1.504(7) for the white powder and 1.505(7) for the gray powder obtained from the Rietveld analysis—further supports the stoichiometric integrity of the material. UV-Vis spectroscopy reveals direct bandgap values of 3.99 eV for the white powder and 3.87 eV for the gray powder. These bandgap values, which are lower than those typically reported for defect-free HAp (5–6 eV), suggest that the optical differences between the powders are driven by surface effects, such as oxygen vacancies or trace impurities. This study highlights how calcination conditions, particularly air exposure, influence surface properties and optical behavior, paving the way for potential applications of fish-scale-derived HAp in electronic and optical materials.
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    Crystallization of Biogenic Hydroxyapatite: Phase Purity and Morphological Control via Successive Annealing and Grinding
    (Sakarya University, 2025) Okur, H. Esma
    Hydroxyapatite (HAp) derived from biogenic sources offers a sustainable and cost-effective alternative to synthetic materials, yet controlling its crystal structure and morphology remains a key challenge. In this study, polycrystalline HAp is extracted from alkali-treated fish scales and subjected to a carefully designed solid-state crystallization protocol involving successive annealing at 320 °C, 500 °C, 600 °C, 700 °C, and 800 °C, with intermediate manual grinding steps. This multi-step thermal approach addresses the limitations of conventional one-step calcination and enables controlled structural evolution. X-ray powder diffraction analyzed via Rietveld refinement reveals a progressive enhancement in crystallinity and phase purity with increasing temperature, with no detectable secondary phases. Well-defined hexagonal facets emerge at 800 °C, indicating advanced structural ordering. Scanning electron microscopy shows a transition from loosely aggregated, irregular clusters to uniform, faceted grains. Fourier-transform infrared spectroscopy confirms the presence of phosphate, hydroxyl, and trace carbonate groups, with carbonate bands decreasing at higher temperatures. Semi-quantitative energy-dispersive X-ray spectroscopy supports carbonate incorporation and suggests trace localized magnesium, without clear evidence of significant elemental substitution. Compared to direct calcination, successive annealing improves thermal regulation, minimizes agglomeration, and preserves nanostructural integrity—features essential for biomedical and catalytic applications. The combined use of alkali pre-treatment, stepwise annealing, and intermediate grinding results in a tunable, solid-state route for tailoring the crystal structure and morphology of biogenic HAp. This optimized method enables the synthesis of phase-pure HAp at moderate temperatures and presents a scalable pathway for advanced applications in bioceramics, environmental remediation, and sustainable materials engineering. © 2025, Sakarya University. All rights reserved.
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    Crystallization of Biogenic Hydroxyapatite: Phase Purity and Morphological Control via Successive Annealing and Grinding
    (2025) Okur, H. Esma
    Hydroxyapatite (HAp) derived from biogenic sources offers a sustainable and cost-effective alternative to synthetic materials, yet controlling its crystal structure and morphology remains a key challenge. In this study, polycrystalline HAp is extracted from alkali-treated fish scales and subjected to a carefully designed solid-state crystallization protocol involving successive annealing at 320 °C, 500 °C, 600 °C, 700 °C, and 800 °C, with intermediate manual grinding steps. This multi-step thermal approach addresses the limitations of conventional one-step calcination and enables controlled structural evolution. X-ray powder diffraction analyzed via Rietveld refinement reveals a progressive enhancement in crystallinity and phase purity with increasing temperature, with no detectable secondary phases. Well-defined hexagonal facets emerge at 800 °C, indicating advanced structural ordering. Scanning electron microscopy shows a transition from loosely aggregated, irregular clusters to uniform, faceted grains. Fourier-transform infrared spectroscopy confirms the presence of phosphate, hydroxyl, and trace carbonate groups, with carbonate bands decreasing at higher temperatures. Semi-quantitative energy-dispersive X-ray spectroscopy supports carbonate incorporation and suggests trace localized magnesium, without clear evidence of significant elemental substitution. Compared to direct calcination, successive annealing improves thermal regulation, minimizes agglomeration, and preserves nanostructural integrity—features essential for biomedical and catalytic applications. The combined use of alkali pre-treatment, stepwise annealing, and intermediate grinding results in a tunable, solid-state route for tailoring the crystal structure and morphology of biogenic HAp. This optimized method enables the synthesis of phase-pure HAp at moderate temperatures and presents a scalable pathway for advanced applications in bioceramics, environmental remediation, and sustainable materials engineering.
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    Fulleride superconductivity tuned by elastic strain due to cation compositional disorder
    (Royal Soc Chemistry, 2024) Okur, H. Esma; Colman, Ross H.; Takabayashi, Yasuhiro; Jeglic, Peter; Ohishi, Yasuo; Kato, Kenichi; Prassides, Kosmas
    Dynamical fluctuations of the elastic strain in strongly correlated systems are known to affect the onset of metal-to-insulator or superconducting transitions. Here we report their effect on the properties of a family of bandwidth-controlled alkali-intercalated fullerene superconductors. We introduce elastic strain through static local structural disorder in a systematic and controllable way in the fcc-structured KxCs3-xC60 (with potassium content, 0.22 <= x(K) <= 2) series of compositions by utilizing the difference in size between the K+ and Cs+ co-dopants. The occurrence of the crossover from the Mott-Jahn-Teller insulating (MJTI) state into the strongly correlated Jahn-Teller metal (JTM) on cooling is evidenced for the compositions with x(K) < 1.28 by both synchrotron X-ray powder diffraction (SXRPD) - anomalous reduction of the unit cell volume - and Cs-133 NMR spectroscopy - sudden suppression in the Cs-133 spin-lattice relaxation rates. The emerging superconducting state with a maximum critical temperature, T-c = 30.9 K shows a characteristic dome-like dependence on the unit-cell volume or equivalently, on the ratio between the on-site Coulomb repulsion, U, and the bandwidth, W. However, compared to the parent Cs3C60 composition in which cation disorder effects are completely absent, the maximum T-c is lower by similar to 12%. The reduction in T-c displays a linear dependence on the variance of the tetrahedral-site cation size, sigma(2)(T), thus establishing a clear link between structural-disorder-induced attenuation of critical elastic strain fluctuations and the electronic ground state.
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    Optimization of Thermal Processing for Enhancing Superconducting Phase Fractions in Ternary Fullerides via a Drop-Quenching Approach
    (İstanbul Üniversitesi, 2025) Okur, H. Esma
    This study explores an optimized thermal protocol for synthesizing ternary superconducting fullerides, focusing on enhancing the face-centered-cubic (fcc) phase fraction while mitigating the formation of competing thermodynamically stable phases. Unlike previous studies, which primarily investigated annealing conditions, this work introduces a drop-quenching approach—rapid cooling in ice/water baths (-3°C to 1°C)—as a potential means to influence phase formation. The expanded ternary fulleride, K0.3Cs2.7C60, was synthesized through a precursor-based solid-state synthetic route. The impact of two annealing temperatures (430°C and 480°C) and two cooling methods (drop-quenching vs. ambient air cooling) was systematically examined. X-ray powder diffraction confirms that K0.3Cs0.7C60 adopts a cubic structure with fcc symmetry. Magnetization measurements reveal that the material exhibits superconductivity with a critical temperature (Tc) of 29.9 K. Structural characterizations shows that increasing the annealing temperature from 430°C to 480°C, combined with intermittent regrinding and pelletization, improved fcc-phase fractions and lattice expansion. However, the difference between quench cooling and natural cooling in air is found to be minimal. These findings suggest that increasing the annealing temperature to 480°C could be beneficial for the synthesis of expanded ternary fullerides by enhancing the superconducting fcc-phase fractions and addressing challenges posed by the increased size mismatch of the substituted cations relative to the interstitial sites in the fcc structure.
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    Pressure effects on the crystal structure of the cubic metallofullerene salt [Li@C60][PF6] to 12 GPa
    (Elsevier, 2022) Colman, Ross H.; Okur, H. Esma; Garbarino, Gaston; Ohishi, Yasuo; Aoyagi, Shinobu; Shinohara, Hisanori; Prassides, Kosmas
    The pressure-dependent structural properties of the metallofullerene salt, Li+@C-60][PF6](-), which has been shown before to adopt a primitive cubic structure at ambient conditions, have been studied by synchrotron X-ray powder diffraction at ambient temperature and at 7 K up to similar to 12 GPa. We find no evidence for a phase transition across the accessed pressure range at either temperature with the structure always remaining strictly primitive cubic (space group Pa (3) over bar). The extracted bulk moduli, K-0 of 17.51(13) GPa and 16.6(2) GPa at 7 K and ambient temperature, respectively are comparable to those of other cubic close-packed fullerene solids implying little influence of the interaction between the endohedral Li+ and the interstitial [PF6](-) unit. Rietveld analysis of the diffraction data shows that the adopted primitive cubic structure incorporates orientationally ordered [Li+@C-60] units - they are rotated anticlockwise by similar to 101 degrees about the three-fold [111] rotation axis. This finding together with the lack of observation of a monomer -> polymer phase transition upon pressurization contrast sharply with the structural behaviour of isostructural pristine C-60 and its intercalated salts, (deely quenched) CsC60 and Na2CsC60, which both incorporate orientationally disordered C60 units and have a tendency to easily polymerize via intermolecular C-C bond formation. Such differences can be understood in terms of the expanded nature of the salt and the steric interactions introduced by the large [PF6](-) unit residing in the octahedral interstices.
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    Rietveld refinement-based structural analysis of biogenic hydroxyapatite and its PVA composite for dye removal
    (Elsevier, 2025) Okur, H. Esma
    The disposal of fish scales is not only an environmental issue but also a missed opportunity to recover valuable resources. In this study, fish-scale waste was transformed into hydroxyapatite (HAp), a material with possible applications in environmental remediation, particularly in removing contaminants from water. While synthetic HAp, with the stoichiometric formula Ca10(PO4)6(OH)2, has been extensively studied, the structural properties of biogenic HAp derived from natural sources, such as fish scales, remain underexplored. To bridge this gap, Rietveld refinement was used to uncover detailed crystallographic information. Using a straightforward calcination process at 800 degrees C, HAp was successfully extracted from recycled European seabass (Dicentrarchus labrax) scales. The Rietveld analysis of the X-ray powder diffraction data confirms crystallization in a hexagonal phase (space group P63/m), with refined lattice parameters comparable to synthetic HAp and a Ca/P molar ratio of 1.474(7). An impurity phase, Mg-whitlockite (15.9(1) % by weight), was also identified, offering potential benefits for bioactivity and osteoconductivity. Further analysis, including Fourier transform infrared spectroscopy, confirms the presence of PO43 - and OH- functional groups, while scanning electron microscopy provides insights into the morphology showing variations in crystallization and particle sizes, ranging from 30 to 100 nm across different regions. Additionally, HAp was incorporated into a polyvinyl alcohol (PVA) matrix, producing composite films with an adsorption capacity of 38-40 % for methylene blue dye, compared to 36-38 % for pure PVA films. This approach provides a sustainable solution for repurposing fish-scale waste into useful materials with notable potential in environmental remediation, especially in the removal of dyes from wastewater.