Researcher:
Yahyaoğlu, Müjde

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PhD Student

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Müjde

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Yahyaoğlu

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Yahyaoğlu, Müjde

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2014 - 201922020 - 20213

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    Ca3[BN2]I3: the first halide-rich alkaline earth nitridoborate with isolated [BN2]3 - units
    (Wiley, 2015) Aydemir, Umut; Drathen, Christina; Akselrud, Lev; Prots, Yurii; Hoehn, Peter; Department of Chemical and Biological Engineering; N/A; Department of Chemistry; Toros, Turna Ezgi; Yahyaoğlu, Müjde; Somer, Mehmet Suat; Undergraduate Student; Master Student; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; College of Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 178882
    The title compound Ca-3[BN2]I-3 was obtained from reactions of mixtures of the starting materials Ca-3[BN2](2) and CaI2 in a 1:4 ratio in sealed Nb tubes at 1223 K. The crystal structure was solved from powder synchrotron diffraction data. Ca-3[BN2]I-3 is the first example of a halide-rich nitridoborate crystallizing in the rhombohedral space group R32 [no. 155, Pearson code: hR96; Z = 12; a = 16.70491(2) angstrom, c = 12.41024(2) angstrom]. The crystal structure is built up by two interpenetrating networks of condensed edge-sharing [BN2]@Ca-6 and [square]@I-6 trigonal antiprisms (square = void). In Ca-3[BN2]I-3 two crystallograhically distinct [BN2](3-) anions are present with d(B1-N) = 1.393(2) angstrom and d(B2-N) = 1.369(9) angstrom. Their bond angles are practically linear, varying only slightly: N-B1-N = 179(1)degrees and N-B2-N = 180 degrees. Vibrational spectra were interpreted based on the D-infinity h symmetry of the discrete linear [N-B-N](3-) moieties, considering the site symmetry reduction and the presence of two distinct [BN2](3-) groups.
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    Synthesis, crystal structure and magnetic properties of Li0.44Eu3[B3N6]
    (Elsevier, 2014) Prots, Yurii; Foerster, Tobias; Sichelschmidt, Jörg; Auffermann, Gudrun; Schnelle, Walter; Schappacher, Falko; Poettgen, Rainer; N/A; Department of Chemistry; N/A; Department of Chemistry; Kokal, İlkin; Aydemir, Umut; Yahyaoğlu, Müjde; Somer, Mehmet Suat; Master Student; Faculty Member; Master Student; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; Graduate School of Sciences and Engineering; College of Science; N/A; 58403; N/A; 178882
    Li0.44Eu3[B3N6] was synthesized from the metathesis reaction of Li-3[BN2] and EuCI3 at 850 degrees C. Li0.44EU3-[B3N6] crystallizes in the trigonal space group R (3) over barc (No. 167) with a=12.0225(2) angstrom, c=6.8556(2) angstrom and Z=6. In the crystal structure, isolated, planar cyclic [B3N6](9-) units are charge-balanced by the mixed-valence Eu3+/Eu2+ and Li+ cations. Li+ occupies partially (44%) the Wyckoff site 6b and is sandwiched between the [B3N6](9-) anions. Mossbauer spectroscopy results show the resonance lines of Eu2+ and Eu3+, respectively, indicating the heterogeneous mixed valency of the Eu atoms. X-Band ESR investigations between 5 and 300 K reveal an intense signal over the whole temperature range originating from Eu2+. Magnetic susceptibility measurements indicate a Curie-Weiss behavior with an experimental effective magnetic moment of (mu eff)=8.28 (mu B) per formula unit.
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    Stress/pressure-stabilized cubic polymorph of Li3Sb with improved thermoelectric performance
    (Royal Society of Chemistry (RSC), 2021) Soldi, Thomas; Candolfi, Christophe; Snyder, G. Jeffrey; N/A; N/A; Department of Chemistry; Yahyaoğlu, Müjde; Özen, Melis; Aydemir, Umut; PhD Student; Master Student; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 58403
    Li3Sb has two polymorphs crystallizing in a face-centered cubic cell (c-Li3Sb; BiF3 structure type; space group Fm3m) and in a hexagonal unit cell (h-Li3Sb; Na3As structure type; space group P6(3)/mmc). c-Li3Sb was predicted to be a promising thermoelectric material based on recent first-principles studies; however, the experimental transport characteristics have remained unknown so far. Herein, successful preparation of c-Li3Sb is reported by stress-induced mechanochemical synthesis (high-energy ball milling) along with its high-temperature thermoelectric properties. Hexagonal Li3Sb (h-Li3Sb) was revealed to be the stable phase at ambient conditions, while it starts unexpectedly transforming to c-Li3Sb by ball milling or under 60 MPa applied pressure at room temperature. The transport properties measurements performed on two polycrystalline specimens evidence that c-Li3Sb behaves as a p-type degenerate semiconductor due to the formation of Li vacancies. In agreement with lattice dynamics calculations, c-Li3Sb exhibits very low lattice thermal conductivity despite the lightweight of Li. A zT value of around 0.3 was obtained at 550 K. Modelling suggests that the hole concentration should be reduced through aliovalent substitutions or under Li-rich conditions for further optimization. Although the strong air sensitivity of Li3Sb makes its use in thermoelectric applications challenging, this simple superionic binary provides an attractive experimental platform to elucidate the effect of stress/pressure on phase transitions as well as that of Fermi surface complexity on thermoelectric properties.
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    Phase-transition-enhanced thermoelectric transport in rickardite mineral Cu3-xTe2
    (American Chemical Society (ACS), 2021) Prots, Yurii; El Hamouli, Oussama; Tshitoyan, Vahe; Ji, Huiwen; Burkhardt, Ulrich; Lenoir, Bertrand; Snyder, G. Jeffrey; Jain, Anubhav; Candolfi, Christophe; N/A; N/A; Department of Chemistry; Yahyaoğlu, Müjde; Özen, Melis; Aydemir, Umut; PhD Student; Master Student; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 58403
    The binary copper chalcogenides Cu2-delta X (X = S, Se, and Te) have recently gained significant interest due to their high thermoelectric performance at moderate temperatures. In an effort to unveil new Cu-based compounds with promising thermoelectric potential, Cu3-xTe2 rickardite mineral emerged as a candidate based on a purely text mining approach applied by a machine learning method. Polycrystalline samples of Cu3-xTe2 within the homogeneity range (x = 0.1, 0.2) were successfully synthesized from the raw elements by a solid-state method. High-temperature powder Xray diffraction combined with differential scanning calorimetry and specific heat measurements showed several reversible phase transitions at around 458, 640, and 647 K. Signatures of these transitions were observed on the electronic and thermal transport properties, measured over a broad range of temperatures (5-733 K). The transition undergone by this compound at 647 K results in a crossover from metallic-like to semiconducting-like properties. The combination of high power factor and low thermal conductivity in the high-temperature phase results in improved thermoelectric performances with a peak dimensionless thermoelectric figure-of-merit zT of similar to 0.14 at 733 K. The synthetic rickardite mineral is an exciting candidate to be used as a phase change material in broad application areas such as in waste heat harvesting and photovoltaic systems.
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    Enhanced thermoelectric performance in Mg3+xSb1.5Bi0.49Te0.01 via engineering microstructure through melt-centrifugation
    (Royal Society of Chemistry (RSC), 2021) Candolfi, Christophe; Veremchuk, Igor; Kaiser, Felix; Burkhardt, Ulrich; Snyder, G. Jeffrey; Grin, Yuri; N/A; N/A; Department of Chemistry; Özen, Melis; Yahyaoğlu, Müjde; Aydemir, Umut; Master Student; PhD Student; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 58403
    N-type Zintl phases with earth-abundant and non-toxic constituent elements have attracted intense research interest thanks to their high thermoelectric efficiencies in the mid-temperature range, exemplified by the recently discovered Mg3Sb2 material. In this study, the liquid phase is expelled from the microstructure of the optimized n-type phase Mg3+xSb1.5Bi0.49Te0.01 by applying a meltcentrifugation technique leading to the formation of lattice dislocations, grain boundary dislocations and increasing porosity. Additional phonon scattering mechanisms were introduced in the microstructure through this manufacturing method, resulting in a significant 50% reduction in the total thermal conductivity from similar to 1 W m(-1) K-1 to similar to 0.5 W m(-1) K-1 at 723 K. Combined with high power factors, this reduced heat transport leads to a dimensionless thermoelectric figure of merit, zT, value of similar to 1.64 at 723 K, 43% higher than the value obtained in untreated Mg3+xSb1.5Bi0.49Te0.01 (zT similar to 1.14 at 723 K). This peak zT value yields a predicted device ZT of 0.95, and a promising theoretical thermoelectric efficiency of about 12%. These results further underline the great potential of the lightweight Mg3Sb2 material for midtemperature energy harvesting via thermoelectric effects.