Scopus İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/395
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Article Citation - WoS: 4Citation - Scopus: 4Very Low Density Amorphous Phase of Zircon(Elsevier Science Bv, 2019-06) Bolat, Suleyman; Durandurdu, MuratUsing a reliable ab initio molecular dynamics method, we investigate the rapid solidification of the zircon melt. Accompanied by amorphization, a drastic volume expansion of 27% is perceived. This value is fairly larger than 18% observed in the metamict zircon. Such a large volume swelling leads to a significant decrease in the mean coordination number of Zr atoms, which is about 5.66 and the lowest one reported so far. On the other hand, the volume expansion is found to have almost no impact on the average coordination number of Si atoms i.e., they maintain their tetragonal coordination. As suggested by earlier investigations, the polymerization of SiO4 units is witnessed but our model shows the highest polymerization with respect to the previous simulations. Based on our findings, we propose that our model does not represent the metamict zircon but a very low density amorphous phase of zircon.Article Citation - WoS: 6Citation - Scopus: 6Tetrahedral Amorphous Boron Nitride: A Hard Material(Wiley, 2019-09-25) Durandurdu, MuratWe generate a tetrahedrally coordinated amorphous boron nitride (BN) model by means of first principles molecular dynamics calculations and report its mechanical and electrical properties in detail. The amorphous configuration is almost free from chemical disorder and consists of about 20% coordination defects, similar to tetrahedral (diamond-like) amorphous carbon. Its theoretical band gap energy is about 2.0 eV, less than 4.85 eV estimated for cubic BN. The bulk modulus and Vickers hardness of tetrahedral amorphous BN are computed as 206 GPa and 28-35 GPa, respectively. Based on these findings, we propose that tetrahedral noncrystalline BN can serve as electronic and hard materials as well.Article Citation - WoS: 4Citation - Scopus: 4Permanent Densification of Amorphous Zinc Oxide Under Pressure: A First Principles Study(Elsevier Science Bv, 2018-02) Tahaoglu, Duygu; Durandurdu, MuratAb initio simulations within a generalized gradient approximation are carried out to investigate the densification mechanism of amorphous zinc oxide (a-ZnO) under hydrostatic pressure. In contrast to the crystalline ZnO, the densification of a-ZnO is found to proceed gradually and is associated with a structural modification from a low density amorphous state to a high density amorphous state. Accompanied by the phase transformation, the mean coordination number increases from similar to 4.0 to similar to 5.5. The high-density amorphous form of ZnO has a local structure, partially comparable with that of the rocksalt type ZnO crystal and presents a semiconducting behavior. The phase change is irreversible because upon pressure release, an amorphous model largely consisting of fivefold coordination is recovered. The decompressed model can be, therefore, classified as an intermediate phase between the wurtzite-like and the rocksalt-like amorphous configurations.Article Citation - WoS: 1Citation - Scopus: 1Liquid and Amorphous States of Boron Subarsenide(Wiley, 2019-08-13) Durandurdu, MuratAb initio molecular dynamics simulations are executed to probe the short-range order and the electrical features of the liquid and amorphous boron subarsenide (B12As2). A drastic volume swelling of similar to 40% is witnessed for the liquid state, relative to the crystal. The density of the melt is found to be close to that of liquid boron. As the temperature applied is gradually decreased, the volume progressively decreases and a glass-transition zone at around 1400 K is observed. About 14% volume expansion is perceived for the amorphous phase. Due to the drastic density (volume) difference between the liquid and amorphous forms, their atomic structure is found to be different from each other. In the liquid phase at 2500 K, the mean coordination number (CN) of B and As atoms is 4.4 and 2.5, correspondingly. During the solidification process, both average CNs steadily increase and reach values of 5.5 (B-atom) and 4.14 (As-atom) at 300 K. The pentagonal pyramid-like motifs barely survive at 2500 K but during the quenching process they develop progressively and some of which lead to the formation of B-12 clusters. In the amorphous state, the chain-like and A7-like As-As clusters are observed. Nonetheless, the noncrystalline state is proposed to be partially similar to the crystalline structure. The liquid state shows a metallic character while the amorphous form presents a semiconducting nature having an energy band gap much smaller than that of the crystalline phase.Article Citation - WoS: 10Citation - Scopus: 11Liquid Boron and Amorphous Boron: An Ab Initio Molecular Dynamics Study(Elsevier, 2015-06) Durandurdu, MuratThe atomic structure of liquid and amorphous boron is investigated using an ab initio molecular dynamics technique. Liquid and amorphous states are found to have notably different microstructures and an average coordination number. Ideal and defective pentagonal pyramidal polyhedrons are the primary building unit of liquid boron but B-12 icosahedra do not exist in the liquid state. During the rapid solidification, more ideal pentagonal pyramids develop progressively, resulting into a gradual formation of B-12 icosahedra. On the basis of our findings, the atomic packing of pure amorphous boron is proposed to be somewhat close to that of the alpha-rhombohedral phase in contrast to the previous suggestions. (C) 2015 Elsevier B.V. All rights reserved.Editorial Citation - WoS: 4Citation - Scopus: 5Hydrogenated Amorphous Boron Nitride: A First Principles Study(Elsevier, 2018-12) Uchoyuk, Tevhide Ayca; Durandurdu, MuratThe influence of hydrogenation on the atomic structure and electronic properties of amorphous boron nitride (alpha-BN) is investigated by using an ab-initio molecular dynamics technique. The structural evaluation of alpha-BN and the hydrogenated (alpha-BN:H) models with four different hydrogen concentrations reveals that although their short-range order is mainly similar to each other, hydrogenation yields some noticeable amendments on the local structure of alpha-BN. Hydrogenation suppresses the formation of twofold coordinated chain-like structures and tetragonal-like rings and leads to more sp(2) and even sp(3) hybridizations. It is also observed that the formation of N-H bonding is more favorable than that of the B-H bonding in the alpha-BN:H configurations. Furthermore hydrogenation is found to have an insignificant impact on the electronic structure of alpha-BN.Article Citation - WoS: 17Citation - Scopus: 17Hexagonal Nanosheets in Amorphous BN: A First Principles Study(Elsevier Science Bv, 2015-11) Durandurdu, MuratAmorphous boron nitrite is modeled by means of first principles molecular dynamics simulations and found to be almost chemically ordered in a stark contrast to the previous predictions. Its average coordination number is 2.97. The main building unit of the amorphous network is hexagonal rings as in the most stable boron nitrite phase but chain-like structures and tetragonal-like rings also exist in amorphous network. The model consists of partially hexagonal nanosheets and hence it is not entirely disordered. Amorphous boron nitrite has a band gap energy of about 2.0 eV. (C) 2015 Elsevier B.V. All rights reserved.Article Citation - WoS: 3Citation - Scopus: 3Ferromagnetism in Amorphous MgO(Taylor & Francis Ltd, 2017-05-10) Durandurdu, MuratWe report, for the first time, the atomic structure of amorphous MgO based on ab initio molecular dynamics simulations. We find that its main building blocks are four-fold and five-fold coordinated configurations, similar to those formed in the liquid state. Its average coordination is estimated to beabout 4.36. The amorphous form having a perfect stoichiometry has a band gap energy of 2.4eV. On the other hand, Mg vacancies induce an insulator to metal transition and ferromagnetism in amorphous MgO whilst O vacancies do not cause such a transition, implying that the magnetism in amorphous MgO is related to the non-stoichiometry and Mg vacancies. With the application of pressure, the stoichiometric and non-stoichiometric (Mg vacancies) models undergo a phase transformation into a rocksalt state, suggesting that the electronic structure of the initial configurations has no influence on the resulting high-pressure phase in amorphous MgO.Article Citation - WoS: 7Citation - Scopus: 7Atomic Structure of Amorphous CDO from First Principles Simulations(Elsevier Science Bv, 2015-03) Durandurdu, MuratAmorphous CdO (a-CdO) is obtained by cooling the liquid at a sufficiently fast cooling rate using first-principles simulations. The topology of the amorphous model is examined using a variety of analyzing techniques. The local structural arrangement of a-CdO is found to be partially similar to that of crystalline phase. The model is chemically ordered but consists of a significant amount of coordination defects. a-CdO is predicted to be a semiconductor with a band gap energy less than the crystalline state. It is likely that a-CdO might serve as a novel electronic material. (C) 2015 Elsevier B.V. All rights reserved.Article Citation - WoS: 3Citation - Scopus: 4An In-Depth Investigation of Mg-Zn Metallic Glasses: A First Principles Study(Elsevier, 2018-10) Erkartal, Mustafa; Durandurdu, MuratThe atomic structures, glass forming evolutions, mechanical properties and high pressure behavior of Mg75Zn20Ca5 and Mg60Zn35Ca5 bulk metallic glasses, which are promising candidates for biomedical implants, have been examined by using ab initio molecular dynamics simulations. The pair-distribution function and coordination number analyses show that increasing Zn content in the alloy results in a decrease in several bond distances and an increase in the total coordination number of each species due to the atomic size difference between Mg and Zn atoms. According to the Voronoi tessellation, bond pair and bond angle distribution analyzes, the fivefold geometrical arrangements (pentagonal-bipyramid) are the most predominant in the first coordination shell, indicating the stability of the amorphous states and their dense atomic packing. The most striking result emerged from the calculations of mechanical properties is that an increase of Zn (>= 30%) content in the alloy yields embrittlement in the alloys. Under uniaxial compressions, both compositions undergo structural failure between 6 and 8 GPa. Under hydrostatic pressure, a diminishing in fcc/hcp ordering and an enlargement of the ideal icosahedral ordering may indicate a more disordered structure. In our view, these results represent a good step toward understanding the atomic structures Mg-Zn-Ca bulk metallic glasses.