WoS İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/394
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Article Citation - WoS: 6Citation - Scopus: 6Uncovering Nanoclusters in Amorphous AlN: An Ab Initio Study(Wiley, 2014-12-22) Durandurdu, MuratAmorphous AlN (a-AlN) is modeled by melt-and-quench technique using ab initio molecular dynamic simulations. For the first time, three-dimensional hexagonal-like nanoclusters embedded in amorphous matrix are proposed for a-AlN. The model is chemically ordered and dominantly fourfold coordinated, but its short-range order is partially different from the crystalline morphology due to the nanoclusters. The model is semiconducting with a theoretical band gap of 1.7eV.Article Citation - WoS: 6Citation - Scopus: 6Two Successive Amorphous-to Phase Transformations in TiO2(Wiley, 2017-05-22) Durandurdu, MuratBased on constant pressure ab initio simulations, we propose, for the first time, two successive amorphous-to-amorphous phase transformations for TiO2. The first one is a gradual phase transformation from a low-density amorphous phase to a high-density amorphous phase, whereas the second one is a first-order phase transformation from the high-density amorphous phase to a very high-density amorphous phase. The low-density amorphous to high-density amorphous phase change is irreversible, whereas the high-density amorphous to very high-density amorphous phase transformation is reversible. The high-density amorphous and very high-density amorphous phases consist of differently coordinated configurations. The sevenfold and ninefold-coordinated arrangements formed in amorphous TiO2 under pressure are similar to the main building motif of the baddeleyite and cotunnite polymorphs of TiO2, respectively, while the eightfold-coordinated configuration is different from the local structure of the cubic TiO2 phase. The electronic structure calculations suggest that both dense amorphous phases present a semiconducting character with a band gap energy less than that of the original low-density amorphous phase.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 Pressure-Induced Quenchable Superhard Tetrahedral Amorphous Phase of BC4N(Wiley, 2025-03-13) Durandurdu, MuratThe high-pressure behavior of an amorphous boron carbon nitride (BC4N) composition is investigated using constant-pressure ab initio molecular dynamics simulations. A first-order phase transformation into a tetrahedral amorphous phase with a high fraction of sp3 bonding is observed. This tetrahedral phase is quenchable and exhibits ultra-high incompressibility and a high Vickers hardness (46 GPa), placing it firmly in the category of superhard materials, comparable to tetrahedral amorphous carbon. Tetrahedral amorphous BC4N demonstrates semiconducting behavior with a narrow bandgap of 0.4 eV, making it suitable for applications requiring both mechanical robustness and moderate electronic conductivity. Thermodynamic analyses confirm the likelihood of a first-order sp2-to-sp3 transition, suggesting that such a transformation could occur around 29 GPa under experimental conditions.Article Citation - WoS: 12Citation - Scopus: 12Polyamorphism in Aluminum Nitride: A First Principles Molecular Dynamics Study(Wiley, 2016-03-02) Durandurdu, MuratThe high-pressure behavior of amorphous aluminum nitride is investigated for the first time by means of ab initio molecular dynamics simulations. It is found to undergo two successive first-order phase transformations with the application of pressure. The first one is a polyamorphic phase transition in which the low-density amorphous phase transforms into a high-density amorphous phase having an average coordination number of about 4.6. The high-density amorphous structure transforms back to a low-coordinated amorphous network upon pressure release but its density is higher than that of the original low-density amorphous phase. The second phase change is the crystallization of the high-density amorphous state into a rocksalt structure. A careful analysis suggests that the hexagonal-like nanoclusters presented in amorphous aluminum nitride prevent the formation of a very dense amorphous phase (about sixfold coordinated) during the first phase transition and they act as a nucleation center for the crystallization process.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: 8Citation - Scopus: 8Hard Boron Rich Boron Nitride Nanoglasses(Wiley, 2017-12-21) Cetin, Aysegul O.; Durandurdu, MuratBoron-rich amorphous boron nitride (BxN1-x, 0.55x0.95) alloys are generated by means of abinitio molecular dynamics simulations and their local structure, mechanical properties and electronic structure are exposed. BN:B phase separations are perceived in all amorphous networks, suggesting that these materials can serve as nanoglass ceramics. The sp(2) hybridization is the main building unit in the BN-rich regions for low boron concentrations, and the models carry locally the signature of the two-dimensional hexagonal BN structure. The amorphous states having both sp(2) and sp(3) hybridizations form for boron contents between 70% and 80%. At higher boron concentrations, sp(3) hybridization with a fraction of similar to 90%-98% is detected as seen in the cubic or wurtize BN crystals. In the boron rich regions, the ideal and defective pentagonal pyramids emerge at 60% boron content, and the first complete B-12 molecule develops at 70% boron concentration. In addition to the B-12 icosahedron, the formation of a cage-like B-16 molecule is, for the first time, discovered in some amorphous alloys. The isolated B-16 molecule is, however, found to be unstable. The Vickers harness calculations reveal that some of these amorphous alloys can serve as hard materials. When their electron properties are considered, all amorphous materials are predicted to be semiconducting.Article Citation - WoS: 1Citation - Scopus: 1Amorphous Zirconia at High Pressure(Wiley, 2018-06-08) Durandurdu, MuratWe show, by means of ab initio calculations, that amorphous zirconia progressively transforms to a high-density amorphous phase with the application of pressure. The average coordination number of Zr and O atoms under pressure rises gradually to 8 and 4, respectively. The main building unit of the dense noncrystalline state is the eightfold-coordinated Zr atoms (62.5%). When the coordinated modification of Zr atoms in the zirconia crystal at high pressure and temperature conditions is considered, it can be perceived that amorphous zirconia follows a transformation mechanism similar to the one observed at high temperature but different than the one detected at high pressure. The dense disordered phase is indeed found to be locally comparable with the high-temperature tetragonal crystal. Upon decompression, some high-pressure arrangements are persevered in the model and a transformation into another amorphous state whose structure is intermediate between uncompressed and dense amorphous phases is observed in the simulations. The high-pressure amorphous structures are found to be semiconductors with a band gap smaller than that of the original model.Article Citation - WoS: 4Citation - Scopus: 4Amorphous Boron Suboxide(Wiley, 2019-02-04) Durandurdu, MuratWe study the atomic structure and the electronic and mechanical properties of amorphous boron suboxide (B6O) using an ab initio molecular dynamic technique. The amorphous network is attained from the rapid solidification of the melt and found to consist of boron and oxygen-rich regions. In the boron-rich regions, boron atoms form mostly perfect or imperfect pentagonal pyramid-like configurations that normally yield the construction of ideal and incomplete B-12 molecules in the model. In addition to the B-12 molecules, we also observe the development of a pentagonal bipyramid (B-7) molecule in the noncrystalline structure. In the oxygen-rich regions, on the other hand, boron and oxygen atoms form threefold and twofold coordinated motifs, respectively. The boron-rich and oxygen-rich regions indeed represent structurally the characteristic of amorphous boron and boron trioxide (B2O3). The amorphous phase possesses a small band gap energy with respect to the crystal. On the bases of the localization of the tail states, we suggest that the p-type doping might be more convenient than the n-type doping in amorphous B6O. Bulk modulus and Vickers hardness of the noncrystalline configuration is estimated are be 106 and 13-18 GPa, respectively, which are noticeably less than those of the crystalline structure. Such a noticeable decrease in the mechanical properties is attributed to the presence of open structured B2O3 glassy domains in the amorphous model.Article Citation - WoS: 3Citation - Scopus: 2Ab Initio Study of Boron-Rich Amorphous Boron Carbides(Wiley, 2023-01-10) Yildiz, Tevhide Ayca; Durandurdu, MuratAmorphous boron carbide compositions having high B contents (BxC1-x, 0.50 <= x <= 0.95) are systematically created by way of ab initio molecular dynamics calculations, and their structural, electrical, and mechanical characteristics are inclusively investigated. The coordination number of both B and C atoms increases progressively with increasing B/C ratio and more close-packed materials having pentagonal pyramid motifs form. An amorphous diamond-like local arrangement is found to be dominant up to 65% B content, and beyond this content, a mixed state of amorphous diamond- and B-like structures is perceived in the models because sp(3) hybridization around C atoms is still leading one for all compositions. The pentagonal pyramid motifs around C atoms are anticipated to appear beyond 65% content. The intericosahedral linear C-B-C chains do not form in any model. All amorphous boron carbides are semiconducting materials. The mechanical properties gradually increase with increasing B concentration, and some amorphous compositions are proposed to be hard materials on the basis of their Vickers hardness estimation.