| 1. |
- Shu, Rui, 1990-, et al.
(författare)
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Stoichiometry Effects on the Chemical Ordering and Superconducting Properties in TiZrTaNbNx Refractory High Entropy Nitrides
- 2023
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Ingår i: Annalen der Physik. - : Wiley-VCH Verlagsgesellschaft. - 0003-3804 .- 1521-3889.
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Tidskriftsartikel (refereegranskat)abstract
- High-entropy materials, an exciting new class of structural materials involvingfive or more elements, are emerging as unexplored ground forsuperconductors. Here, the effects of nitrogen stoichiometry are investigatedon local chemical structure of TiZrNbTa-based thin films by variousX-ray-based techniques. Lattice distortion and short-range order of a set ofTiZrNbTaNxsamples, including bond lengths of different atomic pairs andcoordination numbers of substituting atoms are quantitatively studied. Themaximum superconducting transition temperature Tcis found at 10 K for anear-stoichiometric (TiZrNbTa)N1.08film, which is>8 K measured for ametallic TiZrNbTa film. The underlying electronic structure and chemicalbonding in these high entropy nitrides thus influence the superconductingmacroscopic properties.
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| 2. |
- Bock, Florian, 1994-
(författare)
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Combining ab‐initio and machine learning techniques for theoretical simulations of hard nitrides at extreme conditions
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I focus on combining the high accuracy of first-principles calculations with modern machine learning methods to make large scale investigations of industrially relevant nitride systems reliable and computationally viable. I study the electronic, thermodynamic and mechanical properties of two families of compounds: Ti1−xAlxN alloys at the operational conditions of industrial cutting tools and ReNx systems at crushing pres-sures comparable to inner earth core conditions. Standard first-principles simulations of materials are usually carried out at zero temperature and pressure, and while many state-of-the-art approaches can take these effects into account, they are usually accompanied by a substantial increase in computational demand. In this thesis I therefore explore the possiblities of studying materials at extreme conditions using machine learning methods with extraordinary efficiency without loss of calculational accuracy. Ti1−xAlxN alloy coatings exhibit exceptional properties due to their inherent ability to spinodally decompose at elevated temperature, leading to age-hardening. Since the cubic B1 phase of Ti1−xAlxN is well-studied, available high-accuracy first-principles data served as both a benchmark and data set on which to train a machine learning interatomic potential. Using the reliable moment tensor potentials, an investigation of the accuracy and efficiency of this approach was carried out in a machine learning study. Building upon the success of this technique, implementation of a learning-on-the-fly (active learning) methodology into a workflow to determine accurate material properties with minimal prior knowledge showed great promise, while maintaining a computational demand up to two orders of magnitude lower than comparable first-principles approaches. Investigations of properties of industrially lesser desired, but sometimes present hexagonal alloy phases of Ti1−xAlxN are also included in this thesis, since knowledge and understanding of all competing phases can help guide development toward improving cutting tool lifetime and performance. Furthermore, while w-Ti1−xAlxN may not be able to compete with its cubic counterpart in terms of hardness, it shows promise for other applications due to its electronic and elastic properties. Metastable ReNx phases are high energy materials due to their covalent N-N and Re-N bonds, leading to exceptional mechanical and electronic properties. Just like diamond, the hardest and arguably most famous metastable mate-rial naturally occurring on earth, they are stabilized by extreme pressures and high temperatures, but can be quenched to ambient conditions. Understanding the formation and existence of these non-equilibrium compounds may hold the key to unlocking a new generation of hard materials. In this thesis, all currently known phases of ReNx compounds have been investigated, encompassing both experimentally observed and theoretically suggested structures. Investigations of the convex hulls across a broad pressure range were carried out, coupled with calculations of phonons in the proposed crystals to determine both energetic and dynamical stability. Overall, the studies included in this thesis focused mainly on investigation of the ground state of ReN2 at higher pressure, where experimental results were deviating from earlier theoretical predictions. Additional research focused on specifically exploring properties and stability of novel ReN6 at synthesis conditions using the active learning workflow to train an interatomic potential.
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| 3. |
- Bykov, Maxim, et al.
(författare)
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High-Pressure Synthesis of a Nitrogen-Rich Inclusion Compound ReN8·xN2 with Conjugated Polymeric Nitrogen Chains
- 2018
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Ingår i: Angewandte Chemie International Edition. - : WILEY-V C H VERLAG GMBH. - 1433-7851 .- 1521-3773. ; 57:29, s. 9048-9053
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Tidskriftsartikel (refereegranskat)abstract
- A nitrogen-rich compound, ReN(8)xN(2), was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100GPa, ReN(8)xN(2) is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] that constitute the framework have not been previously observed in any compound. Abinitio calculations on ReN(8)xN(2) provide strong support for the experimental results and conclusions.
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| 4. |
- Dubrovinsky, Leonid, et al.
(författare)
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Materials synthesis at terapascal static pressures
- 2022
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Ingår i: Nature. - London, United Kingdom : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 605:7909, s. 274-278
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions(1,2). Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell(3), producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re7N3-which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime.
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| 5. |
- Laniel, Dominique, et al.
(författare)
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Front Cover: Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α′-P3N5, δ-P3N5 and PN2 (Chem. Eur. J. 62/2022)
- 2022
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Ingår i: Chemistry - A European Journal. - : Wiley-VCH Verlagsgesellschaft. - 0947-6539 .- 1521-3765. ; 28:62
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- For the last 30 years, the lack of a binary phosphorus nitride containing PN6 octahedra formed a scientific chasm between carbon-group and oxygen-group nitrides, both featuring a variety of solids with XN6 units (X being a non-metal element). Now, the discovery of the δ-P3N5 and PN2 phosphorus nitrides—formed under high pressure and both composed of the elusive PN6 octahedron—builds a long-sought-after bridge between these two groups of nitrides. More information can be found in the Research Article by D. Laniel, F. Trybel, and co-workers (DOI: 10.1002/chem.202201998).
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| 6. |
- Laniel, Dominique, et al.
(författare)
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Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α′‐P3N5, δ‐P3N5 and PN2
- 2022
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Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 28:62
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Tidskriftsartikel (refereegranskat)abstract
- Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. These properties mainly rely on maximizing the number of strong covalent bonds, with crosslinked XN6 octahedra frameworks being particularly attractive. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells, and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. delta-P3N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two compounds are ultra-incompressible, having a bulk modulus of K-0=322 GPa for delta-P3N5 and 339 GPa for PN2. Upon decompression below 7 GPa, delta-P3N5 undergoes a transformation into a novel alpha -P3N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of alpha -P3N5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.
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| 7. |
- Laniel, Dominique, et al.
(författare)
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Synthesis of Ultra‐Incompressible and Recoverable Carbon Nitrides Featuring CN4 Tetrahedra
- 2024
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Ingår i: Advanced Materials. - : WILEY-V C H VERLAG GMBH. - 0935-9648 .- 1521-4095. ; 36:3
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Tidskriftsartikel (refereegranskat)abstract
- Carbon nitrides featuring three-dimensional frameworks of CN4 tetrahedra are one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, the high-pressure high-temperature synthesis of three carbon-nitrogen compounds, tI14-C3N4, hP126-C3N4, and tI24-CN2, in laser-heated diamond anvil cells, is reported. Their structures are solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations show that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions.
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| 8. |
- Salamania, Janella, 1992-, et al.
(författare)
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Elucidating dislocation core structures in titanium nitride through high-resolution imaging and atomistic simulations
- 2022
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 224
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Tidskriftsartikel (refereegranskat)abstract
- Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterizedthin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. Byhigh-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besidesthe expected primary a/2{110}h110i dislocation, Shockley partial dislocations a/6{111}h112i and sessileLomer edge dislocations a/2{100}h011i. Density-functional theory and classical interatomic potentialsimulations complement STEM observations by recovering the atomic structure of the different disloca-tion types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core.The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakenedTi-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}h110i and {111}h112idislocations. Our findings underscore that the presence of different dislocation types and their effects onchemical bonding should be considered in the design and interpretations of nanoscale and macroscopicproperties of TiN.
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| 9. |
- Salamania, Janella, 1992-, et al.
(författare)
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High-resolution STEM investigation of the role of dislocations during decomposition of Ti1-xAlxNy
- 2023
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Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- The defect structures forming during high-temperature decomposition of Ti1-xAlxNy films were investigated through high-resolution scanning transmission electron microscopy. After annealing to 950 °C, misfit edge dislocations a/6〈112〉{111} partial dislocations permeate the interface between TiN-rich and AlN-rich domains to accommodate lattice misfits during spinodal decomposition. The stacking fault energy associated with the partial dislocations decreases with increasing Al content, which facilitates the coherent cubic to wurtzite structure transition of AlN-rich domains. The wurtzite AlN-rich structure is recovered when every third cubic {111} plane is shifted by along the [211] direction. After annealing to 1100 °C, a temperature where coarsening dominates the microstructure evolution, we observe intersections of stacking faults, which form sessile locks at the interface of the TiN- and AlN-rich domains. These observed defect structures facilitate the formation of semicoherent interfaces and contribute to hardening in Ti1-xAlxNy.
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| 10. |
- Tidholm, Johan, 1991-
(författare)
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Lattice dynamics : From fundamental research to practical applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The reason to perform calculations in material science usually falls into one of two categories: to predict or explain the origin of material properties. This thesis covers first-principle calculations for solids at extreme conditions, from both of the two mentioned categories. I primarily have studied the effects of high-pressure and high-temperature on lattice dynamics, mechanical and electronic properties. To treat the effects of temperature, ab initio molecular dynamics (AIMD) simulations and self-consistent phonon calculations, based on density functional theory, have been utilised. These approaches account for the temperature effects by considering thermally excited supercells as samples of a statistical ensemble. To extract properties from this representation, I have used methods which maps the supercell data to a unit cell representation or fits it to a simple model Hamiltonian.The small displacement method was used to analyse the dynamical stability for nitrides and polymorphs of silica, synthesised at high-pressure in a diamond anvil cell. The nitride compounds consist of a high amount of nitrogen either as chains, forming a porous framework together with transition metal atoms or as dinitrogen molecules, occupying the channels of the framework. The nitrogen chains consist of single- or double-bonded nitrogen atoms, making these compounds highly energetic. Polymorphs of silica can be used to model deep Earth liquids. These new polymorphs, named coesite-IV and coesite-V, consist of four-, five-, and six-oriented silicon. Some of the octahedra of the six-oriented silicon atoms, of these new phases, are sharing faces, which according to Pauling's third rule would make them highly unstable. My phonon calculations indicate these phases to be dynamically stable. Furthermore, my calculations predict higher compressibility for these new phases compared to the competing ones. By modelling silicate melts with coesite-IV and coesite-V, a more complex and compressible structure is expected, affecting the predicted seismic behaviour.I studied Kohn anomalies for body-centered cubic niobium by simulating this material with self-consistent phonon calculations. The electronic structure was studied by using a band unfolding technique, for which I obtained an effective unit cell representation of the electronic structure at elevated temperatures. Temperature primarily smeared the electronic states but did not induce significant shifts of the bands. In parallel, the anharmonicity of this system was studied using the temperature dependent effective potential method. Even close to the melting temperature, this element is remarkably harmonic. The experimentally observed disappearance of the Kohn anomalies with increased temperature is predominantly dependent, according to my calculations, on the temperature-induced smearing of the electronic states.Using stress-strain relations, accurate high-temperature elastic properties were predicted for Ti0.5Al0.5N. The simulations were performed with AIMD. The stresses were fitted using the least-squares method to a linear expression from which the elastic constants were derived. The results were compared with previously performed calculations that employed additional approximations. The results of the symmetry imposed force constant temperature dependent effective potential (SIFC-TDEP) method agrees well with our results. I also compared my results with TiN calculations that employed a similar methodology. My and the SIFC-TDEP results are reporting lower values for the polycrystalline moduli than the calculations for TiN. The data I generated were also used for a machine learned interatomic potential method, where moment tensor potentials were trained and evaluated, using this data.
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