Mg3Sb2-based thermoelectric materials attract attention for applications near room temperature. Here, Mg-Bi films were synthesized using magnetron sputtering at deposition temperatures from room temperature to 400 °C. Single-phase Mg3Bi2 thin films were grown on c-plane-oriented sapphire and Si(100) substrates at a low deposition temperature of 200 °C. The Mg3Bi2 films grew epitaxially on c-sapphire and fiber-textured on Si(100). The orientation relationships for the Mg3Bi2 film with respect to the c-sapphire substrate are (0001) Mg3Bi2‖(0001) Al2O3 and [112⎯⎯2¯0] Mg3Bi2‖[112⎯⎯2¯0] Al2O3. The observed epitaxy is consistent with the relatively high work of separation, calculated by the density functional theory, of 6.92 J m−2 for the Mg3Bi2 (0001)/Al2O3 (0001) interface. Mg3Bi2 films exhibited an in-plane electrical resistivity of 34 μΩ m and a Seebeck coefficient of +82.5 μV K−1, yielding a thermoelectric power factor of 200 μW m−1 K−2 near room temperature.This work was supported financially by the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program (No. KAW-2020.0196), the Swedish Research Council (VR) under Project Nos. 2016-03365 and 2021-03826, the National Key Research and Development Program of China under Grant No. 2018YFB0703600, the National Natural Science Foundation of China under Grant No. 51872133, the Guangdong Innovative and Entrepreneurial Research Team Program under Grant No. 2016ZT06G587, and the Tencent Foundation through the XPLORER PRIZE, Guangdong Provincial Key Laboratory Program (No. 2021B1212040001) from the Department of Science and Technology of Guangdong Province. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at National Supercomputer Centre (NSC) partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973.
We report structure, vibrational properties, and weak antilocalization-induced quantum correction to magnetoconductivity in single-crystal Bi2GeTe4. Surface band-structure calculations show a single Dirac cone corresponding to topological surface states in Bi2GeTe4. An estimated phase coherence length, lΦ ~ to 143 nm and prefactor α~-1.54 from Hikami-Larkin-Nagaoka fitting of magnetoconductivity describe the quantum correction to conductivity. An anomalous temperature dependence of A1g Raman modes confirms enhanced electron-phonon interactions. Our results establish that electrons of the topological state can interact with the phonons involving the vibrations of Bi-Te in Bi2GeTe4. ; Funding Agencies|CSIRCouncil of Scientific & Industrial Research (CSIR) - India; SERBDepartment of Science & Technology (India)Science Engineering Research Board (SERB), India [CRG/2018/002197]; DST India grant [DST/INT/SWD/VR/P-18/2019]; Swedish Research Council (VR)Swedish Research Council [2016-03365, 2018-07070]; Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program [KAW 2020.0196]; Electron Microscopy Laboratory at Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Energy AgencySwedish Energy AgencyMaterials & Energy Research Center (MERC) [46519-1]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [RIF 14-0074]
We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn2GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method, we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically ordered phases of Mn2GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro-and antiferromagnetic configurations of Mn2GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn2GaC, implying the importance of orbital-dependent localization of the Mn 3d states. We find that Mn2GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by Hunds exchange interaction, and not the Coulomb interaction. ; Funding Agencies|Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish e-Science Research Centre (SeRC); Swedish Research Council (VR)Swedish Research Council [2019-05600]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; state assignment of Minobrnauki of Russia [AAAA-A18-1180201900985]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]
Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature ( T-c) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electron-phonon coupling. It is found that increasing pressure suppresses T-c in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by co-deposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions. ; Funding Agencies|Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Research Council (VR) through International Career GrantSwedish Research Council [2014-6336, 2019-05403]; Marie Sklodowska Curie Actions [INCA 600398]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Office of the Permanent Secretary, Ministry of Higher Education, Science, Research and Innovation: Research Grant for New Scholar [RGNS 63-013]; Swedish National Infrastructure for Computing (SNIC); National Supercomputer Centre (NSC)
A variety of bulk high-entropy alloy superconductors have been recently discovered; however, for thin films, only the TaNbHfZrTi highentropy alloy system has been investigated for its superconducting properties. Here, (TiZrNbTa)1-xWx and (TiZrNbTa)1-xVx superconducting films have been produced by DC magnetron sputtering at different growth temperatures. The phase formation and superconducting behavior of these films depend on the content of alloying x and deposition temperature. A single body-centered cubic (bcc) phase can be formed in the low x range with enough driving energy for crystallinity, but phase transition between amorphous or two bcc structures is observed when increasing x. The highest superconducting transition temperature Tc reaches 8.0 K for the TiZrNbTa film. The superconducting transition temperature Tc of these films deposited at the same temperature decreases monotonically as a function of x. Increasing deposition temperature to 400 °C can enhance Tc for these films while retaining nearly equivalent compositions. Our experimental observations suggest that Tc of superconducting high entropy alloys relate to the atomic radii difference and electronegativity difference of involved elements beyond the valence electron number. ; Funding: Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; VINNOVA Competence Centre FunMat-II [2016-05156]; Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program [KAW-2020.0196]; Swedish Research Council [2021-03826]
Energy level alignment (ELA) at donor-acceptor heterojunctions is of vital importance yet largely undetermined in organic solar cells. Here, authors determine the heterojunction ELA with (mono) layer-by-layer precision to understand the co-existence of efficient charge. Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems. ; Funding Agencies|Swedish Research Council [2016-05498, 2016-05990, 2020-04538, 2018-06048, 2018-07152]; Swedish Energy Agency [45411-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Wallenberg Wood Science Center (WWSC); Stiftelsen for Strategisk Forskning through a Future Research Leader program [FFL18-0322]; Swedish Governmental Agency for Innovation Systems [2018-04969]; Formas [2019-02496]
The experimental study of hydrogen-bonds and their symmetrization under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrization has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution in-situ 1H-NMR experiments in diamond anvil cells investigating a range of systems containing linear O-H ⋯ O units at pressure ranges of up to 90 GPa covering their respective H-bond symmetrization. We found pronounced minima in the pressure dependence of the NMR resonance line-widths associated with a maximum in hydrogen mobility, precursor to a localisation of hydrogen atoms. These minima, independent of the chemical environment of the O-H ⋯ O unit, can be found in a narrow range of oxygen oxygen distances between 2.44 and 2.45 Å, leading to an average critical oxygen-oxygen distance of Å. ; Funding: German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) [DU 393/13-1, DU 393/9-2, STE 1105/13-1, ME 5206/3-1, DU 954/111]; Federal Ministry of Education and Research, Germany (BMBF) [05K19WC1]; Center for High Pressure Science and Technology Advance Research, Beijing, P.R. China; Swedish Research Council (VR) [2019-05600]; Alexander von Humboldt Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]
For the high-power (HP) electronic applications the existing Si-based devices have reached the performance limits governed by the material properties. Hence the device innovation itself is unable to enhance the overall performance. GaN, a semiconductor with wide bandgap, high critical breakdown field, and high electronic saturation velocity is regarded as an alternative of Si. The material properties of GaN make it very suitable for fast-switching HP electronic devices and contribute to the fast growing of GaN technology. The state-of-the-art GaN devices operating up to 650 V have recently become commercially available. Further goal is to reach higher breakdown voltage which can be done via device engineering and material growth optimization. AlxGa1−xN is an ultrawide-bandgap (UWBG) semiconductor which is considered as a natural choice for next generation in the development of GaN-based HP electronic devices. This material attracts particular interest due to the possibility for bandgap tuning from 3.4 eV to 6 eV which allows nonlinear increase of avalanche breakdown field. Furthermore, both n- and p-type conductivity can be achieved on this material permitting variety of device design with reduced energy losses during operation. β−Ga2O3 is also a promising material for HP electronics because of its ultra-wide bandgap (4.8 eV) and a huge value of Baliga's figure of merit (FOM) exceeding by far that of GaN. More interesting feature making this material attractive is the availability of low-cost natural substrates, and then the possibility to obtain high crystal quality of device structures. For the HP electronic devices thermal conductivity is one of the key parameters determining the device's performance. The initial studies have shown that the thermal conductivity of AlxGa1−xN and β−Ga2O3 is quite low comparing with that of GaN. This is one of the biggest challenges slowing the development of these materials for HP device applications. Nevertheless, AlxGa1−xN- and β−Ga2O3-based field-effect transistors and Schottky-barrier diodes have been demonstrated showing performances superior to that of GaN. To optimize and maintain good performance and reliability, heat generated in the device active regions has to be effectively dissipated. Therefore the thermal conductivity of the materials in the device structures needs to be systematically studied and accurately determined. This information is critically important for the thermal management of the devices. Transient thermoreflectance (TTR) is a contactless nondestructive method for measuring of the thermal conductivity of materials. TTR, which is based on a pump-probe technique, has shown its potential in evaluation of the thermal conductivity in bulk crystals as well as in thin layers in hetero-epitaxial structures. The method requires an analysis of experimental data based on the fit of thermoreflectance transients with the solution of the one-dimensional heat transport equations by a least-square minimization of the fitting parameters. Such a procedure allows to extract not only the thermal conductivity of the constituent materials in the structures, but also the thermal boundary resistance at different hetero-interfaces. The main research results of the graduate studies presented in this licentiate thesis are summarized in three scientific papers. Paper I. In this paper thermal conductivity of β−Ga2O3 and high Al-content AlxGa1−xN thin layers was studied. For β−Ga2O3 the the effects of Sn doping and phonon-bondary scattering on the reduction of thermal conductivity were discussed. For the AlxGa1−xN we studied the effect of Al-Ga alloying which gives rise to phonon-alloy scattering. It was found that this scattering process accounts for low thermal conductivity of this material. Finally, a comparison for the thermal conductivity of the two materials was made. Paper II. In this paper the effect of layer thickness on the thermal conductivity of AlxGa1−xN layers grown by HVPE were investigated. Due to Al alloying the thermal conductivity of this material is degraded and reduced by more than one order of magnitude. On top of that we also observed further reduction of thermal conductivity when the layer thickness goes thinner. The mechanism of this phenomenon has been revealed by studying the phonon transport properties in bulk crystal and thin layer. Paper III. This study emphasizes the role of defects in GaN and AlxGa1−xN to the thermal conductivity of these materials. The dislocations, impurities, free carries, and random alloying have been separately studied and discussed. Thermal conductivity of samples containing these defects with various concentrations was measured and the results were interpreted by a theoretical model based on relaxation time approximation (RTA). ; Additional funding agencies: the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009 − 00971
The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity kappa. However, the kappa of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in kappa is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating kappa based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates kappa near the phase transition. Our findings elucidate the influence of structural phase transitions on kappa and provide guidance for design of better thermoelectric materials. ; Funding Agencies|Science Foundation IrelandScience Foundation IrelandEuropean Commission [15/IA/3160, 13/RC/2077]; European Regional Development FundEuropean Commission; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas SeRC
Conducting polymers, such as the p-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n-doped conducting polymers are also needed. Despite major efforts, no n-type equivalents to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n-type conductive ink. BBL:PEI thin films yield an n-type electrical conductivity reaching 8Scm(-1), along with excellent thermal, ambient, and solvent stability. This printable n-type mixed ion-electron conductor has several technological implications for realizing high-performance organic electronic devices, as demonstrated for organic thermoelectric generators with record high power output and n-type organic electrochemical transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance. The development of n-type conductive polymer inks is critical for the development of next-generation opto-electronic devices that rely on efficient hole and electron transport. Here, the authors report an alcohol-based, high performance and stable n-type conductive ink for printed electronics. ; Funding Agencies|Knut and Alice Wallenberg foundationKnut & Alice Wallenberg Foundation; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-03979, 2020-03243]; AForsk [18-313, 19-310]; Olle Engkvists Stiftelse [204-0256]; VINNOVAVinnova [2020-05223]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; National Research Foundation of KoreaNational Research Foundation of Korea [NRF2020M3H4A3081814, 2019R1A6A1A11044070]; National Science FoundationNational Science Foundation (NSF) [DMR-2003518]
Semiconductor quantum dots (QDs) acting as single-photon-emitters are potential building blocks for various applications in future quantum information technology. For such applications, a thorough understanding and precise control of charge states and capture/recombination dynamics of the QDs are vital. In this work, we study the dynamics of QDs spontaneously formed in GaNAsP nanowires, belonging to the dilute nitride material system. By using a random population model modified for these highly mismatched materials, we analyze the results from photoluminescence and photon correlation experiments and show a general trend of disparity in positive and negative trion populations and also a strong dependence of the capture/recombination dynamics and QD charge states on its surroundings. Specifically, we show that the presence of hole-trap defects in the proximity to some QDs facilitates formation of negative trions, which also causes a dramatic reduction of the neutral exciton lifetime. These findings underline the importance of proper understanding of the QD capture and recombination processes and demonstrate the possibility to use highly mismatched materials and defects for charge engineering of QDs. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2019-04312]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]
A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principles calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (also known as point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution(. )A four-parameter semi-quantum model is applied to determine all 16 pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, epsilon(infinity) = 4.05 +/- 0.05. The Lyddane-Sachs-Teller relation then gives access to the static dielectric constant, epsilon(DC) = 10.55 +/- 0.07. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped n-type sample a free electron density of n = (2.81 +/- 0.01) x 10(17) cm(-3), a mobility of mu = (112 +/- 3) cm(2)/(Vs), and an effective mass parameter of (0.208 +/- 0.006)m(e). Density and mobility parameters compare very well with the results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the Gamma point in In2O3 in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In2O3, and our model validates the static dielectric constant obtained from the Lyddane-Sachs-Teller relation. Published under an exclusive license by AIP Publishing. ; Funding Agencies|National Science Foundation (NSF)National Science Foundation (NSF) [NSF DMR 1755479, NSF DMR 1808715]; Nebraska Materials Research Science and Engineering Center, GraFOx, a Leibniz-Science Campus - Leibniz Association-Germany [DMR 1420645]; Air Force Office of Scientific Research (AFOSR)United States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360, FA9550-19-S-0003]; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Governmental Agency for Innovation Systems VINNOVA under the Competence Center Program [2016-05190]; Knut and Alice Wallenbergs Foundation supported grant "Wide-bandgap semiconductors for next generation quantum components"; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Faculty Grant SFO Mat LiU [2009-00971]; University of Nebraska Foundation; J. A. Woollam Foundation
Ab initio simulations of a multi-component alloy using density functional theory (DFT) were combined with experiments on thin films of the same material using X-ray photoelectron spectroscopy (XPS) to study the connection between the electronic and atomic structures of multi-component alloys. The DFT simulations were performed on an equimolar HfNbTiVZr multi-component alloy. Structure and charge transfer were evaluated using relaxed, non-relaxed, as well as elemental reference structures. The use of a fixed sphere size model allowed quantification of charge transfer, and separation into different contributions. The charge transfer was generally found to follow electronegativity trends and results in a reduced size mismatch between the elements, and thus causes a considerable reduction of the lattice distortions compared to a traditional assumption based on tabulated atomic radii. A calculation of the average deviation from the average radius (i.e. the so-called delta-parameter) based on the atomic Voronoi volumes gave a reduction of delta from ca. 6% (using the volumes in elemental reference phases) to ca. 2% (using the volumes in the relaxed multi-component alloy phase). The reliability of the theoretical results was confirmed by XPS measurements of a Hf22Nb19Ti18V19Zr21 thin film deposited by sputter deposition. The experimentally observed core level binding energy shifts (CLS), as well as peak broadening due to a range of chemical surroundings, for each element showed good agreement with the calculated DFT values. The single solid solution phase of the sample was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) including energy dispersive spectroscopy (EDS) with nm-resolution. These observations show that the HfNbTiVZr solid solution phase is non-ideal, and that chemical bonding plays an important part in the structure formation, and presumably also in the properties. Our conclusions should be transferable to other multi-component alloy systems, as well as some other multi-component material systems, and open up interesting possibilities for the design of material properties via the electronic structure and controlled charge transfer between selected metallic elements in the materials. ; Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2018-04834, 2019-05403, 2018-05973, 2019-05487]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Faculty Grant SFO at LiU [2009 00971]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; RFBRRussian Foundation for Basic Research (RFBR) [20-02-00178]; Swedish e-Science Research Centre (SeRC)
In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. A range of inhomogeneities are observed by diffraction and microscopy techniques. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. Density functional theory calculations-considering the spatial and electronic disturbance induced by the favourable nickel vacancy-reveal a nanoscale distortion comparable to STM and TEM observations. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable. ; Funding Agencies|Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; RFBRRussian Foundation for Basic Research (RFBR) [19-29-03021]; Research Facility Center at the ISSP of RAS; Erasmus Plus mobility Grants [2016-1-IE02-KA107-000479, 2017-1-IE02-KA107-000538 2018-1-IE02-KA107-000589]; Ministry of Science and Higher Education of the Russian Federation [K2-2019-001, 211]; Swedish Research Council (VR)Swedish Research Council [2019-05600]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Irish Research Council Laureate AwardIrish Research Council for Science, Engineering and Technology [IRCLA/2019/171]
This thesis addresses theoretical studies of the coupling between electronic, vibrational, configurational and structural degrees of freedom in metal borides. The effect of external conditions of temperature, pressure and composition on the interplay between internal degrees of freedom is investigated. The importance of excitations and disorder of the above types is well-established and known to dictate key materials science concepts such as phase stability, mechanical and electronic properties. Their mutual coupling composes the next level in complexity in understanding what parameters are to be necessarily included in the theoretical modelling of the system. The main tool used for making such predictions herein is density functional theory. It allows us to capture said excitations and disorder, and give accurate results with reasonable computational efficiency. Metal borides are chosen because of technologically interesting combination of both ceramic and metallic properties, like high hardness, melting point, fracture toughness and electrical conductivity, as well as previous reports of interesting fundamental physical phenomena, like the conventional superconductivity of MgB2 and the apparent off-stoichiometry of AlB2. The theoretical approach is chosen because of its ability to controllably couple and decouple different degrees of freedom to study their combined or isolated effect on the desired materials property. The level of theoretical modeling can be adjusted to fit what is reasonable in terms of efficiency, and still well be used for predictions with quantitative or semi-quantitative accuracy. The configurational aspect of phase stability of binary boron compounds has been believed to be trivial to understand as they most often can be constructed by stacking alternating layers of metal and boron atoms. However, a closer inspection of AlB2, the very name-giver of one of the most usual crystal structures within metal borides, shows a surprising existence of ambiguity regarding both its stable composition and the configuration of metal-site vacancies. Here theoretical approaches are used to reveal the configurational thermodynamics of these vacancies, the origin of their stability and coupling to the electronic structure of the compound. Furthermore, we answer the question why ideal stoichiometric AlB2 is unfavourable, why the vacancy stability window is so narrow, and how different arrangements of vacancies couple with the vibrational degree of freedom, including thermal expansion. If a second metal species is introduced the configurational problem becomes more complex. The arrangement of atoms on the metal sublattice is dependent on the bonding chemistry between the metal atoms and temperature driven thermodynamic effects, like entropy and lattice dynamics. The presented work on Ti1-xAlxB2 is an example of this, where mixing and clustering thermodynamics is considered to further investigate the potential for age-hardening of this ternary diboride. In it, the effect of lattice dynamics and configurational clustering on phase stability is discussed. The discovery of MgB2 being a superconductor in 2001 sparked many fruitful experimental and theoretical studies on the topic. It is generally agreed that MgB2 is a two-gap superconductor which originate in the σ (2D character boron px and py) and π (3D character boron pz) bands, respectively. Superconductivity in itself arises mainly from strong coupling between the E2g phonon mode, corresponding to in-plane bond-stretching vibrations of boron atoms, and electrons on the sigma bands. In the work presented, we use the coupling to the global structural parameters and external pressure to compare different theoretical models, with and without explicit treatment of electron-phonon coupling, and their ability to predict the superconducting transition temperature Tc of distorted MgB2. Moreover, a way to experimentally realize such lattice distorted MgB2 through clever nanostructure design is theoretically explored. Epitaxially growing an alternating out-of-plane ordered Mg/M diboride, with M atoms that naturally have clustering tendencies with respect to Mg, is proposed to being able to provide the necessary lattice distortions of both a- and c-parameters that can lead to an increase in Tc. All of the beforementioned covered compounds (TiB2, AlB2, Ti1-xAlxB2 and MgB2) crystallize in the AlB2-type structure with alternating hexagonal and metal 2D layers, that is most common in the diboride family. However, metal borides can also crystallize with puckered boron layers, while preserving the flat hexagonal metal layers, as in the case for ReB2. It is known for its exceptionally high Vickers hardness that varies from 30.1 ± 1.30 to 48.0 ± 5.6 GPa depending on indentation load. While boron-rich transition metal borides often are considered as potential candidates for hard and incompressible materials, rhenium borides with boron content higher than ReB2 have not been experimentally realized. Theoretical work has proposed that ReB4 adopts the same crystal structure as superhard WB4. In the presented papers, we report the successful synthesis of two novel ReB3 and ReB4 phases at high pressure that remain stable when decompressed down to ambient conditions. First-principles calculations are employed to characterize the electronic, dynamic and thermodynamic stability properties of these phases. Furthermore, novel complex modular Re2B5 and Re3B7 structures are synthesized and characterized by hexagonal boron networks interconnected by short B2 dumbbells. The aim of the in-depth investigations contained in this thesis, using state-of-the-art simulation techniques in collaborations with experimental work, is to further the understanding of how the coupling between electrons, vibrations, atomic configuration, disorder and external conditions influences the properties of materials and to share the results with the scientific community. ; Funding agencies: Knut and AliceWallenberg (KAW) Foundation, through Project grant number KAW 2015.0043,the Swedish Research Council (VR) through International Career Grant No. 2014-6336 and Grant No. 2019-05403, from Marie Sklodowska Curie Actions, Cofund,Project INCA 600398, and from the Knut and Alice Wallenberg Foundation (WallenbergScholar Grant No. KAW-2018.0194), as well as support from the SwedishFoundation for Strategic Research through the Future Research Leaders 6 program,FFL 15-0290. Support from the Swedish Government Strategic ResearchArea in Materials Science on Functional Materials at Linköping University (FacultyGrant SFOMatLiU No. 2009 00971)
