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We report the synthesis of three out-of-plane chemically ordered quaternary transition metal borides (o-MAB phases) of the chemical formula M4CrSiB2 (M = Mo, W, Nb). The addition of these phases to the recently discovered o-MAB phase Ti4MoSiB2 shows that this is indeed a new family of chemically ordered atomic laminates. Furthermore, our results expand the attainable chemistry of the traditional M5SiB2 MAB phases to also include Cr. The crystal structure and chemical ordering of the produced materials were investigated using high-resolution scanning transmission electron microscopy and X-ray diffraction by applying Rietveld refinement. Additionally, calculations based on density functional theory were performed to investigate the Cr preference for occupying the minority 4c Wyckoff site, thereby inducing chemical order., Funding Agencies|Knut and Alice Wallenberg (KAW) Foundation for a Fellowship/Scholar Grant; Swedish Foundation for Strategic Research (SSF) [EM16-0004]; Swedish Research Council [2018-05973]; Knut and Alice Wallenberg (KAW) Foundation; Swedish Foundation for Strategic Research (SSF) [EM16-0004]; Swedish Research Council [RIF21-0026]; Goran Gustafsson foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2018-05973]; Swedish National Infrastructure in Advanced Electron Microscopy [200900971, 2021-00171]

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5SiB2 (T2) phases, the finding of a family of laminated quaternary metal borides, M M-4 SiB2, with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4MoSiB2, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4MoSiB2 and select removal of Si and MoB2 sub-layers is validated by derivation of a 2D material, TiOxCly, of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of approximate to 4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications., Funding Agencies|Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research SSFSwedish Foundation for Strategic Research [EM16-0004]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [21805295]; KAW Foundation; Swedish Foundation for Strategic Research (SSF) through the Research Infrastructure Fellow program [RIF 14-0074]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2018-05973]

CrB x thin films with 1.90 < x < 2.08 have been deposited by direct-current magnetron sputtering (DCMS) from a stoichiometric CrB 2 target at 5 and 20 mTorr (0.67 and 2.67 Pa) Ar pressure onto sapphire (0 0 01) substrates. All films, irrespective of deposition conditions, exhibit a (0 0 01) texture. Attesting to the achievement of close-to-stoichiometric composition, epitaxial film growth is observed at 900 ?C, while film growth at 500 ?C yields (0001) fiber texture. Film composition does not depend on substrate temperature but exhibits slightly reduced B content with increasing pressure for samples deposited at 900 ?C. Excess B in the overstoichiometric epitaxial CrB 2.08 films segregates to form B-rich inclusions. Understoichiometry in CrB 1.90 films is accommodated by Cr-rich stacking faults on { 1 1? 00 } prismatic planes. ? 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ), Funding Agencies|Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004, RIF 14-0074]; Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]; KAW

Titanium boride, TiBx thin films are grown in pure Ar discharges by high-power impulse magnetron sputtering (HiPIMS) from a compound TiB2 target Film compositions are determined by time-of-flight elastic recoil detection analysis and Rutherford backscattering spectrometry as a function of deposition temperature (T-s = 25-900 degrees C) and Ar pressure (p(Ar) = 0.67-2.67 Pa, 5-20 mTorr). For reference, films are also grown by direct current magnetron sputtering (dcMS) under similar conditions. The HiPIMS waveform, average target power P-T, and resulting film compositions are strongly dependent not only on P-Ar, but also on T-s. At high pressures the effect of varying T-s on P-T is minimal, while at lower P-Ar the effect of T-s is more pronounced, due to substrate-temperature-induced gas rarefaction. Films grown by HiPIMS at 0.67 Pa are understoichiometric, with B/Ti = 1.4-1.5, while at 2.67 Pa, B/Ti decreases from 2.4 to 1.4 as T-s increases from 25 to 900 degrees C. dcMS-deposited films are overstoichiometric (B/Ti similar or equal to 3) when grown at low pressures, and near-stoichiometric (B/Ti similar or equal to r 1.9-2.2) for higher P-Ar. All experimental results are explained by differences in the ionization potentials of sputtered Ti and B atoms, together with P-Ar- and T-s-dependent gas-phase scattering.

Synthesis of delaminated 2D W1.33C (MXene) has been performed by selectively etching Al as well as Sc/Y from the recently discovered nanolaminated i-MAX phases (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC. Both quaternary phases produce MXenes with similar flake morphology and with a skeletal structure due to formation of ordered vacancies. The measured O, OH, and F terminations, however, differ in amount as well as in relative ratios, depending on parent material, evident from X-ray photoelectron spectroscopy. These findings are correlated to theoretical simulations based on first-principles, investigating the W1.33C, and the effect of termination configurations on structure, formation energy, stability, and electronic structure. The theoretical results indicate a favored F-rich surface composition, though with a system going from insulating/semiconducting to metallic for different termination configurations, suggesting a high tuning potential of these materials. Additionally, free-standing W1.33C films of 2-4 mu m thickness and with up to 10 wt % polymer (PEDOT:PSS) were tested as electrodes in supercapacitors, showing capacitances up to 600 F cm(-3) in 1 M H2SO4 and high capacitance retention for at least 10000 cycles at 10 A g(-1). This is highly promising results compared to other W-based materials to date., Funding Agencies|Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [RIF 14-0074]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swedish Research councilSwedish Research Council [642-2013-8020, 2016-04412]; [KAW 2015.0043]

We report the synthesis and characterization of a new laminated i-MAX phase, (V2/3Sc1/3)(2)AlC, with in-plane chemical ordering between the M-elements. We also present evidence for the solid solution (V2-xScx)(2)AlC, where x amp;lt;= 0.05. Chemical etching of the Al and Sc results in a two-dimensional (2D) MXene counterpart: V2-xC from the latter phase. Furthermore, etching with HF yields single-sheet MXene of flat morphology, while LiF + HCl gives MXene scrolls. We also show a 4x reduction in etching time for (V2-xScx)(2)AlC compared to V2AlC, suggesting that traces of Sc changes the phase stability, and make the material more susceptible to etching. The results show a path for improved control of MXene synthesis and morphology, which may be applicable also for other MAX/MXene systems., Funding Agencies|Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Research Infrastructure Fellow program [RIF 14-0074]; Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Research councilSwedish Research Council [642-2013-8020, 2016-04412]; [KAW 2015.0043]

Introducing point defects in two-dimensional (2D) materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase, wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of solid solution (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases., Funding agencies:We acknowledge support from the Swedish Foundation for Strategic Research through the Synergy Grant FUNCASE and Research Infrastructure Fellowship RIF 14-0074 and from the Knut and Alice Wallenberg (KAW) Foundation for Fellowship Grants, Project funding (KAW 2015.0043), and support toward the Linkoping Electron Microscopy Laboratory. The Swedish Research Council is gratefully acknowledged for Projects 642-2013-8020 and 621-2014-4890. We also acknowledge the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU No. 2009-00971).

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W2/3M1/32)(2)AC, where M-2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only twowith a monoclinic C2/c structureare predicted to be stable: (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W1.33C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications., Funding Agencies|Swedish Research Council [642-2013-8020, 2016-04412]; Knut and Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) through the Synergy Grant FUNCASE; Research Infrastructure Fellow Program [RIF 14-0074]; National Science Foundation through the Catalysis Program in the Division of Chemical, Biological, Environmental and Transport Systems [1602886]

Direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) were used to deposit Ti1-xAlxB2-y metal boride coatings by sputtering from a segmented TiB2-AlB2 target using Ar and Kr as sputtering gas. To probe the influence of chemical composition on the oxidation behavior and mechanical properties five samples were selected. The Al concentration was varied systematically in three samples with a B/(Ti+Al) ratio of 1.3 (y = 0.7) and Al/(Ti+Al) ratios of x Î (0.1, 0.4 and 0.7), while the B concentration was varied for an Al/(Ti+Al) ratio of 0.1 (x = 0.1) with B/(Ti+Al) ratios of y Î (0.1, 0.6 and 0.7). The mechanical properties of Ti1-xAlxB1.3 samples were also investigated, showing a reduction in both hardness and elastic modulus with increasing Al concentration. Correspondingly, the mechanical properties of the Ti0.9Al0.1B2-y samples, with varying B concentration, were measured and showed a superior hardness of 46.2±1.1 GPa and an elastic modulus of 523±7 GPa for Ti0.9Al0.1B1.3. This was higher than the other films, showing a hardness of 43.5±1.0 GPa and 35.8±1.0 GPa, and an elastic modulus of 569±7 GPa and 493±6 GPa, for Ti0.9Al0.1B1.4 and Ti0.9Al0.1B1.9, respectively. Furthermore, oxidation of the Ti0.9Al0.1B2-y samples was investigated through air-annealing at 600 oC for 1 h to 10 h, showing that all films form a protective Al2O3 oxide-scale which significantly reduces the oxidation rate compared to TiBx films, with oxide-scales consisting of porous TiO2. The thinnest oxide-scale after 10 h was found in the B-deficient samples, Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.4, at ~200 nm, which is significantly below that for Ti0.9Al0.1B1.9 at 320 nm., Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFOM at-LiU No 2009 00971)

Transition-metal diboride thin films, which have high melting points, excellent hardness, and good chemical and thermal conductivity, severely suffer from rapid oxidation. Here, we explore the influence of changing B content and nanostructure on the oxidation properties of TiBx thin films, with x = 1.43, 2.20, and 2.70. Results show that all as-deposited layers have columnar structure. The column boundaries of as-deposited TiB2.20 and TiB2.70 films grown by direct current magnetron sputtering (DCMS) are B-rich, while the as-deposited TiB1.43 films grown by high-power impulse magnetron sputtering (HiPIMS) show no apparent grain boundary phases and contain Ti-rich planar defects. The oxidation rate of TiB1.43 air-annealed at 400 oC up to 48 h is significantly lower than that of TiB2.20 and TiB2.70 films. The average oxidation rate of TiB1.43, TiB2.20, and TiB2.70 films was measured at 2.9±1.5, 7.1±1.0, and 20.0±5.0 nm/h, respectively, with no spallation of even as thick oxide-scales as 0.5 µm in any of the films. The improved oxidation resistance can be explained by the absence of B-rich tissue phase at the column boundaries of understoichiometric TiBx films, a phase that interlaces the nanocolumnar TiB2 structures in the corresponding overstoichiometric films, thus eliminating the easy oxidation pathway.

CrB2 thin films with 1.90 ≤ x 2.08 have been synthesized by direct-current magnetron sputtering (DCMS) from a stoichiometric CrB2 target at 5 and 20 mTorr (0.67 and 2.67 Pa) Ar pressure onto sapphire (0001) substrates. All films, irrespective of deposition conditions, exhibit a (0001) texture. Further attesting to the achievement of close-to-stoichiometric composition, epitaxial film growth is observed when increasing the substrate temperature from 500 °C to 900 °C. A columnar microstructure is otherwise present for all films, with smoother surfaces and bulk density, ~5.2 g/cm3, for films grown at the lower pressure. This is explained by more energetic particles impinging on the growth surface due to less gas scattering. The composition in the films show no apparent dependence on substrate temperature, and has a slight dependence on deposition pressure for the samples deposited at 900 °C, with reduced B content for increasing pressure. Excess B in the overstoichiometric CrB2.08 films are accommodated as B-rich inclusions, as opposed to commonly observed B tissue phase in other diboride films like TiB2, while understoichiometric CrB1.90 films displays planar defects comprised of Cr-rich stacking faults., Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO Mat-LiU No 2009 00971)