Guided by the theoretical prediction, a new MAX phase V.sub.2SnC was synthesized experimentally for the first time by reaction of V, Sn, and C mixtures at 1000 degreesC. The chemical composition and ...crystal structure of this new compound were identified by the cross-check combination of first-principles calculations, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and high resolution scanning transmission electron microscopy (HR-STEM). The stacking sequence of V.sub.2C and Sn layers results in a crystal structure of space group P6.sub.3/mmc. The a- and c-lattice parameters, which were determined by the Rietveld analysis of powder XRD pattern, are 0.2981(0) nm and 1.3470(6) nm, respectively. The atomic positions are V at 4f (1/3, 2/3, 0.0776(5)), Sn at 2d (2/3, 1/3, 1/4), and C at 2a (0, 0, 0). A new set of XRD data of V.sub.2SnC was also obtained. Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy, satisfied Born-Huang criteria of mechanical stability, and positive phonon branches over the Brillouin zone. It also has low shear deformation resistance c44 (second-order elastic constant, c.sub.ij) and shear modulus (G), positive Cauchy pressure, and low Pugh's ratio (G/B = 0.500 < 0.571), which is regarded as a quasi-ductile MAX phase. The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond. Keywords: V.sub.2SnC; new MAX phase compound; crystal structure; first-principles calculations
We report muon spin rotation (mu SR) and neutron diffraction on the rare-earth-based magnets (Mo2/3RE1/3)2AlC, also predicted as parent materials for two-dimensional (2D) derivatives, where the rare ...earth (RE) = Nd, Gd (only mu SR), Tb, Dy, Ho, and Er. By crossing information between the two techniques, we determine the magnetic moment (m), structure, and dynamic properties of all compounds. We find that only for RE = Nd and Gd the moments are frozen on a microsecond timescale. Out of these two, the most promising compound for a potential 2D high m magnet is the Gd variant, since the parent crystals are pristine with m = 6.5 +/- 0.5 mu B, Neel temperature of 29 +/- 1 K, and the magnetic anisotropy between out-of- and in-plane coupling is smaller than 10-8. This result suggests that magnetic ordering in the Gd variant is dominated by in-plane magnetic interactions and should therefore remain stable when exfoliated into 2D sheets.
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 (W
M
)
AC, where M
= Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two-with a monoclinic C2/c structure-are predicted to be stable: (W
Sc
)
AlC and (W
Y
)
AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W
C-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.
Abstract
Two‐dimensional (2D) transition metal carbides and carbonitrides, called MXenes, with metallic conductivity and hydrophilic surfaces, show great promise as electrode materials for ...supercapacitors. A major drawback of 2D nanomaterials is the re‐stacking of the nanosheets, which prevents full utilization of surface area and blocks the access of the electrolyte. In this study, a free‐standing nanocomposite paper electrode is realized by combining Mo
1.33
C MXene and positively charged biopolymer lignin (the second most abundant biopolymer in nature, L‐DEA). The self‐assembled layered architecture with alternating polymer and MXene flakes increases the interlayer space to promote ion transport, and with combining charge storage capability of the lignin derivative and MXene in an interpenetrating MXene/L‐DEA nanocomposite, which offers an impressive capacitance of 503.7 F g
−1
. Moreover, we demonstrate flexible solid‐state asymmetric supercapacitors (ASCs) using Mo
1.33
C@L‐DEA as the negative electrode and electrochemically exfoliated graphene with ruthenium oxide (EG@RuO
x
) as the positive electrode. This asymmetric device operates at a voltage window of 1.35 V, which is about two times wider than that of a symmetric Mo
1.33
C@L‐DEA based supercapacitor. Finally, the ASCs can deliver an energy density of 51.9 Wh kg
−1
at a power density of 338.5 W kg
−1
, with 86 % capacitance retention after 10000 charge‐discharge cycles.
Herein, we report on the growth of single crystals of various (Mo2/3RE1/3)(2)AlC (RE = Nd, Gd, Dy, Ho, Er) i-MAX phases and their Raman characterization. Using first principles, the wave numbers of ...the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing i-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.
Research on low-dimensional materials has increased drastically in the last decade, with the discovery of two-dimensional transition metal carbides and nitrides (MXenes) produced by atom-selective ...chemical etching of laminated parent M(n+1)AX(n) (MAX) phases. Here, we apply density functional theory and subsequent materials synthesis and analysis to explore the phase stability and Mo/Sc intermixing on the M site in the chemically ordered quaternary i-MAX phase (MoxSc1-x)(2)AlC. Transmission electron microscopy confirms the theoretical predictions of preferential in-plane ordering of Mo and Sc, with the highest crystal quality obtained for the ideal Mo:Sc ratio of 2:1 (predicted as the most stable), as well as a retained i-MAX structure even for an increased relative Sc content, with Sc partially occupying Mo sites. The results are supported by refined neutron diffraction data, which show space group C2/c (no. 15), and a C occupancy of 1. Subsequent chemical etching produces MXene for x = 0.66, while for x = 0.33 and 0.5 no MXene is observed. These results demonstrate that a precise control of the i-MAX composition is crucial for derivation of MXene, with a MXene quality optimized for a Mo:Sc ratio of 2:1 with minimal intermixing between Mo and Sc.
Rare-earth-based (RE) nanolaminates have attracted attention recently because of their complicated magnetism and their potential as precursors for strongly correlated two-dimensional materials. In ...this work, we synthesized a class of nanolaminates with a Mo4RE4Al7C3 chemistry, where RE = Ce or Pr. Powder samples of both phases were characterized with respect to structure and composition. Single crystals of Mo4Ce4Al7C3 were used for magnetization measurements. The crystal structure was investigated by means of x-ray diffraction and scanning transmission electron microscopy. Magnetization analysis reveals a ferromagnetic ground state with a Curie temperature of similar to 10.5 K. X-ray absorption near-edge structure provides experimental evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only the Ce atoms with 4f(1) configuration occupying one of the two possible sites are ferromagnetically coupled, with a saturation moment of similar to 1.2 mu(B) per atom. We thus classify Mo4Ce4Al7C3 as a ferromagnetic, mixed-valence compound.
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