Assessing the Surface Chemistry of 2D Transition Metal Carbides (MXenes): A Combined Experimental/Theoretical 13C Solid State NMR Approach.

The surface functionalization of 2D transition metal carbides or nitrides, so-called MXenes, is one of the fundamental levers allowing to deeply modify their physicochemical properties. Beyond new approaches to control this pivotal parameter, the ability to unambiguously assess their surface chemistry is thus key to expand the application fields of this large class of 2D materials. Using a combination of experiments and state of the art density functional theory calculations, we show that the NMR signal of the carbon─the element common to all MXene carbides and corresponding MAX phase precursors─is extremely sensitive to the MXene functionalization, although carbon atoms are not directly bonded to the surface groups. The simulations include the orbital part to the NMR shielding and the contribution from the Knight shift, which is crucial to achieve good correlation with the experimental data, as demonstrated on a set of reference MXene precursors. Starting with the Ti3C2Tx MXene benchmark system, we confirm the high sensitivity of the 13C NMR shift to the exfoliation process. Developing a theoretical protocol to straightforwardly simulate different surface chemistries, we show that the 13C NMR shift variations can be quantitatively related to different surface compositions and number of surface chemistry variants induced by the different etching agents. In addition, we propose that the etching agent affects not only the nature of the surface groups but also their spatial distribution. The direct correlation between surface chemistry and 13C NMR shift is further confirmed on the V2CTx, Mo2CTx, and Nb2CTx MXenes.

Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration.

We report room-temperature synthesis of lead- and iodide-deficient α-[HC(NH2)2]PbI3 perovskites (abbreviated d-α-FAPI, FA+ = formamidinium), with the general formula (A',FA)1+ x[Pb1- xI3- x] (with A' = hydroxyethylammonium (HEA+) or thioethylammonium (TEA+) cations, 0.04 ≤ x ≤ 0.15). These materials retain a 3D character of their perovskite network despite incorporation of large HEA+ or TEA+ cations, demonstrating that the Goldschmidt tolerance factor can be bypassed. We found that thin films of (TEA,FA)1+ x[Pb1- xI3- x] ( x = 0.04 and 0.13) show exceptional α-phase stability under ambient conditions, 1 order of magnitude higher than α-FAPI and α-(Cs,FA)PbI3 thin films. d-α-FAPI phases are shown to maintain a direct band gap, which increases monotonously for x ranging from 0 up to 0.20, with characteristics of a p-type semiconductor for low concentrations of vacancies ( x ≤ 0.13) and n-type for larger ones. They offer alternatives to reach the methylammonium- and bromine-free stable α-FAPI-type phase and open new avenues in the field of perovskite solar cells, up to band gap tuning desirable for tandem solar cells.

Preparation-Dependent Composition and O/F Ordering in NbO2F and TaO2F.

Through an analysis combining powder XRD, TGA, and 19F and 1H solid-state NMR, it is confirmed for NbO2F and shown for TaO2F that both contain hydroxyl defects and metal vacancies when prepared by aqueous solution synthesis. The formulations M1-x□xO2-5x(OH,F)1+5x of both the samples are determined. The effects of the usually applied thermal treatments are examined. Obtaining pure NbO2F and TaO2F from these samples, that is, fully removing metal vacancies and hydroxide, while avoiding the formation of M2O5, is not that easy. Since thermal treatments result in dehydroxylation and defluorination, it requires, at least, a larger amount of fluorine than metal initially, which may not be the case. We also confirm that the solid-state synthesis is an efficient method to avoid metal vacancies and hydroxyl defects in NbO2F and then apply it to the synthesis of TaO2F. The local structure of NbO2F and TaO2F is poorly described by an ideal cubic ReO3-type model with O and F randomly distributed over the available anion sites. Since O/F ordering was previously highlighted, NbO2F and TaO2F cubic 3 × 3 × 3 supercells featuring -M-O-M-O-M-F- chains along ⟨100⟩ have been built and geometry optimized. These optimized supercells lead to more realistic structures than the previously proposed models, that is, really disordered structures with angularly and radially distorted MX6 octahedra as expected in disordered compounds. Moreover, the structural modeling of NbO2F and TaO2F by these geometry-optimized supercells is supported by the computed 19F and 93Nb NMR parameters, which give very good agreement with the experimental ones.

A hybrid method using the widely-used WIEN2k and VASP codes to calculate the complete set of XAS/EELS edges in a hundred-atoms system.

Most of the recent developments in EELS modelling has been focused on getting a better agreement with measurements. Less work however has been dedicated to bringing EELS calculations to larger structures that can more realistically describe actual systems. The purpose of this paper is to present a hybrid approach well adapted to calculating the whole set of localised EELS core-loss edges (at the XAS level of theory) on larger systems using only standard tools, namely the WIEN2k and VASP codes. We illustrate the usefulness of this method by applying it to a set of amorphous silicon structures in order to explain the flattening of the silicon L2,3 EELS edge peak at the onset. We show that the peak flattening is actually caused by the collective contribution of each of the atoms to the average spectrum, as opposed to a flattening occurring on each individual spectrum. This method allowed us to reduce the execution time by a factor of 3 compared to a usual-carefully optimised-WIEN2k calculation. It provided even greater speed-ups on more complex systems (interfaces, ∼300 atoms) that will be presented in a future paper. This method is suited to calculate all the localized edges of all the atoms of a structure in a single calculation for light atoms as long as the core-hole effects can be neglected.

Sparse Cyclic Excitations Explain the Low Ionic Conductivity of Stoichiometric Li_{7}La_{3}Zr_{2}O_{12}.

We have performed long time scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion electrolyte Li_{7}La_{3}Zr_{2}O_{12} (LLZO), using a first-principles parametrized interatomic potential. Collective lithium transport was analyzed by identifying dynamical excitations: persistent ion displacements over distances comparable to the separation between lithium sites, and stringlike clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO (cubic) are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO (tetragonal) are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.

Elucidation of the Na(2/3)FePO4 and Li(2/3)FePO4 intermediate superstructure revealing a pseudouniform ordering in 2D.

Based on TEM, synchrotron X-ray diffraction, DFT calculations, and Mössbauer spectroscopy, a unified understanding of the Na and Li intercalation process in FePO4 is proposed. The key to this lies in solving the highly sought-after intermediate A(2/3)FePO4 (A = Na, Li) superstructures that are characterized by alkali ions as well as Fe(II)/Fe(III) charge orderings in a monoclinic three-fold supercell. Formation energies and electrochemical potential calculations confirm that Na(2/3)FePO4 and Li(2/3)FePO4 are stable and metastable, respectively, and that they yield insertion potentials in fair agreement with experimental values. The 2/3 Na(Li) and 1/3 vacancy sublattice of the intermediate phases forms a dense (101)(Pnma) plane in which the atom/vacancy ordering is very similar to that predicted for the most uniform distribution of 1/3 of vacancies in a 2D square lattice. Structural analysis strongly suggests that the key role of this dense plane is to constrain the intercalation in the diffusion channels to operate by cooperative filling of (bc)(Pnma). From a practical point of view, this generalized mechanism highlights the fact that an interesting strategy for obtaining high-rate FePO4 materials would consist in designing grains with an enhanced (101) surface area, thereby offering potential for substantial improvements with respect to the performance of rechargeable Li and Na batteries.

NMR parameters in column 13 metal fluoride compounds (AlF3, GaF3, InF3 and TlF) from first principle calculations.

The relationship between the experimental (19)F isotropic chemical shift and the (19)F isotropic shielding calculated using the gauge including projector augmented-wave (GIPAW) method with PBE functional is investigated in the case of GaF3, InF3, TlF and several AlF3 polymorphs. It is shown that the linear correlation between experimental and DFT-PBE calculated values previously established on alkali, alkaline earth and rare earth of column 3 basic fluorides (Sadoc et al., Phys. Chem. Chem. Phys. 13 (2011) 18539-18550) remains valid in the case of column 13 metal fluorides, indicating that it allows predicting (19)F solid state NMR spectra of a broad range of crystalline fluorides with a relatively good accuracy. For the isostructural α-AlF3, GaF3 and InF3 phases, PBE-DFT geometry optimization leads to noticeably overbended M-F-M bond angles and underestimated (27)Al, (71)Ga and (115)In calculated quadrupolar coupling constants. For the studied compounds, whose structures are built of corner shared MF6 octahedra, it is shown that the electric field gradient (EFG) tensor at the cationic sites is not related to distortions of the octahedral units, in contrast to what previously observed for isolated AlF6 octahedra in fluoroaluminates.

Synthesis and structure resolution of RbLaF4.

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.

NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations.

(19)F isotropic chemical shifts for alkali, alkaline earth and rare earth of column 3 basic fluorides are measured and the corresponding isotropic chemical shieldings are calculated using the GIPAW method. When using the PBE exchange-correlation functional for the treatment of the cationic localized empty orbitals of Ca(2+), Sc(3+) (3d) and La(3+) (4f), a correction is needed to accurately calculate (19)F chemical shieldings. We show that the correlation between experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows us to predict (19)F NMR spectra of crystalline compounds with a relatively good accuracy. In addition, we experimentally determine the quadrupolar parameters of (25)Mg in MgF(2) and calculate the electric field gradients of (25)Mg in MgF(2) and (139)La in LaF(3) using both PAW and LAPW methods. The orientation of the EFG components in the crystallographic frame, provided by DFT calculations, is analysed in terms of electron densities. It is shown that consideration of the quadrupolar charge deformation is essential for the analysis of slightly distorted environments or highly irregular polyhedra.

DFT-NMR Investigation and (51)V 3QMAS experiments for probing surface oh ligands and the hydrogen-bond network in a polyoxovanadate cluster: the case of Cs(4)[H(2)V(10)O(28)].4H(2)O.

This work shows that the combination of first-principles calculations and (51)V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs(4)[H(2)V(10)O(28)].4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs(4)[H(2)V(10)O(28)].4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature (1)H MAS NMR spectra. First-principles calculations of (51)V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five (51)V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the (51)V quadrupolar parameters. These results are fully consistent with (51)V 3QMAS NMR spectra recorded with and without (1)H decoupling. Finally, correlations are established between local octahedral VO(6) deformations and (51)V NMR parameters (C(q) and Deltadelta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

Alpha- to beta-(dmes)BiI(5) (dmes = dimethyl(2-ethylammonium)sulfonium dication): umbrella reversal of sulfonium in the solid state and short I...I interchain contacts--crystal structures, optical properties, and theoretical investigations of 1D iodobismuthates.

Syntheses, X-ray structural characterization, optical properties, and electronic structures of 1D metal(III) iodide hybrids, namely, alpha-((CH(3))(2)S(CH(2))(2)NH(3))BiI(5) (1a), beta-((CH(3))(2)S(CH(2))(2)NH(3))BiI(5) (1b), ((CH(3))(2)S(CH(2))(2)NH(3))SbI(5) (2), and (HO(2)C(C(6)H(4))CH(2)NH(3))BiI(4) (3), are reported. According to the results of single-crystal X-ray diffraction analyses, the 1D inorganic chains are constructed by corner-shared M(III)I(6) octahedra in 1a, 1b, and 2 and by edge-shared ones in 3. In polymorphs 1a, 1b, and 2, the polymeric BiI(5)(2-) anionic chains are charge-balanced by the dimethyl(2-ethylammonium)sulfonium (dmes) dications. Complex 1a crystallizes in the polar space group of P2(1)cn. A spectacular umbrella reversal of half sulfonium parts together with the conformational change of half polymeric anions in the crystal structure of 1a occurs at moderate temperature (73 degrees C), leading to the beta-phase 1b, through a reversible single-crystal-to-single-crystal process. Complex 1b, as well as the isotype structure of 2, crystallize in the nonpolar acentric space group of P2(1)2(1)2(1). Because of their acentric structural characteristic, second harmonic generation (SHG) optical properties are observed in the polycrystalline powder samples of 1a, 1b, and 2. It is notable that the SHG signal of 1a is much stronger than that of 1b and 2 owing to the polarity of 1a. Remarkably, the peculiar dissymmetrical dication of dmes is able to modify the bonding features of the inorganic frameworks through shortening I...I distances between adjacent chains (d(I...I) < 4A). The structure of 3, which crystallizes in the triclinic space group P1, features a polymeric anionic chain constructed from edge-shared BiI(6) octahedra. The charge is balanced by the pairs of trans-4-(ammoniummethyl)-cyclohexane-carboxylic acid, which are linked together via the H bonding between the carboxylic groups to form a pseudodication. The results of DFT calculations based on the structures of 1a and 3 indicate that the narrower band gap in 1 appears to be associated on the one hand with a sigma* I-p/Bi-s interaction that moves the Fermi level to higher energy and on the other hand with the interchain I...I contacts.