CH3NH3+ Dynamics in CH3NH3PbBr3 by Combining Solid-State NMR and Molecular Dynamics Calculations

International audienceThe dynamical behavior of the organic cations in hybrid halide perovskites has triggered a wealth of experimental and theoretical investigations in the recent years. The intrinsic dynamical and electrical (dipolar) properties of the organic part were suggested to be responsible of some of the most noticeable features of these materials. Previous investigations [Phys. Chem. Chem. Phys. 2016, 18, 27133] have shown that static theoretical calculations of quadrupolar parameters (2H) in the orthorhombic phase of deuterated-MAPbBr3 (MA = CH3NH3+), lead to an overestimation of the linewidth broadening. This was rationalized in terms of thermally activated internal rotational dynamics of the molecular cations. Impact of the dynamics is further inspected in the present paper by a joint experimental and theoretical effort with low temperature solid-state NMR measurements (< 25K). Starting from extensive Ab initio molecular dynamics trajectories performed on large (4x4x4) supercells for both the tetragonal and orthorhombic phases [J. Phys. Chem. C 2017, 121, 20729] we use a specifically designed procedure to account for the effect of the thermally activated cation dynamics in the calculated NMR parameters, which includes the quadrupolar lineshapes of the NMR spectra

Computing of 93Nb NMR Parameters of Solid-State Niobates. The Geometry Matters

International audienceThis work aims at studying the influence of structural parameters on the computations of 93 Nb quadrupolar interaction and chemical shift parameters in various niobates using first-principles approaches. We demonstrate that some of the computed NMR parameters, especially the isotropic chemical shift and the quadrupolar coupling constant, may differ either the X-ray crystal structure or a relaxed structure are used for the calculation of the spectroscopic properties

Structure and Dynamics of a Tsai‐type quasicrystal and approximant from 45Sc and 67Zn NMR studies

International audienceNMR is a recognized complementary method to X-rays/Neutron diffusion for structure and dynamics understanding, dueto different space-time correlation function probe range, and atomic selectivity. Both benefit from Large Scale Facilities (synchrotron,high field NMR, neutron reactors …) allowing unprecedented space-time resolution. For instance, the family of icosahedralquasicrystals of Tsai type [1,2] contains binary stable i-QC with low chemical disorder that offer the opportunity of precise structuralanalysis [3] to reconstruct the atomic structure from a 6-dimensional superspace projection, using 1/1 and 2/1 approximant periodicstructures as starting models. However, the complexity of superspace crystallography requires assumptions and approximations inorder to answer to the question "where are the atoms". Thus, major questions about local structure, dynamics and possible phasetransition in the quasicrystal are still to be answered. To the best of our knowledge, no NMR studies have been performed on suchiQc.In this presentation, solid-state NMR experiments of 45Sc (I=7/2, 100%) and 67Zn (I=5/2, 4.1%) nuclei at different fields andtemperatures, in the 1/1 approximant Zn6Sc and quasicrystal ZnScAg are reported and discussed in the light of structural studies. Bothnuclei carry complementary information to probe intershell coupling and tetrahedron induced distorsions. Indeed, 12 scandium atomsconstitute the second shell of a Tsai cluster, composed of a tetrahedron (4Zn), dodecahedron (20Zn), Sc icosahedron and larger shellsof zinc( Fig.1). Another type of building brick is the double Friauf polyhedron, that exists only in the 2/1 approximant and iQc. Wewill discuss how our NMR results provide insight in the structure (double Friauf and icosahedral sites) and dynamics (frozen state inthe iQc), from comparison between the 1/1 approximant and iQc.References[1] A. P. Tsai, J. Q. Guo, E. Abe, H. Takakura, T. J. Sato, Nature, 2000, 408, 537[2] A.P. Tsai, Chem. Soc. Rev., 2013, 42, 5352.[3] H. Takakura, C. Pay Gomez, A. Yamamoto, M. de Boissieu, A. P. Tsai, Nat. Mater.,2007, 6,58Acknowledgements: We acknowledge IRRMN FR3050 CNRS for support of High Field NMR experiments in Lille. This researchreceived FEDER financial support (FEDER 34722-Prin2Tan) for funding NMR spectrometers in Rennes

Contribution de la RMN haute résolution Proton à l’étude de l’ordre local dans des pérovskites hybrides

National audienceLes cellules solaires à base de pérovskites hybrides apparaissent depuis quelques années comme des candidats de choix pour concurrencer les cellules conventionnelles à base de silicium cristallin1. En effet, les rendements photovoltaïques obtenus avec les pérovskites halogénées dépassent aujourd’hui 22 % et leur stabilité temporelle sous irradiation lumineuse ne cesse de s’améliorer2. Ces performances s’obtiennent grâce à l’emploi de solutions solides de plus en plus complexes. Dans le cas particulier des pérovskites à réseau tri-dimensionnel formule générique APbX3, A est un petit cation organique (Methylammonium=MA, Formamidinium=FA,...) ou inorganique (Cs, Rb,...) et X un halogène (Cl, Br ou I).2 Dans ces solutions solides, l’influence des substitutions de A et de X sur la structure et la dynamique est peu appréhendée et la RMN peut s’avérer être un outil de choix.Récemment nous avons montré le fort potentiel de la RMN pour ces matériaux et notamment celle du plomb-207 pour sonder l’effet de la substitution de l’halogène dans MAPbX3 (X=Cl, Br, I)3. Cet exposé vise à discuter des performances de la RMN pour analyser les implications de la substitution du cation organique MA. Nous montrerons quà température ambiante, la RMN du plomb-207 présente peu d’intérêt. Elle est peu sensible à la substitution du cation A et présente des temps de relaxations transversales courts qui empêchent d’envisager des expériences de corrélations. En revanche, le proton apparait comme un candidat de choix pour l’étude de l’ordre local. Les temps de relaxation sont favorables et permettent l’utilisation du couplage dipolaire pour sonder les proximités spatiales (RFDR, BABA)

Coordination Polymers Based on Heterohexanuclear Rare Earth Complexes: Toward Independent Luminescence Brightness and Color Tuning

Reactions in solvothermal conditions between hexanuclear rare earth complexes and H₂bdc, where H₂bdc symbolizes terephthalic acid, lead to a family of monodimensional coordination polymers in which hexanuclear complexes act as metallic nodes. The hexanuclear cores can be either homometallic with general chemical formula [Ln₆O­(OH)₈(NO₃)₆]²⁺ (Ln = Pr–Lu plus Y) or heterometallic with general chemical formula [Ln_6<i>x</i>Ln′_{6–6<i>x</i>}O­(OH)₈(NO₃)₆]²⁺ (Ln and Ln′ = Pr–Lu plus Y). Whatever the hexanuclear entity is, the resulting coordination polymer is iso-structural to [Y₆O­(OH)₈(NO₃)₂(bdc)­(Hbdc)₂·2NO₃·H₂bdc]_∞, a coordination polymer that we have previously reported. The random distribution of the lanthanide ions over the six metallic sites of the hexanuclear entities is demonstrated by ⁸⁹Y solid state NMR, X-ray diffraction (XRD), and luminescent measurements. The luminescent and colorimetric properties of selected compounds that belong to this family have been studied. These studies demonstrate that some of these compounds exhibit very promising optical properties and that there are two ways of modulating the luminescent properties: (i) playing with the composition of the heterohexanuclear entities or (ii) playing with the relative ratio between two different hexanuclear entities. This enables the independent tuning of luminescence intensity and color

Experimental and Theoretical Insights into the Structure of Tellurium Chloride Glasses

The structure of the binary chalcohalide glasses Te_1–<i>x</i>Cl_<i>x</i> (0.35 ≤ <i>x</i> ≤ 0.65) is considered by combining experimental and theoretical results. The structural network properties are influenced by a competition between ionic and covalent bonding in such glasses. At first, a focus is placed on the detailed information available by using the complementary high-energy X-ray and the neutron diffractions in both the reciprocal and real spaces. The main characteristic suggested by the structure factors <i>S</i>(<i>Q</i>) concerns the presence of three length scales in the intermediate range order. The total correlation function <i>T</i>(<i>r</i>) lets us also suppose that the structure of these glasses is more complicated than Te-chain fragments with terminal Cl as demonstrated in crystalline Te₃Cl₂. Molecular dynamics simulations were subsequently performed on Te₃Cl₂ and Te₂Cl₃, and coupled with the experimental data, a highly reticulated network of chalcogen atoms, with a fair amount of chlorine atoms bonded in a bridging mode, is proposed. The simulations clearly lead to a glass description that differs markedly from the simple structural model based on only Te atom chains and terminal Cl atoms. Solid-state NMR experiments and NMR parameters calculations allowed validation of the presence of Te highly coordinated with chlorine in these glasses

Characterization and Luminescence Properties of Lanthanide-Based Polynuclear Complexes Nanoaggregates

For the first time, hexanuclear complexes with general chemical formula [Ln₆O­(OH)₈­(NO₃)₆(H₂O)_<i>n</i>]²⁺ with <i>n</i> = 12 for Ln = Sm–Lu and Y and <i>n</i> = 14 for Ln = Pr and Nd were stabilized as nanoaggregates in ethylene glycol (EG). These unprecedented nanoaggregates were structurally characterized by ⁸⁹Y and ¹H NMR spectroscopy, UV–vis absorption and luminescence spectroscopies, electrospray ionization mass spectrometry, diffusion ordered spectroscopy, transmission electron microscopy, and dynamic light scattering. These nanoaggregates present a 200 nm mean solvodynamic diameter. In these nanoaggregates, hexanuclear complexes are isolated and solvated by EG molecules. The replacement of ethylene glycol by 2-hydroxybenzyl alcohol provides new nanoaggregates that present an antenna effect toward lanthanide ions. This results in a significant enhancement of the luminescence properties of the aggregates and demonstrates the suitability of the strategy for obtaining highly tunable luminescent solutions

Evaluation of ⁹⁵Mo Nuclear Shielding and Chemical Shift of [Mo₆X₁₄]^2– Clusters in the Liquid Phase

[Mo₆X₁₄]^2– octahedral molybdenum clusters are the main building blocks of a large range of materials. Although ⁹⁵Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the ⁹⁵Mo chemical shift of three clusters: [Mo₆Cl₁₄]^2–, [Mo₆Br₁₄]^2–, and [Mo₆I₁₄]^2–. In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the ⁹⁵Mo nuclear magnetic shielding of the experimental reference, namely, MoO₄^2– in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the ⁹⁵Mo nuclear shieldings. The results demonstrate that, despite the difficulty to obtain accurate ⁹⁵Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ∼50 ppm the agreement between theory and experiment. Finally, the standard deviation of ∼70 ppm that we calculate for the ⁹⁵Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the ⁹⁵Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements