Accurate structure models and absolute configuration determination using dynamical effects in continuous-rotation 3D electron diffraction data

Continuous-rotation 3D electron diffraction methods are increasingly popular for the structure analysis of very small organic molecular crystals and crystalline inorganic materials. Dynamical diffraction effects cause non-linear deviations from kinematical intensities that present issues in structure analysis. Here, a method for structure analysis of continuous-rotation 3D electron diffraction data is presented that takes multiple scattering effects into account. Dynamical and kinematical refinements of 12 compounds—ranging from small organic compounds to metal–organic frameworks to inorganic materials—are compared, for which the new approach yields significantly improved models in terms of accuracy and reliability with up to fourfold reduction of the noise level in difference Fourier maps. The intrinsic sensitivity of dynamical diffraction to the absolute structure is also used to assign the handedness of 58 crystals of 9 different chiral compounds, showing that 3D electron diffraction is a reliable tool for the routine determination of absolute structures. [Figure not available: see fulltext.]

Band structure of CuMnAs probed by optical and photoemission spectroscopy

5 pages, 5 figures + Supplementary InformationTetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-orbit torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys.Rev.B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell.We acknowledge support from National Grid Infrastructure MetaCentrum provided under the programme “Projects of Large Research, Development, and Innovations Infrastructures” (CESNET LM2015042); Grant Agency of the Czech Republic under Grant No. 15-13436S; CEDAMNF (CZ.02.1.01/0.0/0.0/15_003/0000358) of the Czech ministry of education (MŠMT) as well as its LM2015087 and LNSMLNSpin grants; Cariplo Foundation, Grant No. 2013-0726 (MAGISTER); Spanish MINECO under MAT2015-67593-P project and the ‘Severo Ochoa’ Programme (SEV-2015-0496); EU FET Open RIA Grant No. 766566; Engineering and Physical Sciences Research Council Grant No. EP/P019749/1. P.W. acknowledges support from the Royal Society through a University Research Fellowship.Peer reviewe

Application de la précession des électrons en mode tomographie à l'étude de phases apériodiques et de films minces d'oxydes

The advancement in electron crystallography techniques allows today to face new challenges in terms of structural analyzes. Particularly, this work uses the precession electron diffraction tomography method (PEDT) which is a procedure similar to that of the rotating crystal X-ray diffraction and consistent with crystals of a few tens of nanometers. The first part concerns the study of structural and dielectric properties (ferroelectric relaxor type) of a new family of modulated incommensurate phases derived to Aurivillius phases. A structural model could be established by PEDT and confirmed by neutron powder diffraction to the series of ABi7Nb5O24 compounds (A = Ba, Sr, Ca, Pb). The crystallochemical analysis of these new materials was then used to extend the family to other compositions in both BiO1,5-NbO2,5-BaO and BiO1,5-NbO2,5-WO3 systems. If the first part attests the power of PEDT data analysis to solve complex structures within the kinematic approximation, reliable refinement of the resulting model is problematic. In the second part of this thesis, a new refinement procedure from PEDT data involving the dynamic theory of diffraction is presented. First thoroughly testing on CaTiO3 and PrVO3 perovskite compounds (powder), this approach has proved particularly promising. The challenge was then to use the new approach in order to describe the slight structural variations of these compounds deposited as epitaxial thin films. Keywords: Transmission electron microscopy, Precession, Tomography, Oxides, Thin films, CrystallographyL’avancement des techniques de cristallographie aux électrons permet aujourd’hui de relever de nouveaux défis en termes d’analyses structurales. Ce travail de thèse utilise en particulier la diffraction par précession des électrons en mode tomographie (PEDT) qui est une procédure s’apparentant à celle du cristal tournant en diffraction des rayons X mais applicable à des cristaux de quelques dizaines de nanomètres. La première partie concerne l’étude des caractéristiques structurales et des propriétés diélectriques (ferroélectriques de type relaxeurs) d’une nouvelle famille constituée de phases modulées incommensurables dérivées des phases d’Aurivillius. Un modèle structural a pu être établi par PEDT et confirmé par diffraction de neutrons sur poudre pour la série de composés ABi7Nb5O24 (A=Ba, Sr, Ca, Pb). L’analyse cristallochimique de ces nouveaux matériaux a ensuite permis d’étendre la famille à d’autres compositions dans les systèmes BiO1,5NbO2,5-BaO et BiO1,5-NbO2,5-WO3. Si cette première partie démontre que l’analyse des données PEDT permet de résoudre des structures complexes dans le cadre de l’approximation cinématique, l’affinement de façon fiable du modèle obtenu reste problématique. Dans la seconde partie de cette thèse, une nouvelle procédure d’affinement des données PEDT faisant intervenir la théorie dynamique de la diffraction est présentée. Tout d’abord testée sur des perovskite CaTiO3 et PrVO3 (poudre), cette approche s’est révélée particulièrement prometteuse. Le challenge a alors été d’utiliser la nouvelle approche pour décrire finement les faibles variations structurales de ces mêmes composés déposés sous forme de films minces épitaxiés. Mots clés : Microscopie électronique en transmission, Précession, Tomographie, Oxydes, Couches minces, Cristallographi

Application de la précession des électrons en mode tomographie à l'étude de phases apériodiques et de films minces d'oxydes

The advancement in electron crystallography techniques allows today to face new challenges in terms of structural analyzes. Particularly, this work uses the precession electron diffraction tomography method (PEDT) which is a procedure similar to that of the rotating crystal X-ray diffraction and consistent with crystals of a few tens of nanometers. The first part concerns the study of structural and dielectric properties (ferroelectric relaxor type) of a new family of modulated incommensurate phases derived to Aurivillius phases. A structural model could be established by PEDT and confirmed by neutron powder diffraction to the series of ABi7Nb5O24 compounds (A = Ba, Sr, Ca, Pb). The crystallochemical analysis of these new materials was then used to extend the family to other compositions in both BiO1,5-NbO2,5-BaO and BiO1,5-NbO2,5-WO3 systems. If the first part attests the power of PEDT data analysis to solve complex structures within the kinematic approximation, reliable refinement of the resulting model is problematic. In the second part of this thesis, a new refinement procedure from PEDT data involving the dynamic theory of diffraction is presented. First thoroughly testing on CaTiO3 and PrVO3 perovskite compounds (powder), this approach has proved particularly promising. The challenge was then to use the new approach in order to describe the slight structural variations of these compounds deposited as epitaxial thin films. Keywords: Transmission electron microscopy, Precession, Tomography, Oxides, Thin films, CrystallographyL’avancement des techniques de cristallographie aux électrons permet aujourd’hui de relever de nouveaux défis en termes d’analyses structurales. Ce travail de thèse utilise en particulier la diffraction par précession des électrons en mode tomographie (PEDT) qui est une procédure s’apparentant à celle du cristal tournant en diffraction des rayons X mais applicable à des cristaux de quelques dizaines de nanomètres. La première partie concerne l’étude des caractéristiques structurales et des propriétés diélectriques (ferroélectriques de type relaxeurs) d’une nouvelle famille constituée de phases modulées incommensurables dérivées des phases d’Aurivillius. Un modèle structural a pu être établi par PEDT et confirmé par diffraction de neutrons sur poudre pour la série de composés ABi7Nb5O24 (A=Ba, Sr, Ca, Pb). L’analyse cristallochimique de ces nouveaux matériaux a ensuite permis d’étendre la famille à d’autres compositions dans les systèmes BiO1,5NbO2,5-BaO et BiO1,5-NbO2,5-WO3. Si cette première partie démontre que l’analyse des données PEDT permet de résoudre des structures complexes dans le cadre de l’approximation cinématique, l’affinement de façon fiable du modèle obtenu reste problématique. Dans la seconde partie de cette thèse, une nouvelle procédure d’affinement des données PEDT faisant intervenir la théorie dynamique de la diffraction est présentée. Tout d’abord testée sur des perovskite CaTiO3 et PrVO3 (poudre), cette approche s’est révélée particulièrement prometteuse. Le challenge a alors été d’utiliser la nouvelle approche pour décrire finement les faibles variations structurales de ces mêmes composés déposés sous forme de films minces épitaxiés. Mots clés : Microscopie électronique en transmission, Précession, Tomographie, Oxydes, Couches minces, Cristallographi

Stairlike Aurivillius Phases in the Pseudobinary Bi 5 Nb 3 O 15 –ABi 2 Nb 2 O 9 (A = Ba and Sr) System: A Comprehensive Analysis Using Superspace Group Formalism

International audienc

Stacking sequence variations in vaterite resolved by precession electron diffraction tomography using a unified superspace model

International audienceAs a metastable phase, vaterite is involved in the first step of crystallization of several carbonate-forming systems including the two stable polymorphs calcite and aragonite. Its complete structural determination would consequently shed important light to understand scaling formation and biomineralization processes. While vaterite’s hexagonal substructure (a0 ~ 4.1 Å and c0 ~ 8.5 Å) and the organization of the carbonate groups within a single layer is known, conflicting interpretations regarding the stacking sequence remain and preclude the complete understanding of the structure. To resolve the ambiguities, we performed precession electron diffraction tomography (PEDT) to collect single crystal data from 100 K to the ambient temperature. The structure was solved ab initio and described over all the temperature range using a unified modulated structure model in the superspace group C12/c1(α0γ)00 with a = a0 = 4.086(3) Å, b = 3−−√a0 = 7.089(9) Å, c = c0 = 8.439(9) Å, α = β = γ = 90° and q = 23a* + γc*. At 100 K the model presents a pure 4-layer stacking sequence with γ = 12 whereas at the ambient temperature, ordered stacking faults are introduced leading to γ < 12. The model was refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement

Stacking sequence variations in vaterite resolved by precession electron diffraction tomography using a unified superspace model

International audienceAs a metastable phase, vaterite is involved in the first step of crystallization of several carbonate-forming systems including the two stable polymorphs calcite and aragonite. Its complete structural determination would consequently shed important light to understand scaling formation and biomineralization processes. While vaterite’s hexagonal substructure (a0 ~ 4.1 Å and c0 ~ 8.5 Å) and the organization of the carbonate groups within a single layer is known, conflicting interpretations regarding the stacking sequence remain and preclude the complete understanding of the structure. To resolve the ambiguities, we performed precession electron diffraction tomography (PEDT) to collect single crystal data from 100 K to the ambient temperature. The structure was solved ab initio and described over all the temperature range using a unified modulated structure model in the superspace group C12/c1(α0γ)00 with a = a0 = 4.086(3) Å, b = 3−−√a0 = 7.089(9) Å, c = c0 = 8.439(9) Å, α = β = γ = 90° and q = 23a* + γc*. At 100 K the model presents a pure 4-layer stacking sequence with γ = 12 whereas at the ambient temperature, ordered stacking faults are introduced leading to γ < 12. The model was refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement

Stairlike Aurivillius Phases in the Pseudobinary Bi₅Nb₃O₁₅–ABi₂Nb₂O₉ (A = Ba and Sr) System: A Comprehensive Analysis Using Superspace Group Formalism

We report the possibility of extending the so-called stairlike Aurivilius phases in the pseudobinary Bi₅Nb₃O₁₅–ABi₂Nb₂O₉ (A = Ba and Sr) over a wide range of compositions. These phases are characterized by a discontinuous stacking of [Bi₂O₂] slabs and perovskite blocks, leading to long-period intergrowths stabilized as a single phase. When analyses from precession electron diffraction tomography and X-ray and neutron powder diffraction are combined, the monoclinic incommensurately modulated structure with <b>q</b> = α<b>a</b>* + γ<b>c</b>* previously proposed for the ABi₇Nb₅O₂₄ composition could be generalized to the Bi₅Nb₃O₁₅–ABi₂Nb₂O₉ (A = Ba and Sr) compounds. Considering the compositions expressed as (A,Bi)_1–<i>x</i>Nb_<i>x</i>O_{3–3<i>x</i>}, the stacking sequence associated with compositions ranging from <i>x</i> = ²/₅ to ³/₈ is governed by the component γ of the modulation vector and can be predicted following a Farey tree hierarchy independently to the A cation. The length of the steps, characteristic of the stairlike nature, is controlled by the α component and depends on the substitution ratio A/Bi and the nature of A (A = Ba and Sr). This study highlights the compositional flexibility of stairlike Aurivillius phases

Exotic Compositional Ordering in Manganese–Nickel–Arsenic (Mn-Ni-As) Intermetallics

International audienceIn this work we benefited from recent advances in tools for crystal-structure analysis that enabled us to describe an exotic nanoscale phenomenon in structural chemistry. The Mn0.60Ni0.40As sample of the Mn1−xNixAs solid solution, exhibits an incommensurate compositional modulation intimately coupled with positional modulations. The average structure is of the simple NiAs type, but in contrast to a normal solid solution, we observe that manganese and nickel segregate periodically at the nano-level into ordered MnAs and NiAs layers with thickness of 2–4 face-shared octahedra. The detailed description was obtained by combination of 3D electron diffraction, scanning transmission electron microscopy, and neutron diffraction. The distribution of the manganese and nickel layers is perfectly described by a modulation vector q=0.360(3) c*. Displacive modulations are observed for all elements as a consequence of the occupational modulation, and as a means to achieve acceptable Ni–As and Mn–As distances. This modulated evolution of magnetic MnAs and non-magnetic NiAs-layers with periodicity at approximately 10 Å level, may provide an avenue for spintronics