Pierre Fertey

The modulated structure of Ba0.39Sr0.61Nb2O6. I. Harmonic solution

The structure of a crystal of Sr(0.61)Ba(0.39)Nb(2)O(6) has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(pp1/2,p - p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) A, modulation vectors q(1) = 0.3075 (6) (a* + b*), q(2) = 0.3075 (6) (a* - b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 A, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary.

Charge ordering, symmetry and electronic structure issues and Wigner crystal structure of the quarter-filled band Mott insulators and high pressure metals δ-(EDT-TTF-CONMe2)2X, X = Br and AsF6

We report on the synthesis and application of an internal chemical pressure to effectively control, and reduce, the Mott gap in the system δ-(EDT-TTF-CONMe2)2X, X = Br, AsF6; the detailed accounts of its Pmna, averaged room temperature structure and reversible phase transition at ca. 190 K towards a low temperature P21/a structure; the synthesis of (13C-EDT-TTF-CONMe2)2Br, where one carbon atom of the inner double bond is 100% 13C-enriched and single crystal 13C solid state NMR spectroscopy and relaxation revealing that charge ordering occurs at room temperature and ambient pressure; the discovery of weak superstructure Bragg reflections in δ-(EDT-TTF-CONMe2)2Br and subsequent analysis of the superstructure symmetry and refinement of an exhaustive synchrotron radiation data set; suggesting an alternation at room temperature of neutral and oxidized molecules along both the stacking a and transverse b directions in orthorhombic, non-centrosymmetric space groupP2nn, a CO pattern compatible with ferroelectricity. The charge disproportionation and long range order crystallization of the electron gas onto every other molecular site within a three-dimensional Wigner lattice is coupled to a concerted activation-deactivation of large collections of transverse Csp2–H⋯O hydrogen bonds and an anti-phase, static modulation of the bromide anions displacements along b. Despite the occurrence of charge ordering, the stacks remain essentially uniform, in agreement with the rich low temperature Mott physics of the system.

Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector

A new experimental approach to perform in situ electric field diffraction on single crystals using an on-then-off pump–probe mode in situ (i.e. the field-switching method) with a synchrotron or a laboratory X-ray source is presented. Taking advantage of the fast readout of the XPAD hybrid pixel two-dimensional detector and its programmable functionalities, the operation mode of the detector has been customized to significantly increase the efficiency of the method. The very weak electric field-induced structural response of a piezoelectric crystal can be accurately measured. This allows the piezoelectric tensor to be precisely obtained from Δθ shifts while the structural variations can be modelled using a full set of ΔI/I data. The experimental method and methodology are detailed and tested as a case study on pure piezoelectric compounds belonging to the α-quartz family (SiO2 and GaAsO4 single crystals). Using the two scan modes developed, it is demonstrated that tiny Bragg angle shifts can be measured as well as small field-induced Bragg intensity variations (<1%). The relevance of measurements performed with an X-ray laboratory source is demonstrated: partial data sets collected at synchrotrons can be completed, but more interestingly, a large part of the study can now be realized in the laboratory (medium to strong intensity reflections) in a comparable data collection time.

Investigating the Case of Titanium(IV) Carboxyphenolate Photoactive Coordination Polymers

The reactivity of 2,5-dihydroxyterephthalic acid (H4DOBDC) with titanium(IV) precursors was thoroughly investigated for the synthesis of metal-organic frameworks under solvothermal conditions. Four crystalline phases were isolated whose structures were studied by a combination of single-crystal or powder X-ray diffraction and solid-state NMR. The strong coordination ability of the phenolate moieties was found to favor the formation of isolated TiO6 octahedra bearing solely organic ligands in the resulting structures, unless hydrothermal conditions and precondensed inorganic precursors are used. It is worth noting that these solids strongly absorb visible light, as a consequence of the ligand-to-metal charge transfer (LMCT) arising from Ti-phenolate bonds. Preliminary photocatalytic tests suggest that one compound, namely, MIL-167, presents a higher activity for hydrogen evolution than the titanium carboxylate MIL-125-NH2 but that such an effect cannot be directly correlated with its improved light absorption feature.

Studies of electric field induced structural and electron‐density modifications by X‐ray diffraction

During the last two decades, a number of X-ray diffraction studies on the response of a crystal to an applied electric field have been carried out. In a few cases, the electron-density polarizations could be determined. The analysis of the induced variations of the structural properties on an atomic scale are of prime importance in order to acquire a better understanding of physical properties like the piezoelectric and dielectric properties of crystals. This article reviews the experimental technique used and the modelling methods of the Bragg scattering variations induced by the field. Some noteworthy results are presented that illustrate the possibility of detecting subtle structural changes, for example as small as 0.1 degrees in bond angles arising from applying a strong field, 10-40 kV cm(-1), as well as the pitfalls of such an approach for clarifying the relevance of the structural properties in physical mechanisms.

Crystal structures of α-quartz homeotypes boron phosphate and boron arsenate: structure-property relationships

Abstract The structures of single crystals of the α-quartz homeotypes boron phosphate and boron arsenate prepared under high-pressure, high-temperature conditions in a belt-type apparatus were determined by X-ray diffraction. Both structures are more distorted with respect to the β-quartz structure type than α-quartz itself with average tetrahedral tilt angles δ of 19.7° and 24.6° for BPO4 and BAsO4, respectively. The structure of BAsO4 is one of the most highly distorted found among α-quartz homeotypes. It is shown that for the known α-quartz homeotypes, the density, which can be related to the frequency constant for piezoelectric resonators, scales with the intertetrahedral bridging angle θ.

Anharmonicity and isomorphic phase transition: a multi-temperature X-ray single-crystal and powder diffraction study of 1-(2′-aminophenyl)-2-methyl-4-nitroimidazole

Multi-temperature single-crystal and powder diffraction experiments on 1-(2′-aminophenyl)-2-methyl-4-nitroimidazole show that this crystal undergoes an isomorphic phase transition with the coexistence of two phase domains over a wide temperature range. The anharmonic approach was the only way to model the resulting disorder.

Can a Formally Zwitterionic Rhodium(I) Complex Emulate the Charge Density of a Cationic Rhodium(I) Complex? A Combined Synchrotron X-ray and Theoretical Charge-Density Study

The molecular electron densities of structurally related cationic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indene)Rh(COD)](CF(3)SO(3)), [1c](CF(3)SO(3))) and formally zwitterionic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indenide)Rh(COD)], 1z) complexes were accurately determined using synchrotron bright-source X-ray radiation at 30 K followed by multipolar refinement (COD = η(4)-1,5-cyclooctadiene). The densities were also obtained from density functional theory calculations with a large, locally dense basis set. A 28-electron ([Ar]3d(10)) core of the Rh atom was modeled by an effective core potential to obtain a density that was then augmented with relativistic cores according to the Keith-Frisch approximation. Calculations were performed at the experimental geometry and after vacuum-phase geometry optimization starting from the experimental geometry. Experimental and calculated geometries and electron-density distributions show that the electron density and electronic structure in the region of the Rh center are not significantly altered by protonation of the aromatic ring and that formal removal of CF(3)SO(3)H from [1c](CF(3)SO(3)) affords a complex 1z possessing substantial zwitterionic character (with a charge separation of ca. 0.9 electronic charge) featuring a negatively charged aromatic indenide framework. Further, the molecular electrostatic potentials of 1c and 1z exhibit similar topography around the metal, despite being drastically different in the vicinity of the indene or indenide portion of the cation (1c) and zwitterion (1z), respectively. Collectively, these observations obtained from high-level experimental and theoretical electron-density analysis confirm, for the first time, that appropriately designed zwitterionic complexes can effectively emulate the charge distribution found within ubiquitous cationic platinum-group metal catalyst complexes, in keeping with recent catalytic investigations.

Modulated structure of potassium sodium strontium barium niobates (KNSBN): harmonic solution

The structures of two specimens of the (K(1 - y)Na(y))(2F - 2)(Sr(x)Ba(1 - x))(2 - F)Nb2O6 family (KNSBN) have been solved and refined (x(I) = 0.6, y(I) = 0.45, F(I) = 1.07 and x(II) = 0.72, y(II) = 0.45, F(II) = 1.08). The KNSBN compounds appear to be incommensurately modulated, as described in a five-dimensional superspace approach with the tetragonal P4bm (pp1/2, p - p1/2) superspace group. Their modulation wavevectors are almost independent of the composition. The description of the modulation is restricted to first-order harmonics in order to limit the number of refined parameters. The modulation mainly affects the positions of the O atoms and their displacements depend on the Sr/Ba ratio. The structural models for the two compounds are not only almost identical, but also analogous to the structural model of the Sr(x)Ba(1 - x)Nb2O6 parent compound: doping with K+ and Na+ cations has no significant influence on the incommensurate modulation, contrary to the strong dependence observed for the physical properties.

Atomic structure and phason modes of the Sc–Zn icosahedral quasicrystal

The detailed atomic structure of the binary icosahedral ScZn7.33 quasicrystal has been investigated by means of high-resolution synchrotron single-crystal X-ray diffraction and absolute scale measurements of diffuse scattering.