Grafting Spiropyran Molecular Switches on TiO₂: A First-Principles Study

To explore the optoelectronic properties of spiropyran molecular switches adsorbed onto TiO₂ anatase surfaces, we performed a density functional theory (DFT)/time-dependent density functional theory (TD-DFT) study considering the two isomeric forms of the photochromes anchored by both their sides. A comparison between the features of the hybrid and isolated systems is proposed to probe the adsorption effects on both subsystems. This comparison considered, on the one hand, the density of states and the alignment of the energy levels, and, on the other hand, the UV–visible spectra of these systems. We show that several electronic and optical characteristics of the hybrid systems are modulated by the open/closed state of the photochromes. These properties are also modified by the localization of the anchor group on the photochrome

How Adsorption Onto TiO₂ Modifies the Properties of Multiswitchable DTE Systems: Theoretical Insights

In order to best employ multiphotochromes as complex molecular gates, each isomer should ideally have a distinct optical profile to be selectively addressable. In this ab initio DFT and TD-DFT study, we have modeled the electronic and optical properties of a series of dithienylethene (DTE) dimers grafted onto an anatase (101) surface. We seek to investigate how grafting onto a TiO₂ surface modifies the energy levels and UV–visible spectra of the dimers and enhances the asymmetry of the isomers. By extracting information from the density of states, we have qualified the distinct degrees of interaction between the substrate and each isomeric configuration as CO > CC > OC > OO in order of decreasing electronic coupling. We subsequently use this information to interpret the UV–vis spectra computed for the isomers. The results show that the grafted systems present new peaks and shifted <i>S</i>₁ energies compared with the isolated photochrome, suggesting that adsorption onto a TiO₂ surface may induce an asymmetric character in the DTE dyad

Elucidation of the Na_2/3FePO₄ and Li_2/3FePO₄ 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_2/3FePO₄ (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_2/3FePO₄ and Li_2/3FePO₄ 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 (101̅)_<i>Pnma</i> 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 (<i>bc</i>)_<i>Pnma</i>. 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

Simulation of NMR Fermi Contact Shifts for Lithium Battery Materials: The Need for an Efficient Hybrid Functional Approach

In the context of the development of NMR Fermi contact shift calculations for assisting structural characterization of battery materials, we propose an accurate, efficient, and robust approach based on the use of an all electron method. The full-potential linearized augmented plane wave method, as implemented in the WIEN2k code, is coupled with the use of hybrid functionals for the evaluation of hyperfine field quantities. The WIEN2k code uses an autoadaptive basis set that is highly accurate for the determination of the hyperfine field. Furthermore the implementation of an onsite version for the Hartree–Fock exchange offers the possibility to use hybrid functional schemes at no additional computational cost. In this paper, NMR Fermi contact shifts for lithium are studied in different classes of paramagnetic materials that present an interest in the field of Li-ion batteries: olivine LiMPO₄ (M = Mn, Fe, Co, and Ni), anti-NASICON type Li₃M₂(PO₄)₃ (M = Fe and V), and antifluorite-type Li₆CoO₄. Making use of the possibility to apply partial hybrid functionals either only on the magnetic atom or also on the anionic species, we evidence the role played by oxygen atoms on polarization mechanisms. Our method is quite general for an application on various types of materials. Furthermore, it is very competitive compared to the other methods recently proposed that are based either on a plane wave basis set with a PAW implementation or on an LCAO one with a full potential description

Structural Investigation of α- and β-Sodium Hexafluoroarsenate, NaAsF₆, by Variable Temperature X-ray Powder Diffraction and Multinuclear Solid-State NMR, and DFT Calculations

We report the phase transition between the α- and β-phases of NaAsF₆ monitored by DTA, variable temperature ¹⁹F solid-state NMR and temperature controlled X-ray powder diffraction (XRPD) as well as their crystalline structures determined from XRPD data. The structural type of β-NaAsF₆ has been determined thanks to ¹⁹F and ⁷⁵As solid-state NMR experiments. ¹⁹F, ²³Na, and ⁷⁵As NMR parameters, including ¹⁹F–⁷⁵As ¹<i>J</i> coupling, have been measured for both phases. The ¹⁹F, ²³Na, and ⁷⁵As solid-state NMR investigations are in full agreement with both crystalline structures from a qualitative point of view. Chemical shielding tensors have been calculated from the gauge including projector augmented wave approach. The electric field gradient tensors of ²³Na and ⁷⁵As have been calculated in α-NaAsF₆ from the all-electrons method and the projector augmented-wave approach. Two difficulties were encountered: the libration of the rigid and regular AsF₆^– anions in the β-phase, highlighted by the atomic anisotropic displacement parameters for F, which leads to erroneous shortened As–F bond length, and the overestimation of the As–F bond length with the PBE functional used in the density functional theory calculations. We show that both difficulties can be overcome by full optimization and rescaling of the cell parameters of the crystalline structures. Additionally, a linear correlation is observed between experimental ²³Na δ_iso values and calculated ²³Na σ_iso values from previously reported data and from our own measurements and calculations

Interplay between TiO₂ Surfaces and Organic Photochromes: A DFT Study of Adsorbed Azobenzenes and Diarylethenes

Because photochromes act as light-induced molecular switches, there is considerable interest in exporting their molecular functionality in the solid state in order to develop photoresponsive materials. In that context, we have carried out a DFT investigation of a series of photochromic derivatives of <i>trans</i>/<i>cis</i>-azobenzene and open/closed-diarylethene adsorbed onto rutile (110) and anatase (101) slabs. By varying the auxochromes and the photochromic state, we examine the trends in the surface–adsorbate interplay in terms of the electronic structure, adsorption geometries, and Bader charge transfer. Using principal components analysis, we demonstrate how the dipole moment of the isolated photochrome in the direction of the anchoring group is the most important predictor of the electronic structure of the adsorbed system. A key point of this paper is to show how the energy levels of the isolated photochrome and bare slab change as a result of adsorption and how first-principles modeling helps to rationalize and predict these trends

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La 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 RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La 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 RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated