7 research outputs found

    Large-Scale Computation of Nuclear Magnetic Resonance Shifts for Paramagnetic Solids Using CP2K

    No full text
    Large-scale computations of nuclear magnetic resonance (NMR) shifts for extended paramagnetic solids (pNMR) are reported using the highly efficient Gaussian-augmented plane-wave implementation of the CP2K code. Combining hyperfine couplings obtained with hybrid functionals with g-tensors and orbital shieldings computed using gradient-corrected functionals, contact, pseudocontact, and orbital-shift contributions to pNMR shifts are accessible. Due to the efficient and highly parallel performance of CP2K, a wide variety of materials with large unit cells can be studied with extended Gaussian basis sets. Validation of various approaches for the different contributions to pNMR shifts is done first for molecules in a large supercell in comparison with typical quantum-chemical codes. This is then extended to a detailed study of g-tensors for extended solid transition-metal fluorides and for a series of complex lithium vanadium phosphates. Finally, lithium pNMR shifts are computed for Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, for which detailed experimental data are available. This has allowed an in-depth study of different approaches (e.g., full periodic versus incremental cluster computations of g-tensors and different functionals and basis sets for hyperfine computations) as well as a thorough analysis of the different contributions to the pNMR shifts. This study paves the way for a more-widespread computational treatment of NMR shifts for paramagnetic materials

    Characterizing Oxygen Local Environments in Paramagnetic Battery Materials via <sup>17</sup>O NMR and DFT Calculations

    No full text
    Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of <sup>17</sup>O NMR in the same materials has not yet been reported. In this work, we present a combined <sup>17</sup>O NMR and hybrid density functional theory study of the local O environments in Li<sub>2</sub>MnO<sub>3</sub>, a model compound for layered Li-ion batteries. After a simple <sup>17</sup>O enrichment procedure, we observed five resonances with large <sup>17</sup>O shifts ascribed to the Fermi contact interaction with directly bonded Mn<sup>4+</sup> ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the <sup>17</sup>O shifts of environments similar to the 4i and 8j sites in pristine Li<sub>2</sub>MnO<sub>3</sub>, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li<sub>2</sub>MnO<sub>3</sub> structure. From the ratio of the intensities of the different <sup>17</sup>O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying <sup>17</sup>O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides

    Large-Scale Computation of Nuclear Magnetic Resonance Shifts for Paramagnetic Solids Using CP2K

    No full text
    Large-scale computations of nuclear magnetic resonance (NMR) shifts for extended paramagnetic solids (pNMR) are reported using the highly efficient Gaussian-augmented plane-wave implementation of the CP2K code. Combining hyperfine couplings obtained with hybrid functionals with g-tensors and orbital shieldings computed using gradient-corrected functionals, contact, pseudocontact, and orbital-shift contributions to pNMR shifts are accessible. Due to the efficient and highly parallel performance of CP2K, a wide variety of materials with large unit cells can be studied with extended Gaussian basis sets. Validation of various approaches for the different contributions to pNMR shifts is done first for molecules in a large supercell in comparison with typical quantum-chemical codes. This is then extended to a detailed study of g-tensors for extended solid transition-metal fluorides and for a series of complex lithium vanadium phosphates. Finally, lithium pNMR shifts are computed for Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, for which detailed experimental data are available. This has allowed an in-depth study of different approaches (e.g., full periodic versus incremental cluster computations of g-tensors and different functionals and basis sets for hyperfine computations) as well as a thorough analysis of the different contributions to the pNMR shifts. This study paves the way for a more-widespread computational treatment of NMR shifts for paramagnetic materials

    Local Environments of Dilute Activator Ions in the Solid-State Lighting Phosphor Y<sub>3ā€“<i>x</i></sub>Ce<sub><i>x</i></sub>Al<sub>5</sub>O<sub>12</sub>

    No full text
    The oxide garnet Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (YAG), when substituted with a few percent of the activator ion Ce<sup>3+</sup> to replace Y<sup>3+</sup>, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce<sup>3+</sup> ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce<sup>3+</sup> local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching

    Identifying the Critical Role of Li Substitution in P2ā€“Na<sub><i>x</i></sub>[Li<sub><i>y</i></sub>Ni<sub><i>z</i></sub>Mn<sub>1ā€“<i>y</i>ā€“<i>z</i></sub>]O<sub>2</sub> (0 < <i>x</i>, <i>y</i>, <i>z</i> < 1) Intercalation Cathode Materials for High-Energy Na-Ion Batteries

    No full text
    Li-substituted layered P2ā€“Na<sub>0.80</sub>[Li<sub>0.12</sub>Ni<sub>0.22</sub>Mn<sub>0.66</sub>]Ā­O<sub>2</sub> is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different states of charge/discharge to track Li-ion site occupation changes. Surprisingly, Li is found to be mobile, some Li ions migrate from the TM layer to the Na layer at high voltage, and yet this process is highly reversible. Novel design principles for Na cathode materials are proposed on the basis of an atomistic level understanding of the underlying electrochemical processes. These principles enable us to devise an optimized, high capacity, and structurally stable compound as a potential cathode material for high-energy Na-ion batteries

    Investigation of the Orderā€“Disorder Rotator Phase Transition in KSiH<sub>3</sub> and RbSiH<sub>3</sub>

    No full text
    The Ī²ā€“Ī± (orderā€“disorder) transition in the silanides ASiH<sub>3</sub> (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (<i>C</i><sub><i>p</i></sub>), solid-state <sup>2</sup>H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of Ī±-ASiH<sub>3</sub> corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH<sub>3</sub><sup>ā€“</sup> moieties. At temperatures below 200 K ASiH<sub>3</sub> exist as hydrogen-ordered (Ī²) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH<sub>3</sub> and KSiH<sub>3</sub>, respectively. The transition is accompanied by a large molar volume increase of about 14%. The <i>C</i><sub><i>p</i></sub>(<i>T</i>) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH<sub>3</sub><sup>ā€“</sup> rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In Ī±-ASiH<sub>3</sub>, the SiH<sub>3</sub><sup>ā€“</sup> anions undergo rotational diffusion with average relaxation times of 0.2ā€“0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20ā€“40 K). <sup>2</sup>H NMR revealed that the Ī±-form can coexist, presumably as 2ā€“4 nm (sub-Bragg) sized domains, with the Ī²-phase below the phase transition temperatures established from <i>C</i><sub><i>p</i></sub> measurements. The reorientational mobility of H atoms in undercooled Ī±-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases Ī±-ASiH<sub>3</sub> near room temperature and the presence of dynamical disorder even in the low-temperature Ī²-phases imply that SiH<sub>3</sub><sup>ā€“</sup> ions are only weakly coordinated in an environment of A<sup>+</sup> cations. The orientational flexibility of SiH<sub>3</sub><sup>ā€“</sup> can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations
    corecore