7 research outputs found
Large-Scale Computation of Nuclear Magnetic Resonance Shifts for Paramagnetic Solids Using CP2K
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
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
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>
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 P2āNa<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
Li-substituted
layered P2āNa<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 OrderāDisorder Rotator Phase Transition in KSiH<sub>3</sub> and RbSiH<sub>3</sub>
The
Ī²āĪ± (orderādisorder) 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 0.2ā0.3 ps between successive
H jumps. The first-order reconstructive phase transition is characterized
by a large hysteresis (20ā40 K). <sup>2</sup>H NMR revealed
that the Ī±-form can coexist, presumably as 2ā4 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