Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO₃ Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

Hydrated BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} (0.10 ≤ <i>x</i> ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The ¹¹⁹Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to <i>x</i> = 0.20, while at higher substitution levels, Y–O–Y linkages are avoided, leading, at <i>x</i> = 0.50, to strict Y–O–Sn alternation of B-site cations. These results are confirmed by the absence of a “Y–O–Y” ¹⁷O resonance and supported by the ¹⁷O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by ⁸⁹Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by ¹¹⁹Sn and ⁸⁹Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at <i>x</i> = 0.50, which is ascribed to the presence of residual Sn–O–Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature ¹¹⁹Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y–O–Sn cation ordering, which prevents proton trapping on the more basic Y–O–Y sites

Identification of Cation Clustering in Mg–Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy

A combined X-ray diffraction and magic angle spinning nuclear magnetic resonance (MAS NMR) study of a series of layered double hydroxides (LDHs) has been utilized to identify cation clustering in the metal hydroxide layers. High resolution (multiple quantum, MQ) ²⁵Mg NMR spectroscopy was successfully used to resolve different Mg local environments in nitrate and carbonate-containing layered double hydroxides with various Al for Mg substitution levels, and it provides strong evidence for cation ordering schemes based around Al–Al avoidance (in agreement with ²⁷Al NMR), the ordering increasing with an increase in Al content. ¹H MAS double quantum NMR spectroscopy verified the existence of small Mg₃OH and Mg₂AlOH clusters within the same metal hydroxide sheet and confirmed that the cations gradually order as the Al concentration is increased to form a honeycomb-like Al distribution throughout the metal hydroxide layer. The combined use of these multinuclear NMR techniques provides a structural foundation with which to rationalize the effects of different cation distributions on properties such as anion binding and retention in this class of materials

Understanding the Conduction Mechanism of the Protonic Conductor CsH₂PO₄ by Solid-State NMR Spectroscopy

Local dynamics and hydrogen bonding in CsH₂PO₄ have been investigated by ¹H, ²H, and ³¹P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by ¹H and ²H NMR, and their chemical shifts (¹H) and quadrupolar coupling constants (²H) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature ¹H MAS NMR and <i>T</i>₁ spin–lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. ³¹P NMR line shapes are measured under MAS and static conditions, and spin–lattice relaxation time measurements have been performed as a function of temperature. Although the ³¹P line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the ³¹P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the <i>T</i>₁ measurements along with considerable line narrowing of both the ¹H and the ³¹P NMR signals

Ion Dynamics in Li₂CO₃ Studied by Solid-State NMR and First-Principles Calculations

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO₂ looping, where the material reacts reversibly with CO₂ to form Li₂CO₃, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li₂CO₃ has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li₂CO₃ over the temperature range of 293–973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li₂CO₃. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations

Structural Study of La_1–<i>x</i>Y_<i>x</i>ScO₃, Combining Neutron Diffraction, Solid-State NMR, and First-Principles DFT Calculations

The solid solution La_1–<i>x</i>Y_<i>x</i>ScO₃ (<i>x</i> = 0, 0.2, 0.4, 0.6, 0.8, and 1) has been successfully synthesized using conventional solid-state techniques. Detailed structural characterization has been undertaken using high-resolution neutron powder diffraction and multinuclear (⁴⁵Sc, ¹³⁹La, ⁸⁹Y, and ¹⁷O) solid-state NMR and is supported by first-principles density functional theory calculations. Diffraction data indicate that a reduction in both the unit cell parameters and unit cell volume is observed with increasing <i>x</i>, and an orthorhombic perovskite structure (space group <i>Pbnm</i>) is retained across the series. ⁴⁵Sc multiple-quantum (MQ) MAS NMR spectra proved to be highly sensitive to subtle structural changes and, in particular, cation substitutions. NMR spectra of La_1–<i>x</i>Y_<i>x</i>ScO₃ exhibited significant broadening, resulting from distributions of both quadrupolar and chemical shift parameters, owing to the disordered nature of the material. In contrast to previous single-crystal studies, which reveal small deficiencies at both the lanthanide and oxygen sites, the powder samples studied herein are found to be stoichiometric

Dynamic Nuclear Polarization Enhanced Natural Abundance ¹⁷O Spectroscopy

We show that natural abundance oxygen-17 NMR of solids could be obtained in minutes at a moderate magnetic field strength by using dynamic nuclear polarization (DNP). Electron spin polarization could be transferred either directly to ¹⁷O spins or indirectly via ¹H spins in inorganic oxides and hydroxides using an oxygen-free solution containing a biradical polarization agent (bTbK). The results open up a powerful method for rapidly acquiring high signal-to-noise ratio solid-state NMR spectra of ¹⁷O nuclear spins and to probe sites on or near the surface, without the need for isotope labeling

Joint Experimental and Computational ¹⁷O and ¹H Solid State NMR Study of Ba₂In₂O₄(OH)₂ Structure and Dynamics

A structural characterization of the hydrated form of the brownmillerite-type phase Ba₂In₂O₅, Ba₂In₂O₄(OH)₂, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H₂O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. (Solid State Ionics 2004, 170, 25−32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba₂In₂O₄(OH)₂ yield a set of low energy structures; GIPAW NMR calculations on these configurations yield ¹H and ¹⁷O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct ¹H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. ¹H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The ¹⁷O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by ¹H–¹⁷O double resonance experiments

Revealing Local Dynamics of the Protonic Conductor CsH(PO₃H) by Solid-State NMR Spectroscopy and First-Principles Calculations

A joint study incorporating multinuclear solid-state NMR spectroscopy and first-principles calculations has been performed to investigate the local structure and dynamics of the protonic conductor CsH­(PO₃H) in the paraelectric phase. The existence of the superprotonic phase (>137 °C) is clearly confirmed by NMR, in good agreement with the literature. The variable-temperature ¹H, ²H, and ³¹P NMR data further reveal a distribution of motional correlation times, with isotropic rotation of the phosphite ion being observed below the superprotonic phase transition for a small but gradually increasing subset of anions. This isotropic rotation is associated with fast local protonic motion, with the distribution of correlation times being tentatively assigned to internal defects or surface adsorbed H₂O. The phosphite ion dynamics of the majority slower subset of phosphite ions is quantified through analysis of variable-temperature ¹⁷O spectra recorded from 34 to 150 °C, by considering a model for the pseudo <i>C</i>₃ rotation of the phosphite ion around the P–H bond axis below the phase transformation. An extracted activation energy of 0.24 ± 0.08 eV (23 ± 8 kJ mol^–1) for this model was obtained, much lower than that reported from proton conductivity measurements, implying that no strong correlation exists between long-range protonic motion and <i>C</i>₃ rotations of the phosphite. We conclude that proton conduction in CsH­(PO₃H) in the paraelectric phase is governed by the activation energy for exchange between donor and acceptor oxygen sites, rotation of the phosphite units, and the lack of isotropic rotation of the phosphite ion. Surprisingly, coalescence of ¹⁷O NMR resonances, as would be expected for rapid isotropic reorientations of all phosphite groups, is not observed above the transition. Potential reasons for this are discussed

Long-Range-Ordered Coexistence of 4‑, 5‑, and 6‑Coordinate Niobium in the Mixed Ionic-Electronic Conductor γ‑Ba₄Nb₂O₉

In a study combining high-resolution single-crystal neutron diffraction and solid-state nuclear magnetic resonance, the mixed ionic-electronic conductor γ-Ba₄Nb₂O₉ is shown to have a unique structure type, incorporating niobium in 4-, 5-, and 6-coordinate environments. The 4- and 5-coordinate niobium tetrahedra and trigonal bipyrimids exist in discrete layers, within and among which their orientations vary systematically to form a complex superstructure. Through analysis and comparison of data obtained from hydrated versus dehydrated samples, a mechanism is proposed for the ready hydration of the material by atmospheric water. This mechanism, and the resulting hydrated structure, help explain the high protonic and oxide ionic conductivity of γ-Ba₄Nb₂O₉

Drug–Polymer Interactions in Acetaminophen/Hydroxypropylmethylcellulose Acetyl Succinate Amorphous Solid Dispersions Revealed by Multidimensional Multinuclear Solid-State NMR Spectroscopy

The bioavailability of insoluble crystalline active pharmaceutical ingredients (APIs) can be enhanced by formulation as amorphous solid dispersions (ASDs). One of the key factors of ASD stabilization is the formation of drug–polymer interactions at the molecular level. Here, we used a range of multidimensional and multinuclear nuclear magnetic resonance (NMR) experiments to identify these interactions in amorphous acetaminophen (paracetamol)/hydroxypropylmethylcellulose acetyl succinate (HPMC-AS) ASDs at various drug loadings. At low drug loading (1H–13C through-space heteronuclear correlation experiments identify proximity between aromatic protons in acetaminophen with cellulose backbone protons in HPMC-AS. We also show that 14N–1H heteronuclear multiple quantum coherence (HMQC) experiments are a powerful approach in probing spatial interactions in amorphous materials and establish the presence of hydrogen bonds (H-bond) between the amide nitrogen of acetaminophen with the cellulose ring methyl protons in these ASDs. In contrast, at higher drug loading (40 wt %), no acetaminophen/HPMC-AS spatial proximity was identified and domains of recrystallization of amorphous acetaminophen into its crystalline form I, the most thermodynamically stable polymorph, and form II are identified. These results provide atomic scale understanding of the interactions in the acetaminophen/HPMC-AS ASD occurring via H-bond interactions