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The addition of neutral ligand NH₃ is known to increase the Mg²⁺ ionic conductivity in Mg(BH₄)₂·NH₃ as compared to the parent compound Mg(BH₄)₂. Using inelastic neutron scattering, quasielastic neutron scattering, synchrotron X-ray powder diffraction, impedance spectroscopy, and density functional theory, the structure, the dynamics, and the Mg²⁺ ionic conductivity were investigated. The results show that the introduction of the NH₃ ligand not only enhances the Mg²⁺ ionic conductivity but also significantly increases the reorientational mobility of the BH₄^– anions. Thus, the results suggest that there may be a link between the two. Furthermore, the results show that Mg(BH₄)₂·NH₃ exhibits two coordination environments for the BH₄^– anions, which act as either bridging or terminal anions, in contrast to Mg(BH₄)₂, which only exhibits bridging anions. The different coordination environments in Mg(BH₄)₂·NH₃ lead to a clear difference in dynamics where the terminal anions have a much lower reorientational energy barrier (∼65 meV), as compared to the bridging anions (∼280 meV), and thus become dynamically active at much lower temperatures. The results show that the NH₃ ligands also exhibit reorientational dynamics and that these are even faster than the dynamics of the BH₄^– anions, with the NH₃ ligands having a reorientational energy barrier of ∼10 meV. In addition to the reorientational dynamics, the NH₃ ligands undergo quantum mechanical rotational tunneling below 50 K. In summary, this study provides a detailed characterization of both the structure and the dynamics of Mg(BH₄)₂·NH₃ and suggests that the rapidly reorienting terminal BH₄^– anions may be behind the increased Mg²⁺ ionic conductivity upon ligand incorporation.

Solid-state batteries are one of the most recent iterations of electrochemical energy storage, and the technology can potentially provide safer and more-energy-dense batteries. The metal closo- and nido-(carba)borates show promise as versatile solid electrolytes and have been shown to have some of the highest ionic conductivities as well as wide electrochemical stability windows. In the present study, we investigate the four potassium nido-(carba)borates KB11H14, K-7-CB10H13, K-7,8-C2B9H12, and K-7,9-C2B9H12, and a total of eight new crystal structures were solved. All four compounds transition from a low-temperature, ordered phase to a high-temperature, disordered phase with the space group Fm-3m. In the high-temperature polymorphs, the anions are disordered and undergo rapid reorientational dynamics, which is confirmed by quasielastic neutron scattering experiments. Reorientational activation energies of 0.151(2), 0.146(32), and 0.143(3) eV were determined for K-7-CB10H13, K-7,8-C2B9H12, and K-7,9-C2B9H12, respectively. Additionally, such rotationally fluid anions are concomitant with fast potassium-ion conductivity. The highest ionic conductivity is observed for K-7,8-C2B9H12 with 1.7·10–2 S cm–1 at 500 K and an activation energy of 0.28 eV in the disordered state. The differences in phase transition temperatures, reorientational dynamics, and ionic conductivities among the potassium nido-(carba)borates illustrate a strong correlation between the K+ cationic mobility and the local cation–anion interactions, anion dynamics, and the specific positions of the carbon atoms in the nido-(carba)borate anion cages.

The Prussian blue analogues (PBAs) Na2-xFe[Fe(CN)6]·z H2O (x,z = 0–2) exhibit many phase transitions as a function of the sodium and water content, which involves large volume changes that can negatively affect its energy storage performance in a battery. However, the presence of water helps stabilize the PBA framework and thus diminishes these volume changes. To improve the material for its desired applications, a deeper fundamental understanding of the interactions between water, sodium, and the PBA framework is needed. Here, the local structure and vibrational dynamics of water were studied using inelastic neutron scattering, neutron diffraction, and theoretical calculations. When the sodium content is high, the material exhibits well-defined water environments that become less defined when the sodium content is lower. It was shown that the positions of sodium and water are more complex than suggested by previous diffraction and computational studies. Most of the water in the high sodium sample occupies the center of the PBA subcube, while only a small fraction is located close to the window site of the subcube. For the low sodium sample, the results suggest that a large distribution of local water environments is present. These results lay the groundwork for unraveling the ionic transport in PBAs and the development of improved energy storage materials.

Prussian blue analogues (PBAs) are interesting cathode materials for sodium-ion batteries, especially the iron-based, [Fe(CN)₆]ⁿ⁻ vacancy-free PBA Na_2–xFe[Fe(CN)₆]·zH₂O. However, the presence of water has an opposing role in the application of PBAs as electrode materials: the water provides structural stability ensuring minimum volume changes during sodium extraction and insertion, however, water can react with the electrolyte leading to unwanted side reactions. Therefore, water must be replaced with another compatible small molecule to ensure optimal performance. To achieve this, insights into the dynamics of water are crucial. Two samples with compositions of Na_1.90(9)Fe_0.90(7)²⁺Fe_0.10(3)³⁺[Fe²⁺(CN)₆]·2.12(2)H₂O and Na_0.34(5)Fe³⁺[Fe^2.66(5)+(CN)₆]·0.360(4)H₂O were investigated using quasi-elastic neutron scattering (QENS). The results show that the water dynamics strongly depend on the sodium content. The water was found to diffuse within a spherical cavity in the porous framework with a radius of 2.6 Å for the high sodium-containing sample and 1.8 Å for the low sodium-containing sample consistent with the pore sizes in the crystal structures. In addition to the water diffusing within the pores, it was found that a small fraction of the water exhibits a rattling or rotational motion suggesting that this water strongly interacts and binds to the sodium ions. For the high sodium-containing sample, this rattling or rotational motion transforms into quantum rotational tunneling of the water below 75 K. These results give new fundamental insight into the role of water in PBAs, laying the groundwork for substituting water with another small molecule compatible with nonaqueous battery systems while also ensuring structural stability.

Rare earth monogallide (REGa) Zintl phases are attractive for their properties in hydrogen storage and magnetic cooling. However, the magnetic effects upon hydrogen additions in REGa are not well understood. This study aims to explore the magnetic effects in REGaHx using SQUID magnetometry and neutron powder diffraction. To avoid challenges due to absorption and high incoherent scattering in the neutron diffraction experiments, the compound NdGaDx (x = 0, 0.9, or 1.6) was chosen for examination. It was found that NdGa exhibits two ferromagnetic structures below the Curie temperature of 42 K. Just below 42 K the magnetic moments are oriented along the crystallographic c axis, and at 20 K a spin reorientation occurs where the moments turn similar to 30 degrees toward the a axis. Upon partial deuteration (x = 0.9), the magnetization decreases and two magnetic phases are observed, one intermediate incommensurate phase, and one canted ferromagnetic phase with the net magnetization aligning along the b axis. For the full deuteride (x = 1.6) only one incommensurate magnetic phase is observed at low temperatures. Magnetometry also reveals that there are no isotope effects when absorbing H or D. The absorption of H or D changes the Nd-Nd distances as well as the electronic structure, which results in a drastic change in the magnetic properties as compared to NdGa.

The reorientational dynamics of Y(BH₄)₃·xNH₃ (x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH₃ ligands drastically alters the reorientational mobility of the BH₄^– anion. From the QENS experiments, it was determined that the BH₄^– anion performs 2-fold reorientations around the C₂ axis in Y(BH₄)₃, 3-fold reorientations around the C₃ axis in Y(BH₄)₃·3NH₃, and either 2-fold reorientations around the C₂ axis or 3-fold reorientations around the C₃ axis in Y(BH₄)₃·7NH₃. The relaxation time of the BH₄^– anion at 300 K decreases from 2 × 10^–7 s for x = 0 to 1 × 10^–12 s for x = 3 and to 7 × 10^–13 s for x = 7. In addition to the reorientational dynamics of the BH₄^– anion, it was shown that the NH₃ ligands exhibit 3-fold reorientations around the C₃ axis in Y(BH₄)₃·3NH₃ and Y(BH₄)₃·7NH₃ as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH₄·xNH₃ and Mg(BH₄)₂·xCH₃NH₂.

NdGa hydride and deuteride phases were prepared from high-quality NdGa samples and their structures characterized by powder and single-crystal X-ray diffraction and neutron powder diffraction. NdGa with the orthorhombic CrBtype structure absorbs hydrogen at hydrogen pressures <= 1 bar until reaching the composition NdGaH(D)(1.1), which maintains a CrB-type structure. At elevated hydrogen pressure additional hydrogen is absorbed and the maximum composition recovered under standard temperature and pressure conditions is NdGaH(D) (1.6) with the Cmcm LaGaH1.66-type structure. This structure is a threefold superstructure with respect to the CrB-type structure. The hydrogen atoms are ordered and distributed on three fully occupied Wyckoff positions corresponding to tetrahedral (4c, 8g) and trigonal-bipyramidal (8g) voids in the parent structure. The threefold superstructure is maintained in the H-deficient phases NaGaH(D)(x) until 1.6 >= x >= 1.2. At lower H concentrations, coinciding with the composition of the hydride obtained from hydrogenation at atmospheric pressure, the unit cell of the CrB-type structure is resumed. This phase can also display H deficiency, NdGaH(D) (y) (1.1 >= y >= 0.9), with H(D) exclusively situated in partially empty tetrahedral voids. The phase boundary between the threefold superstructure (LaGaH1.66 type) and the onefold structure (NdGaH1.1 type) is estimated on the basis of phase-composition isotherms and neutron powder diffraction to be x = 1.15.

Reorientational dynamics in solid electrolytes can significantly enhance the ionic conductivity, and understanding these dynamics can facilitate the rational design of improved solid electrolytes. Additionally, recent investigations on metal hydridoborate-based solid electrolytes have shown that the addition of a neutral ligand can also have a positive effect on the ionic conductivity. In this study, we investigate the dynamics in monomethylamine magnesium borohydride (Mg(BH₄)₂·CH₃NH₂) with quasielastic and inelastic neutron scattering, density functional theory calculations, and molecular dynamics simulations. The results suggest that the addition of methylamine significantly speeds up the reorientational frequency of the BH₄^– anion compared to Mg(BH₄)₂. This is likely part of the explanation for the high Mg-ion transport observed for Mg(BH₄)₂·CH₃NH₂. Furthermore, while the dynamics of both the BH₄^– anion and the CH₃ group of the methylamine ligand is rapid, the NH₂ group of the methylamine ligand exhibits much slower reorientations, as confirmed by both experimental and computational investigations. Notably, molecular dynamics calculations reveal mean square displacements of 0.387 Ų for NH₂, 1.503 Ų for CH₃, and 1.856 Ų for BH₄^– using a trajectory of 10 ps. This study confirms the simultaneous presence of fast dynamics and high ionic conductivity in a metal borohydride-based system and can function as an experimental foundation for future studies on dynamics in hydrogen-rich solid electrolytes.

Dense systems of magnetic nanoparticles may exhibit dipolar collective behavior. However, two fundamental questions remain unsolved: i) whether the transition temperature may be affected by the particle anisotropy or it is essentially determined by the intensity of the interparticle dipolar interactions, and ii) what is the minimum ratio of dipole-dipole interaction (E-dd) to nanoparticle anisotropy (KefV, anisotropy.volume) energies necessary to crossover from individual to collective behavior. A series of particle assemblies with similarly intense dipolar interactions but widely varying anisotropy is studied. The K-ef is tuned through different degrees of cobalt-doping in maghemite nanoparticles, resulting in a variation of nearly an order of magnitude. All the bare particle compacts display collective behavior, except the one made with the highest anisotropy particles, which presents "marginal" features. Thus, a threshold of KefV/E-dd approximate to 130 to suppress collective behavior is derived, in good agreement with Monte Carlo simulations. This translates into a crossover value of approximate to 1.7 for the easily accessible parameter T-MAX(interacting)/T-MAX(non-interacting) (ratio of the peak temperatures of the zero-field-cooled magnetization curves of interacting and dilute particle systems), which is successfully tested against the literature to predict the individual-like/collective behavior of any given interacting particle assembly comprising relatively uniform particles.

In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth (R) ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center R magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral R magnetic moments are less affected by the cluster-center R, while the averaged cluster-center R magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.