Die Chloride Na3xM2-xCl6 (M = La?Sm) und NaM2Cl6 (M = Nd, Sm): Derivate des UCl3-Typs. Synthese, Kristallstruktur und R�ntgenabsorptionsspektroskopie (XANES)

LaCl3-based sodium halide solid electrolytes with high ionic conductivity for all-solid-state batteries

To enable high performance of all solid-state batteries, a catholyte should demonstrate high ionic conductivity, good compressibility and oxidative stability. Here, a LaCl3-based Na+ superionic conductor (Na1-xZrxLa1-xCl4) with high ionic conductivity of 2.9 × 10-4 S cm-1 (30 °C), good compressibility and high oxidative potential (3.80 V vs. Na2Sn) is prepared via solid state reaction combining mechanochemical method. X-ray diffraction reveals a hexagonal structure (P63/m) of Na1-xZrxLa1-xCl4, with Na+ ions forming a one-dimensional diffusion channel along the c-axis. First-principle calculations combining with X-ray absorption fine structure characterization etc. reveal that the ionic conductivity of Na1-xZrxLa1-xCl4 is mainly determined by the size of Na+-channels and the Na+/La3+ mixing in the one-dimensional diffusion channels. When applied as a catholyte, the NaCrO2||Na0.7Zr0.3La0.7Cl4||Na3PS4||Na2Sn all-solid-state batteries demonstrate an initial capacity of 114 mA h g-1 and 88% retention after 70 cycles at 0.3 C. In addition, a high capacity of 94 mA h g-1 can be maintained at 1 C current density.

Superionic Conducting Halide Frameworks Enabled by Interface-Bonded Halides

The revival of ternary halides Li-M-X (M = Y, In, Zr, etc.; X = F, Cl, Br) as solid-state electrolytes (SSEs) shows promise in realizing practical solid-state batteries due to their direct compatibility toward high-voltage cathodes and favorable room-temperature ionic conductivities. Most of the reported superionic halide SSEs have a structural pattern of [MCl₆]^x- octahedra and generate a tetrahedron-assisted Li⁺ ion diffusion pathway. Here, we report a new class of zeolite-like halide frameworks, SmCl₃, for example, in which 1-dimensional channels are enclosed by [SmCl₉]^6- tricapped trigonal prisms to provide a short jumping distance of 2.08 Å between two octahedra for Li⁺ ion hopping. The fast Li⁺ diffusion along the channels is verified through ab initio molecular dynamics simulations. Similar to zeolites, the SmCl₃ framework can be grafted with halide species to obtain mobile ions without altering the base structure, achieving an ionic conductivity over 10^-4 S cm^-1 at 30 °C with LiCl as the adsorbent. Moreover, the universality of the interface-bonding behavior and ionic diffusion in a class of framework materials is demonstrated. It is suggested that the ionic conductivity of the MCl₃/halide composite (M = La-Gd) is likely in correlation with the ionic conductivity of the grafted halide species, interfacial bonding, and framework composition/dimensions. This work reveals a potential class of halide structures for superionic conductors and opens up a new frontier for constructing zeolite-like frameworks in halide-based materials, which will promote the innovation of superionic conductor design and contribute to a broader selection of halide SSEs.

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore

As potential nuclear waste host matrices, two series of uranium-doped Nd2Zr2O7 nanoparticles were successfully synthesized using an optimized molten salt method in an air atmosphere. Our combined X-ray diffraction, Raman and X-ray absorption fine-structure (XAFS) spectroscopy studies reveal that uranium ions can precisely substitute the Nd site to form an Nd2-xUxZr2O7+δ (0 ≤ x ≤ 0.2) system and the Zr site to form an Nd2Zr2-yUyO7+δ (0 ≤ y ≤ 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd3m) to defect fluorite (Fm3m) structures in both series of U-doped Nd2Zr2O7. The XAFS analysis indicates that uranium exists in the form of high-valent U6+ in all samples. To balance the extra charge for substituting Nd3+ or Zr4+ by U6+, additional oxygen is introduced accompanied by a large structural distortion; however, the Nd2Zr1.6U0.4O7+δ sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd2Zr2O7 host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd2Zr2O7 synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.

Hexagonal-phase NaREF4 upconversion nanocrystals: the matter of crystal structure

The hexagonal-phase (β) of NaREF₄ upconversion nanocrystals (RE = rare earth elements) has been widely employed because of the outstanding luminescence performance, yet less is known about the essence of this superior property. The current understanding of this issue is raised from the advantage of weak electron-vibration interactions in fluoride systems, while the interpretability of this statement is controversial and contradictory results are commonly reported. One feasible way to solve this puzzle is from the aspect of "structure-property" relationship, yet even after decades of investigation, the structural details of β-NaREF₄ are still under debate. Herein, the reported results relevant to this topic are reviewed, and the conflicting viewpoints are summarized. The similarities and differences between different lattice templates are assessed, and the reasons underlying the divergence are analysed. Based on these discussions, it is realized that the crystal structure of β-NaREF₄ should be more reliably depicted as one flexible lattice framework with complex characteristics, and the structural disorder induced by atom displacements in the lattice is probably the key to supporting the superior luminescence properties of β-NaREF₄ nanocrystals.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl_n; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mⁿ⁺) on redox reactions accessible to lanthanide ions; Ln³⁺ + e^1- → Ln²⁺ (Ln = Eu, Yb, Sm; e^1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu²⁺ instantaneously formed when Eu³⁺ dissolved in molten chloride salts that had strongly polarizing cations (like Ca²⁺ from CaCl₂) via the Eu³⁺ + Cl^1- → Eu²⁺ + ½Cl₂ reaction. Conversely, molten salts with less polarizing outer-sphere M¹⁺ cations (e.g., K¹⁺ in KCl) stabilized Ln³⁺. For instance, the Eu³⁺/Eu²⁺ reduction potential was >0.5 V more positive in CaCl₂ than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mⁿ⁺ cations (high polarization power) inductively removed electron density from Lnⁿ⁺ across Ln-Cl⋯Mⁿ⁺ networks and stabilized electron-rich and low oxidation state Ln²⁺ ions. Conversely, less polarizing Mⁿ⁺ cations (like K¹⁺) left electron density on Lnⁿ⁺ and stabilized electron-deficient and high-oxidation state Ln³⁺ ions.

The complete series of sodium rare-earth metal(III) chloride oxotellurates(IV) Na2 RE 3Cl3[TeO3]4 (RE = Y, La–Nd, Sm–Lu)

The complete series of sodium rare-earth metal(III) chloride oxotellurates(IV) with the composition Na2 RE 3Cl3[TeO3]4 (RE = Y, La–Nd, Sm–Lu) has been synthesized via solid-state reactions. For these conversions mixtures of the respective rare-earth metal(III) oxides, tellurium dioxide and sodium chloride as flux and reactant were prepared, intimately ground and heated for 5 days at 1225 K. The almost colorless single crystals were characterized via single-crystal X-ray diffractometry. In the monoclinic crystal structure of these compounds two crystallographically different rare-earth metal(III), but only one sodium cation sites occur. [REO8]13− polyhedra around both RE 3+ positions as well as sodium-centered polyhedra [NaO4Cl4]8− form layers via different connectivity modes. These layers spread out parallel to the (001) plane and arrange alternatingly resulting in the three-dimensional network of the Na2 RE 3Cl3[TeO3]4 structure, where the Te4+ lone-pair cations at two different sites work as linkers by forming isolated ψ 1-tetrahedra [TeO3]2−. Some of these compounds were represented before in different settings of space group C2/c. Now the complete series of the Na2 RE 3Cl3[TeO3]4 representatives with RE = Y, La–Nd, Sm–Lu is described consistently for a better comparison and understanding. Additionally, a single crystal of Na2Pr3Cl3[TeO3]4 was measured via energy dispersive X-ray analysis to verify the included elements, powder samples of Na2Nd3Cl3[TeO3]4 were characterized by X-ray diffractometer data for a phase-purity check and a single-crystal Raman spectrum of Na2Yb3Cl3[TeO3]4 served for proving the signature of discrete [TeO3]2− anions.

Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31- using XANES spectroscopy and DFT calculations

The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f n 5d1 configurations (not 4f n+1 5d0). In these 4f n 5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f n 5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.

Higher order speciation effects on plutonium L(3) X-ray absorption near edge spectra

Pu L(3) X-ray near edge absorption spectra for Pu(0-VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.