Multimodal Structure Solution with 19F NMR Crystallography of Spin Singlet Molybdenum Oxyfluorides.

Complex crystal structures with subtle atomic-scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging. Identifying unambiguous atomic models for oxyfluorides, needed for materials design and structure-property control, is often still a considerable challenge despite their advantageous optical responses and applications in energy storage systems. In this work, NMR crystallography and single-crystal X-ray diffraction are combined for the complete structure solution of three new compounds featuring a rare triangular early transition metal oxyfluoride cluster, [Mo3O4F9]5-. After framework identification by single-crystal X-ray diffraction, 1D and 2D solid-state 19F NMR spectroscopy supported by ab initio calculations are used to solve the structures of K5[Mo3O4F9]·3H2O (1), K5[Mo3O4F9]·2H2O (2), and K16[Mo3O4F9]2[TiF6]3·2H2O (3) and to assign the nine distinct fluorine sites in the oxyfluoride clusters. Furthermore, 19F NMR identifies selective fluorine dynamics in K16[Mo3O4F9]2[TiF6]3·2H2O. These dual scattering and spectroscopy methods are used to demonstrate the generality and sensitivity of 19F shielding to small changes in bond length, on the order of 0.01 Å or less, even in the presence of hydrogen bonding, metal-metal bonding, and electrostatic interactions. Starting from the structure models, the nature of chemical bonding in the molybdates is explained by molecular orbital theory and electronic structure calculations. The average Mo-Mo distance of 2.505 Å and diamagnetism in 1, 2, and 3 are attributed to a metal-metal bond order of unity along with a 1a21e4 electronic ground state configuration for the [Mo3O4F9]5- cluster, leading to a rare trimeric spin singlet involving d2 Mo4+ ions. The approach to structure solution and bonding analysis is a powerful strategy for understanding the structures and chemical properties of complex fluorides and oxyfluorides.

Lithium Diffusion in Niobium Tungsten Oxide Shear Structures.

Niobium tungsten oxides with crystallographic shear structures form a promising class of high-rate Li-ion anode materials. Lithium diffusion within these materials is studied in this work using density functional theory calculations, specifically nudged elastic band calculations and ab initio molecular dynamics simulations. Lithium diffusion is found to occur through jumps between 4-fold coordinated window sites with low activation barriers (80-300 meV) and is constrained to be effectively one-dimensional by the crystallographic shear planes of the structures. We identify a number of other processes, including rattling motions with barriers on the order of the thermal energy at room temperature, and intermediate barrier hops between 4-fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO3-like block units of the structures, cavities at the corners and edges host more isolated diffusion tunnels than those in the interior. Diffusion coefficients are found to be in the range of 10-12 to 10-11 m2 s-1 for lithium concentrations of 0.5 Li/TM. Overall, the results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure-property relationships identified in this work can be generalized to the entire family of crystallographic shear phases.

Ionic and Electronic Conduction in TiNb2O7.

TiNb2O7 is a Wadsley-Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in LixTiNb2O7) is rapid with low activation barriers from NMR and DLi = 10-11 m2 s-1 at the temperature of the observed T1 minima of 525-650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100-200 meV down the tunnels but ca. 700-1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in LixTiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li+ mobility.

Cation Disorder and Lithium Insertion Mechanism of Wadsley-Roth Crystallographic Shear Phases from First Principles.

Wadsley-Roth crystallographic shear phases form a family of compounds that have attracted attention due to their excellent performance as lithium-ion battery electrodes. The complex crystallographic structure of these materials poses a challenge for first-principles computational modeling and hinders the understanding of their structural, electronic and dynamic properties. In this article, we study three different niobium-tungsten oxide crystallographic shear phases (Nb12WO33, Nb14W3O44, Nb16W5O55) using an enumeration-based approach and first-principles density-functional theory calculations. We report common principles governing the cation disorder, lithium insertion mechanism, and electronic structure of these materials. Tungsten preferentially occupies tetrahedral and block-central sites within the block-type crystal structures, and the local structure of the materials depends on the cation configuration. The lithium insertion proceeds via a three-step mechanism, associated with an anisotropic evolution of the host lattice. Our calculations reveal an important connection between long-range and local structural changes: in the second step of the mechanism, the removal of local structural distortions leads to the contraction of the lattice along specific crystallographic directions, buffering the volume expansion of the material. Niobium-tungsten oxide shear structures host small amounts of localized electrons during initial lithium insertion due to the confining effect of the blocks, but quickly become metallic upon further lithiation. We argue that the combination of local, long-range, and electronic structural evolution over the course of lithiation is beneficial to the performance of these materials as battery electrodes. The mechanistic principles we establish arise from the compound-independent crystallographic shear structure and are therefore likely to apply to niobium-titanium oxide or pure niobium oxide crystallographic shear phases.