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International Symposium on Fusion Nuclear Technology This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document was prepared as an account of work sponsored by an agency of the United States Government.

Monoclinic lithium metatitanate, β-Li 2 TiO 3 , is a member of the Li 2 MO 3 (M = Ti, Mn, Sn, Ru, and/or Ir) series and an important cation conductor for various energy applications such as Li-ion batteries and nuclear fusion reactors. Comprehensive knowledge of the crystal structure is vital to understand the Li-ion diffusion mechanism, and several possibilities were proposed previously. However, the exact crystal structure and Li-ion diffusion paths of β-Li 2 TiO 3 are still unclear. Here, the results of a neutron diffraction study of high-purity 7 Li-enriched β-Li 2 TiO 3 are reported. The occupancy factor 0.033(3) and the atomic coordinates of the interstitial Li ion in the Li−O layer are successfully refined by Rietveld analysis of the time-of-flight neutron diffraction data. The three-dimensional network of Li-ion diffusion pathways is visualized by a combined technique of high-temperature neutron-diffraction and maximum-entropy methods. An interstitialcy diffusion mechanism, in which a lithium ion migrates through both the interstitial tetrahedral and lattice octahedral sites, is proposed for the Li 2 MO 3 series.

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Hydrogen (proton) induced switchable multiphase transformations in d-band electron-correlated materials recently opened a new field for exploring merging multifunctional proton-gated electronic devices, [1] synaptic plasticity, [2,3] sensors, [4] and novel energy conversion devices. [5] Compared to other dopant elements, hydrogen is small in radius and has high ionic mobility. [1,[4][5][6] Therefore, the proton distribution is highly adjustable via external electric fields, and the respective tuning of the physical properties of materials is feasible. This is, in particular, the case for the hydrogen-induced sharp transitions in electron orbital configurations and the magnetoelectric/spintronic states for d-band electron-correlated materials, such as SmNiO 3 , [1][2][3][4][5] SrCoO 3−δ , [6] and VO 2 . [7] As a typical example, the hydrogenation of the perovskite-structured SmNiO 3 d-band electron-correlated system results in an abrupt electronic transition of the e g orbital from the electron-itinerant Ni 3+ t e 2g 6 g 1 state to the electron-localized Ni 2+ t e 2g 6 g 2 The discovery of hydrogen-induced electron localization and highly insulating states in d-band electron correlated perovskites has opened a new paradigm for exploring novel electronic phases of condensed matters and applications in emerging field-controlled electronic devices (e.g., Mottronics). Although a significant understanding of doping-tuned transport properties of single crystalline correlated materials exists, it has remained unclear how dopingcontrolled transport properties behave in the presence of planar defects. The discovery of an unexpected high-concentration doping effect in defective regions is reported for correlated nickelates. It enables electronic conductance by tuning the Fermi-level in Mott-Hubbard band and shaping the lower Hubbard band state into a partially filled configuration. Interface engineering and grain boundary designs are performed for H x SmNiO 3 /SrRuO 3 heterostructures, and a Mottronic device is achieved. The interfacial aggregation of hydrogen is controlled and quantified to establish its correlation with the electrical transport properties. The chemical bonding between the incorporated hydrogen with defective SmNiO 3 is further analyzed by the positron annihilation spectroscopy. The present work unveils new materials physics in correlated materials and suggests novel doping strategies for developing Mottronic and iontronic devices via hydrogen-doping-controlled orbital occupancy in perovskite heterostructures.