CdO has been studied for decades as a prototypical wide band gap transparent conducting oxide with excellent n-type ability. Despite this, uncertainty remains over the source of conductivity in CdO and over the lack of p-type CdO, despite its valence band maximum (VBM) being high with respect to other wide band gap oxides. In this article, we use screened hybrid DFT to study intrinsic defects and hydrogen impurities in CdO and identify for the first time the source of charge carriers in this system. We explain why the oxygen vacancy in CdO acts as a shallow donor and does not display negative-U behavior similar to all other wide band gap n-type oxides. We also demonstrate that p-type CdO is not achievable, as n-type defects dominate under all growth conditions. Lastly, we estimate theoretical doping limits and explain why CdO can be made transparent by a large Moss-Burstein shift caused by suitable n-type doping.
The defect structure and ionic diffusion processes within the anion-deficient, fluorite structured system Ce1–x Y x O2–x/2 have been investigated at high temperatures (873 K–1073 K) as a function of dopant concentration, x, using a combination of neutron diffraction studies, impedance spectroscopy measurements, and molecular dynamics (MD) simulations using interionic potentials developed from ab initio calculations. Particular attention is paid to the short-range ion–ion correlations, with no strong evidence that the anion vacancies prefer, at high temperature, to reside in the vicinity of either cationic species. However, the vacancy–vacancy interactions play a more important role, with preferential ordering of vacancy pairs along the ⟨111⟩ directions, driven by their strong repulsion at closer distances, becoming dominant at high values of x. This effect explains the presence of a maximum in the ionic conductivity in the intermediate temperature range as a function of increasing x. The wider implications of these conclusions for understanding the structure–property relationships within anion-deficient fluorite structured oxides are briefly discussed, with reference to complementary studies of yttria and/or scandia doped zirconia published previously.
We have performed long time-scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion-electrolyte Li7La3Zr2O12 (LLZO), using a first-principles parameterised interatomic potential. Collective lithium transport was analysed by identifying dynamical excitations; persistent ion displacements over distances comparable to the separation between lithium sites, and string-like clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO, and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.Conventional lithium-ion batteries rely on unstable liquid-organic polymer electrolytes, which pose practical limitations in terms of flammability, miniaturization, and safe disposal. A possible solution is to replace liquid electrolytes with inorganic ceramics that are electrochemically stable and non-flammable. The family of garnetlike oxides with general formula Li x M 3 M 2 O 12 , where M = La and M = Nb, Ta or Zr, have attracted significant attention in this regard due to their high lithiumion conductivity, high electrochemical stability window, and chemical stability with respect to metallic lithium [1,2]. The highly stuffed garnet Li 7 La 3 Zr 2 O 12 (LLZO) is the most studied member of this family, and can be considered prototypical. LLZO exhibits two phases with strikingly different ionic conductivities: a cubic phase (c-LLZO) that is adopted at high temperature (> 600 K) or stabilized by doping [3,4] with σ ≈ 10 −4 S cm −1 , and a tetragonal (t-LLZO) phase with σ ≈ 10 −6 S cm −1 that is favoured in the pure system at ambient temperature.The large difference in ionic conductivity between c-LLZO and t-LLZO is interesting from a mechanistic perspective because the pathways available for lithium transport are topologically identical in the two phases. Lithium ions move through an open three dimensional network of rings. Each ring comprises twelve alternating tetrahedral and octahedral sites (Fig. 1) [5], and the tetrahedral sites act as nodes connecting adjacent rings. In stoichiometric LLZO each ring accommodates on average eight lithium ions, which preferentially occupy all six octahedra and two of the tetrahedra. In the cubic phase the tetrahedral sites are equivalent and the lithium ions are disordered. In the tetragonal phase the tetrahedra are inequivalent due to the reduced crystal symmetry, and lithium occupies tetrahedral pairs aligned along Triangles represent tetrahedra, and rectangles represent octahedra. Arrows indicate neighbouring octahedra within conjoined rings. In c-LLZO lithium resides at off-center positions in the octa...
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