Browsing by Author "Kim, YI"

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    Crystal structure analysis of tungsten bronzes beta-SrTa(2)O(6) and beta '-SrTa(2)O(6) by synchrotron X-ray and neutron powder diffraction
    (Academic Press Inc Elsevier Science, 2012-07-01) Lee, E; Park, CH; Shoemaker, DP; Avdeev, M; Kim, YI
    Strontium ditantalum oxide SrTa(2)O(6) exists in alpha-, beta-, and beta'-polymorphs. Herein the crystal structures of the latter two were studied using synchrotron X-ray and constant-wavelength neutron powder diffraction. While beta'-SrTa(2)O(6) [space group P4/mbm, a=12.47099(1) angstrom, c=3.898210(5) angstrom, V=606.271(2) angstrom(3), Z=5] belongs to the regular tetragonal tungsten bronze (TTB) family, it contains locally disordered strontium atoms within the pentagonal channel. beta-SrTa(2)O(6) [space group Pnam, a = 12.36603(2) angstrom, b=12.43467(2) angstrom, c=7.72403(1) angstrom, V=1187.705(4) angstrom(3), Z=10] can be described as an orthorhombic modification of the TTB, where the octahedral tilting distortion effectively alleviates the bonding strains around TaO(6) and SrO(13) polyhedra. For comparison, rynersonite type alpha-SrTa(2)O(6) [space group Prima, a=11.00610(6) angstrom, b=7.63397(3) angstrom, c=5.62634(3) angstrom, V=472.727(5) angstrom(3), and Z=4] is built from edge-shared dimer units of TaO(6) octahedra. As measured by diffuse-reflection absorption spectroscopy, alpha-, beta- and beta'-SrTa(2)O(6) have indirect band gap energies of 4.4, 4.0, and 3.8 eV, respectively. © 2012, Elsevier Ltd.
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    Crystal structure and optical property of complex perovskite oxynitrides ALi0.2Nb0.8O2.8N0.2, ANa0.2Nb0.8O2.8N0.2, and AMg0.2Nb0.8O2.6N0.4 (A = Sr, Ba)
    (Elsevier, 2017-10-01) Moon, KH; Avdeev, M; Kim, YI
    Oxynitride type complex perovskites AM0.2Nb0.8O3−xNx (A = Sr, Ba; M = Li, Na, Mg) were newly synthesized by the solid state diffusion of Li+, Na+, or Mg2+ into the layered oxide, A5Nb4O15, with concurrent O/N substitution. Neutron and synchrotron X-ray Rietveld refinement showed that SrLi0.2Nb0.8O2.8N0.2, SrNa0.2Nb0.8O2.8N0.2, and SrMg0.2Nb0.8O2.6N0.4 had body-centered tetragonal symmetry (I4/mcm), while those with A = Ba had simple cubic symmetry (Pm ̅ m). In the tetragonal Sr-compounds, the nitrogen atoms were localized on the c-axial 4a site. However, the octahedral cations, M/Nb (M = Li, Na, Mg) were distributed randomly in all six compounds. The lattice volume of AM0.2Nb0.8O3−xNx was dependent on various factors including the type of A and the electronegativity of M. Compared to the simple perovskites, ANbO2N (A = Sr, Ba), AM0.2Nb0.8O3−xNx had wider band gaps (1.76–2.15 eV for A = Sr and 1.65–2.10 eV for A = Ba), but significantly lower sub-gap absorption. © 2017 Elsevier Inc.
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    Crystal structures and color properties of new complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba)
    (Royal Society of Chemistry, 2016-02-26) Moon, KH; Kim, JM; Sohn, Y; Cho, DW; Kim, YI; Avdeev, M
    New complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba) were synthesized by reacting A5Ta4O15 with MgCl2 in flowing NH3. The formation of AMg0.2Ta0.8O2.6N0.4 can be described as the cooperative insertion of Mg2+ + 2N3− and the release of 2O2− from the layered oxide, A5Ta4O15. Rietveld refinement of the neutron and synchrotron X-ray diffraction patterns indicated that BaMg0.2Ta0.8O2.6N0.4 is simple cubic (Pm[3 with combining macron]m) and SrMg0.2Ta0.8O2.6N0.4 is body-centered tetragonal (I4/mcm), isostructural with BaM0.2Ta0.8O2.8N0.2 and SrM0.2Ta0.8O2.8N0.2 (M = Li, Na), respectively. The nitrogen in SrMg0.2Ta0.8O2.6N0.4 is partially ordered favoring the c-axial site over the ab-plane of the tetragonal cell, which is a different O/N order pattern from that of SrLi0.2Ta0.8O2.8N0.2 and SrNa0.2Ta0.8O2.8N0.2. The group of simple and complex perovskites, ATaO2N, ALi0.2Ta0.8O2.8N0.2, ANa0.2Ta0.8O2.8N0.2, and AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba) covers the band gap range, 1.9–2.4 eV, and the color ranges from yellow to dark brown. © Royal Society of Chemistry 2021
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    Defect control and ionic conductivity of oxynitride perovskite Sr0.83Li0.17Ta0.83O1.88N0.74
    (Elsevier, 2022-01) Kim, YI; Avdeev, M
    Perovskite lattice was tailored by introducing site vacancies and mixed anion composition, to produce Sr0.83Li0.17Ta0.83O1.88N0.74 (Li02N). Further, Li02N was converted to a defect oxide Sr0.83Li0.17Ta0.83O3 (Li02O) by applying an optimized treatment: heating in air at 1173 K for 2 h. According to the neutron Rietveld refinement, Li02N and Li02O are tetragonal and orthorhombic, respectively, where the lattice volume of Li02O is significantly smaller than that of Li02N. The ionic conductivity (σion) of Li02N and Li02O was evaluated by the ac impedance spectroscopy and the equivalent circuit analysis. Both Li02N (σion = 10−5.5 S/cm at 671 K) and Li02O (σion = 10−6.2 S/cm at 667 K) exhibited an Arrhenius behavior of ionic conductivity with activation energies of 0.87 eV and 0.75 eV, respectively. It is interpreted that the nitride component enhances the ionic conduction of Li02N, while the vacancy of the anion lattice makes an opposite effect. © 2021 Elsevier Ltd and Techna Group S.r.l.
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    Intercalation route to complex perovskites AM0.2Ta0.8O2.8N0.2 (A = Sr, Ba; M = Li, Na): neutron diffraction and nuclear magnetic resonance study
    (American Chemical Society, 2014-12-03) Kim, YI; Paik, Y; Avdeev, M
    Oxynitride-type complex perovskites, AM0.2Ta0.8O2.8N0.2 (A = Sr, Ba; M = Li, Na), were synthesized by the ammonolytic heating of a layered perovskite, A5Ta4O15, with 0.5M2CO3. A Rietveld refinement of the synchrotron X-ray and neutron powder diffraction patterns confirmed the complete structural transformation from a hexagonal layered-perovskite to a three-dimensional perovskite type, as well as the stabilization of alkali cations on the octahedral sites rather than on the dodecahedral sites in the latter. In all four compounds, M+ and Ta5+ were disordered completely despite a charge difference as much as 4. The crystal symmetry of the average structure depended on the size of the dodecahedral cation: simple cubic for BaM0.2Ta0.8O2.8N0.2 and body-centered tetragonal for SrM0.2Ta0.8O2.8N0.2. This trend coincides with the symmetry transition from BaTaO2N (Pm3̅m) to SrTaO2N (I4/mcm). In both SrM0.2Ta0.8O2.8N0.2, nitrogen atoms preferentially occupied the c-axial 4a site of the tetragonal cell. Solid state magic angle spinning nuclear magnetic resonance spectroscopy showed that SrNa0.2Ta0.8O2.8N0.2 and BaNa0.2Ta0.8O2.8N0.2 exhibited marked downfield shifts of 23Na, manifesting an octahedral coordination. On the other hand, the 7Li NMR of SrLi0.2Ta0.8O2.8N0.2 and BaLi0.2Ta0.8O2.8N0.2 indicated a highly symmetrical coordination environment of Li. © 2014 American Chemical Society
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    Li+ conductivity of tungsten bronze LixSr1−0.5xTa2O6 studied by neutron diffraction analysis
    (Royal Society of Chemistry, 2018-05-04) Han, HD; Avdeev, M; Kim, YI
    The crystal structures of tetragonal tungsten bronze, LixSr1−0.5xTa2O6 (x = 0.08, 0.17, 0.25) were investigated by neutron diffraction analysis, focusing on the geometry of the oxide framework as well as the Li distribution. The Rietveld refinement and Fourier mapping of nuclear density indicated that Li atoms are distributed in the pentagonal and rectangular channels of the tungsten bronze lattice, which provide 15-fold and 12-fold coordinated cavities, respectively. Those cavities are interconnected to form a three-dimensional network, which can serve as the Li+ conduction pathway in LixSr1−0.5xTa2O6. It is proposed that the rhombic faces of the 12-coordinated polyhedra (distorted cuboctahedra) act as the bottleneck for long-range Li+ migration.© Open Access CC-NC licence - The Royal Society of Chemistry 2018
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    Synthesis, crystal structure, and magnetic properties of oxynitride perovskites SrMn0.2M0.8O2.6N0.4 (M = Nb, Ta)
    (Royal Society of Chemistry, 2020-04-28) Kim, YI; Avdeev, M
    Complex perovskites SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were synthesized; these are rare examples of octahedral Mn2+ in oxynitride perovskites. Joint Rietveld refinement of neutron and X-ray data revealed that SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 had orthorhombic symmetries, in contrast with those of analogous perovskites SrM′0.2M0.8O3−xNx (M′ = Li, Na, Mg; M = Nb, Ta), which are all tetragonal. Both SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 exhibited paramagnetic behavior with effective magnetic moments of 5.60μB and 5.94μB, respectively, consistent with a high-spin Mn2+ (d5, S = 5/2) state. The Weiss constants were −24.7 K for SrMn0.2Nb0.8O2.6N0.4 and −15.4 K for SrMn0.2Ta0.8O2.6N0.4, indicating the presence of weak antiferromagnetic spin–spin interactions. The band gaps of SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were determined to be 1.75 eV and 2.2 eV, respectively, suggesting that the Mn 3d electrons were essentially localized. © The Royal Society of Chemistry 2020
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    Synthesis, structural distortion, and magnetic property of complex perovskites AMn0.2M0.8O2.6N0.4 (A = Sr, Ba; M = Nb, Ta)
    (Elsevier, 2022-01-01) Kim, YI; Avdeev, M
    Oxynitride complex perovskites, BaMn0.2M0.8O2.6N0.4 (M ​= ​Nb, Ta), were prepared by reacting the layered oxides Ba5M4O15 with MnCl2, in the NH3 atmosphere. Both BaMn0.2M0.8O2.6N0.4 phases crystallized in orthorhombic symmetries, in contrast with previous Ba-based perovskite oxynitrides, BaMO2N and BaM'0.2M0.8O3−xNx (M′ ​= ​Li, Na, Mg; M ​= ​Nb, Ta), all of which were cubic. The thermogravimetry (TG) and differential scanning calorimetry (DSC) of AMn0.2M0.8O2.6N0.4 (A ​= ​Sr, Ba; M ​= ​Nb, Ta) suggested that the phase stability is higher for A ​= ​Ba and M ​= ​Ta than for A ​= ​Sr and M ​= ​Nb, respectively. Both BaMn0.2Nb0.8O2.6N0.4 and BaMn0.2Ta0.8O2.6N0.4 exhibited paramagnetic behavior with effective magnetic moments of 5.75 μB and 5.90 μB, respectively, well consistent with the high-spin Mn2+ state. All the four members of AMn0.2M0.8O2.6N0.4 had negative Weiss constants (θW's), indicative of the antiferromagnetic interactions. The variations of the θW among AMn0.2M0.8O2.6N0.4 were attributable to the differences in the Mn–O bond lengths (A ​= ​Sr vs. Ba) or to the distinct lattice covalency (M ​= ​Nb vs. Ta). © 2022 Elsevier Inc
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    Tunnel structure of tetragonal tungsten bronzes BaTa2O6, Ba0. 8Ta2O5.8, and Ba0.5Ta2O5.5 studied using synchrotron x-ray and neutron diffraction
    (Elsevier, 2019-09-24) Kim, NG; Avdeev, M; Kim, YI
    The complex oxide system of Ba-Ta-O presents interesting cases of tetragonal tungsten bronze (TTB) phases being formed over a wide compositional range of Ba1−xTa2O6−x, 0 ≤ x ≤ 0.65 [X. Kuang et al., Inorg. Chem., 2013, 52, 13244−13252]. Compared to the common formalism of the TTB phase AB2O6, Ba1−xTa2O6−x is highly off stoichiometric and is presumed to involve substantial structural modifications. In this study, Ba1−xTa2O6−x samples were prepared with the chemical compositions of BaTa2O6 (x = 0), Ba0.8Ta2O5.8 (x = 0.2), and Ba0.5Ta2O5.5 (x = 0.5). Rietveld refinement and Fourier electron density calculation, using high-resolution synchrotron X-ray diffraction data, revealed that the above three compositions have distinct local structural features, and the irregularity escalates with the increase of x. In all the three compositions, the Ba position inside the pentagonal tunnel (A2 site) was lightly split, reflecting the oversize polyhedral cavity. Additionally, Ba0.8Ta2O5.8 and Ba0.5Ta2O5.5 contained segments of (TaO)3+ chains in the pentagonal tunnel and Ta5+ in the triangular tunnel (A3 site). Hence, the unit cell structure of BaTa2O6, Ba0.8Ta2O5.8, and Ba0.5Ta2O5.5 can be represented as Ba5[Ta10O30], Ba4.26(TaO)0.04Ta0.22[Ta10O30], and Ba2.84(TaO)1.19Ta0.18[Ta10O30], respectively. Dielectric constants (κ) of Ba1−xTa2O6−x were measured in the temperature range of 30–450 K and at frequencies of 100 kHz−1 MHz. Compared to BaTa2O6, which displayed high κ (≈100 at 300 K) and broad relaxation peaks below 50 K, Ba0.8Ta2O5.8 and Ba0.5Ta2O5.5 exhibited markedly reduced lattice polarizations. © 2019 Elsevier B.V.