Crystal and Magnetic Structures of the Chain Antiferromagnet CaFe₄Al₈

The crystal structure of CaFe₄Al₈ was studied by X-ray single crystal and powder diffraction as well as high-resolution neutron powder diffraction. CaFe₄Al₈ crystallizes with a tetragonal CeMn₄Al₈-type structure, an ordered variant of the ThMn₁₂-type (Pearson symbol <i>tI</i>26, space group <i>I</i>4/<i>mmm</i>, <i>a</i> = 8.777(1), <i>c</i> = 5.077(1) Å). Similarly to the well-known A15-type superconductors, the structure of CaFe₄Al₈ contains one-dimensional chains of <i>d</i>-metal atoms, which are parallel to the crystallographic fourfold axis. CaFe₄Al₈ is paramagnetic at room temperature and exhibits long-range antiferromagnetic ordering at about 180 K, combined with a short-range ordered spin arrangement. The magnetic structure, determined by powder neutron diffraction at 4 K, shows that the magnetic moments on the Fe atoms form mirror-inverted chains along the <i>c</i>-direction and are slightly canted from the axis

Correlating Transport and Structural Properties in Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) Prepared from Aqueous Solution

Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe₂(PO₄)₃ and AlPO₄. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li⁺ incorporation in the material and the actual Li⁺ content in the sample increases with the nominal Li⁺ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ solid solution

Correlating Transport and Structural Properties in Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) Prepared from Aqueous Solution

Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe₂(PO₄)₃ and AlPO₄. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li⁺ incorporation in the material and the actual Li⁺ content in the sample increases with the nominal Li⁺ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ solid solution

Lithium Diffusion Pathways in 3R-Li_<i>x</i>TiS₂: A Combined Neutron Diffraction and Computational Study

Layered lithium transition-metal sulfides have long been discussed as early electrode materials for lithium-ion batteries. However, fundamental knowledge of lithium-ion migration in these solids is still lacking. In this study, we report on the diffusion dynamics in lithium-deficient high-temperature polymorphs of lithium titanium sulfides (3R-Li_<i>x</i>TiS₂; <i>x</i> = 0.7, 0.9) as analyzed using powder neutron diffractometry and density functional theory (DFT) climbing-image nudged-elastic-band (cNEB) calculations. Two classes of probable migration pathways have been identified from the scattering-length density distributions (filtered using the maximum-entropy method [MEM]) and the probability density functions (PDFs, modeled from anharmonic Debye–Waller factors): direct diffusion in the (001) plane as the major mechanism and indirect diffusion through adjacent tetrahedral voids as a minor mechanism. Calculated activation barriers agree well with one-particle potentials (OPPs) derived from measurements for Li_0.7TiS₂ (0.484[14] and 0.88[4] eV) but deviate for Li_0.9TiS₂. The discrepancy at low defect concentration is attributed to the failure of the OPP derivation and the different nature of the methods (space-time averaged vs individual-ion perspective). This work elucidates the pathways of lithium-ion diffusion in 3R-Li_<i>x</i>TiS₂ and points out pitfalls in established experimental/computational methods

Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Li-oxide garnets such as Li₇La₃Zr₂O₁₂ (LLZO) are among the most promising candidates for solid-state electrolytes to be used in next-generation Li-ion batteries. The garnet-structured cubic modification of LLZO, showing space group <i>Ia</i>-3<i>d</i>, has to be stabilized with supervalent cations. LLZO stabilized with Ga³⁺ shows superior properties compared to LLZO stabilized with similar cations; however, the reason for this behavior is still unknown. In this study, a comprehensive structural characterization of Ga-stabilized LLZO is performed by means of single-crystal X-ray diffraction. Coarse-grained samples with crystal sizes of several hundred micrometers are obtained by solid-state reaction. Single-crystal X-ray diffraction results show that Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂ with <i>x</i> > 0.07 crystallizes in the acentric cubic space group <i>I</i>-43<i>d</i>. This is the first definite record of this cubic modification for LLZO materials and might explain the superior electrochemical performance of Ga-stabilized LLZO compared to its Al-stabilized counterpart. The phase transition seems to be caused by the site preference of Ga³⁺. ⁷Li NMR spectroscopy indicates an additional Li-ion diffusion process for LLZO with space group <i>I</i>-43<i>d</i> compared to space group <i>Ia</i>-3<i>d</i>. Despite all efforts undertaken to reveal structure–property relationships for this class of materials, this study highlights the potential for new discoveries

Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Li-oxide garnets such as Li₇La₃Zr₂O₁₂ (LLZO) are among the most promising candidates for solid-state electrolytes to be used in next-generation Li-ion batteries. The garnet-structured cubic modification of LLZO, showing space group <i>Ia</i>-3<i>d</i>, has to be stabilized with supervalent cations. LLZO stabilized with Ga³⁺ shows superior properties compared to LLZO stabilized with similar cations; however, the reason for this behavior is still unknown. In this study, a comprehensive structural characterization of Ga-stabilized LLZO is performed by means of single-crystal X-ray diffraction. Coarse-grained samples with crystal sizes of several hundred micrometers are obtained by solid-state reaction. Single-crystal X-ray diffraction results show that Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂ with <i>x</i> > 0.07 crystallizes in the acentric cubic space group <i>I</i>-43<i>d</i>. This is the first definite record of this cubic modification for LLZO materials and might explain the superior electrochemical performance of Ga-stabilized LLZO compared to its Al-stabilized counterpart. The phase transition seems to be caused by the site preference of Ga³⁺. ⁷Li NMR spectroscopy indicates an additional Li-ion diffusion process for LLZO with space group <i>I</i>-43<i>d</i> compared to space group <i>Ia</i>-3<i>d</i>. Despite all efforts undertaken to reveal structure–property relationships for this class of materials, this study highlights the potential for new discoveries

Flux Synthesis, Crystal Structures, and Magnetic Ordering of the Rare-Earth Chromium(II) Oxyselenides RE₂CrSe₂O₂ (RE = La–Nd)

The rare-earth chromium­(II) oxyselenides RE₂CrSe₂O₂ (RE = La–Nd) were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined by single-crystal and powder X-ray diffraction (Pb₂HgCl₂O₂-type, <i>C</i>2/<i>m</i>, <i>Z</i> = 2). The magnetic structure of La₂CrSe₂O₂ was solved and refined from neutron powder diffraction data. Main building blocks are chains of edge-sharing CrSe₄O₂ octahedra linked together by two edge-sharing ORE₃Cr tetrahedra forming infinite ribbons. The Jahn–Teller instability of divalent Cr²⁺ (d⁴) leads to structural phase transitions at 200 and 130 K in La₂CrSe₂O₂ and Ce₂CrSe₂O₂, respectively. RE₂CrSe₂O₂ are Curie–Weiss paramagnetic above <i>T</i>_N ≈ 14–17 K. Neutron powder diffraction reveals anti-ferromagnetic ordering of the Cr²⁺ moments in La₂CrSe₂O₂ below <i>T</i>_N = 12.7(3) K with an average ordered moment of 3.40(4) μ_B/Cr²⁺ at 4 K, which was confirmed by muon spin rotation experiments