New Kagome Metal Sc₃Mn₃Al₇Si₅ and Its Gallium-Doped Analogues: Synthesis, Crystal Structure, and Physical Properties

We report the synthesis, crystal structure, and basic properties of the new intermetallic compound Sc₃Mn₃Al₇Si₅. The structure of the compound was established by single-crystal X-ray diffraction, and it crystallizes with a hexagonal structure (Sc₃Ni₁₁Si₄ type) with Mn atoms forming the Kagome nets. The dc magnetic susceptibility measurements reveal a Curie–Weiss moment of ∼0.51 μ_B/Mn; however, no magnetic order is found for temperatures as low as 1.8 K. Electrical resistivity and heat capacity measurements show that this compound is definitively metallic, with a relatively large specific heat Sommerfeld coefficient, indicating strong electronic correlations. Intriguingly, these features have revealed Sc₃Mn₃Al₇Si₅ as a possible quantum spin liquid. With chemical and lattice disorder introduced by doping, a spin liquid to spin glass transition is observed in the highest Ga-doped compounds. The roles of the geometrically frustrated structure and Mn-ligand hybridization in the magnetism of the title compounds are also discussed

Coordination Site Disorder in Spinel-Type LiMnTiO₄

LiMnTiO₄ was prepared through solid-state syntheses employing different heating and cooling regimes. Synchrotron X-ray and neutron powder diffraction data found quenched LiMnTiO₄ to form as single phase disordered spinel (space group <i>Fd</i>3̅<i>m</i>), whereas slowly cooled LiMnTiO₄ underwent partial phase transition from <i>Fd</i>3̅<i>m</i> to <i>P</i>4₃32. The phase behavior of quenched and slowly cooled LiMnTiO₄ was confirmed through variable-temperature synchrotron X-ray and neutron powder diffraction measurements. The distribution of Li between tetrahedral and octahedral sites was determined from diffraction data. Analysis of the Mn/Ti distribution in addition required Mn and Ti K-edge X-ray absorption near-edge structure spectra. These revealed the presence of Mn³⁺ in primarily octahedral and Ti⁴⁺ in octahedral and tetrahedral environments, with very slight variations depending on the synthesis conditions. Magnetic measurements indicated the dominance of antiferromagnetic interactions in both the slowly cooled and quenched samples below 4.5 K

Local Structure, Dynamics, and the Mechanisms of Oxide Ionic Conduction in Bi₂₆Mo₁₀O₆₉

We report the results of a computational and experimental study into the stabilized fluorite-type δ-Bi₂O₃-related phase Bi₂₆Mo₁₀O₆₉ aimed at clarifying the local and average structure, for which two distinct models have previously been proposed, and the oxide ionic diffusion mechanism, for which three distinct models have previously been proposed. Concerning the structure, we propose a new model in which some molybdenum atoms have higher coordination numbers than 4; that is, some MoO₅ trigonal bipyramids coexist with MoO₄ tetrahedra. This accounts for the additional oxygen required to achieve the nominal composition (a tetrahedron-only model gives Bi₂₆Mo₁₀O₆₈) without invoking a previously proposed unbonded interstitial site, which we found to be energetically unfavorable. All these MoO_<i>x</i> units are rotationally disordered above a first-order transition at 310 °C, corresponding to a first-order increase in conductivity. Concerning oxide ionic diffusion above that transition temperature, we found excellent agreement between the results of ab initio molecular dynamics simulations and quasielastic neutron scattering experiments. Our results indicate a mechanism related to that proposed by Holmes et al. (<i>Chem. Mater.</i> <b>2008</b>, <i>20</i>, 3638), with the role previously assigned to partially occupied interstitial oxygen sites played instead by transient but stable MoO₅ trigonal bipyramids and with more relaxed requirements in terms of the orientation and timing of the diffusive jumps

Synthesis and Characterization of the Crystal Structure and Magnetic Properties of the New Fluorophosphate LiNaCo[PO₄]F

The new compound LiNaCo­[PO₄]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo­[PO₄]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo­[PO₄]F crystallizes with orthorhombic symmetry, space group <i>Pnma</i>, with <i>a</i> = 10.9334(6), <i>b</i> = 6.2934(11), <i>c</i> = 11.3556(10) Å, and <i>Z</i> = 8. The structure consists of edge-sharing CoO₄F₂ octahedra forming CoFO₃ chains running along the <i>b</i> axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie–Weiss behavior above 60 K with θ = −21 K. The specific heat and magnetization measurements show that LiNaCo­[PO₄]F undergoes a three-dimensional magnetic ordering at <i>T</i>_<i>mag</i> = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO₃ chain along the <i>b</i>-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the <i>a</i>-direction but have a noncollinear arrangement along the <i>c</i>-direction. The noncollinear spin arrangement implies the presence of spin conflict along the <i>c</i>-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations

Giant Magnetoelastic Effect at the Opening of a Spin-Gap in Ba₃BiIr₂O₉

As compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba₃BiIr₂O₉, which contains 5d Ir⁴⁺ (<i>S</i> = 1/2) dimerized into isolated face-sharing Ir₂O₉ bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at <i>T</i>* = 74 K. The resulting first-order transition is characterized by a remarkable 4% increase in Ir–Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature)

Giant Magnetoelastic Effect at the Opening of a Spin-Gap in Ba₃BiIr₂O₉

As compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba₃BiIr₂O₉, which contains 5d Ir⁴⁺ (<i>S</i> = 1/2) dimerized into isolated face-sharing Ir₂O₉ bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at <i>T</i>* = 74 K. The resulting first-order transition is characterized by a remarkable 4% increase in Ir–Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature)

Key Role of Bismuth in the Magnetoelastic Transitions of Ba₃BiIr₂O₉ and Ba₃BiRu₂O₉ As Revealed by Chemical Doping

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉ has been confirmed by systematically replacing bismuth with La³⁺ and Ce⁴⁺. Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉ have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L₃-edge spectrum of Ba₃BiRu₂O₉). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials