Is La3Ni2O6.5 a Bulk Superconducting Nickelate?

Superconducting states onsetting at moderately high temperatures have been observed in epitaxially stabilized RENiO2-based thin films. However, recently, it has also been reported that superconductivity at high temperatures is observed in bulk La3Ni2O7-δ at high pressure, opening further possibilities for study. Here we report the reduction profile of La3Ni2O7 in a stream of 5% H2/Ar gas and the isolation of the metastable intermediate phase La3Ni2O6.45, which is based on Ni2+. Although this reduced phase does not superconduct at ambient or high pressures, it offers insights into the Ni-327 system and encourages future study of nickelates as a function of oxygen content.

The pressure-stabilized polymorph of indium triiodide.

A layered rhombohedral polymorph of indium(III) triiodide is synthesized at high pressure and temperature. The unit cell symmetry and approximate dimensions are determined by single crystal X-ray diffraction. Its R3̄ crystal structure, with a = 7.214 Å, and c = 20.47 Å, is refined by the Rietveld method on powder X-ray diffraction data. The crystal structure is based on InI6 octahedra sharing edges to form honeycomb lattice layers, though with considerable stacking defects. Different from ambient pressure InI3, which has a monoclinic molecular structure and a light-yellow color, high pressure InI3 is layered and has an orange color. The band gaps of both the monoclinic and rhombohedral variants of InI3 are estimated from diffuse reflectance measurements.

Large off-diagonal magnetoelectricity in a triangular Co2+-based collinear antiferromagnet.

Magnetic toroidicity is an uncommon type of magnetic structure in solid-state materials. Here, we experimentally demonstrate that collinear spins in a material with R-3 lattice symmetry can host a significant magnetic toroidicity, even parallel to the ordered spins. Taking advantage of a single crystal sample of CoTe6O13 with an R-3 space group and a Co2+ triangular sublattice, temperature-dependent magnetic, thermodynamic, and neutron diffraction results reveal A-type antiferromagnetic order below 19.5 K, with magnetic point group -3' and k = (0,0,0). Our symmetry analysis suggests that the missing mirror symmetry in the lattice could lead to the local spin canting for a toroidal moment along the c axis. Experimentally, we observe a large off-diagonal magnetoelectric coefficient of 41.2 ps/m that evidences the magnetic toroidicity. In addition, the paramagnetic state exhibits a large effective moment per Co2+, indicating that the magnetic moment in CoTe6O13 has a significant orbital contribution. CoTe6O13 embodies an excellent opportunity for the study of next-generation functional magnetoelectric materials.

Impersonating a Superconductor: High-Pressure BaCoO3, an Insulating Ferromagnet.

We report the high-pressure synthesis (6 GPa, 1200 °C) and ambient-pressure characterization of hexagonal HP-BaCoO3. The material (with a 2H crystal structure) has a short intrachain Co-Co distance of about 2.07 Å. Our magnetization investigation revealed robust diamagnetic behavior below approximately 130 K when the material was exposed to weak applied magnetic fields (10 Oe) and a distinct "half-levitation" phenomenon below that temperature, as is often observed for superconductors. Its field-dependent magnetization profile, however, unveils the characteristics of ferromagnetism, marked by a substantial magnetic retentivity of 0.22(1) µB/Co at a temperature of 2 K. Electrical resistivity measurements indicate that HP-BaCoO3 is a ferromagnetic insulator, not a superconductor.

Erbium-Excess Gallium Garnets.

A series of garnets of formula Er3+xGa5-xO12 are described, for which we report the crystal structures for both polycrystalline and single-crystal samples. The x limit in the garnet phase is between 0.5 and 0.6 under our conditions, with the Er fully occupying the dodecahedral (24c) garnet site plus some of the octahedral site (16a) in place of the Ga normally present. Long-range antiferromagnetic order with spin-ice-like frustration is suggested by the transition temperature (TN ≈ 0.8 K) being lower than the Curie-Weiss theta. The magnetic ordering temperature does not depend on the Er excess, but there is increasing residual entropy as the Er excess is increased, highlighting the potential for unusual magnetic behavior in this system. The field-dependent magnetic entropy trend is consistent with the reported behavior for frustrated triangular magnetic systems: an increasing transition temperature with a broader hump as the applied field increases [Xing, J.; Phys. Rev. Mater. 2019, 3(11), 114413;Filippi, J.; Solid State Commun. 1977, 23(9), 613-616; Bloxsom, J. A. Thermal and Magnetic Studies of Spin Ice Compounds. University College London, 2016].

Electron doping of a double-perovskite flat-band system.

Electronic structure calculations indicate that the Sr2FeSbO6 double perovskite has a flat-band set just above the Fermi level that includes contributions from ordinary subbands with weak kinetic electron hopping plus a flat subband that can be attributed to the lattice geometry and orbital interference. To place the Fermi energy in that flat band, electron-doped samples with formulas Sr2-xLaxFeSbO6 (0 ≤ x ≤ 0.3) were synthesized, and their magnetism and ambient temperature crystal structures were determined by high-resolution synchrotron X-ray powder diffraction. All materials appear to display an antiferromagnetic-like maximum in the magnetic susceptibility, but the dominant spin coupling evolves from antiferromagnetic to ferromagnetic on electron doping. Which of the three subbands or combinations is responsible for the behavior has not been determined.

Chemical Pressure Tuning Magnetism from Pyrochlore to Triangular Lattices.

Geometrically frustrated lattices combined with magnetism usually host quantum fluctuations that suppress magnetic orders and generate highly entangled ground states. Three-dimensionally (3D) frustrated magnets generally exist in the diamond and pyrochlore lattices, while two-dimensionally (2D) frustrated geometries contain Kagomé, triangular, and honeycomb lattices. In this work, we reported using chemical pressure to tune the magnetism of the pyrochlore lattice in LiYbSe2 into a triangular lattice by doping Ga or In. Li3-xGaxYb3Se6 and Li3-xInxYb3-yInySe6/Li3-xInxYb3-y□ySe6 crystallize in a trigonal α-NaFeO2 structure-type (space group R3̅m) and can be synthesized using either LiCl or Se flux. In Li3-xGaxYb3Se6, Ga3+ and Li+ are mixed, leaving Yb3+ on the triangular plane. Instead of just Li+ being replaced in Li3-xGaxYb3Se6, In3+ was observed in both the Li+ and Yb3+ layers in Li3-xInxYb3-yInySe6 depending on the reaction conditions. Dominant antiferromagnetic interactions are revealed by magnetic measurements in both Li3-xGaxYb3Se6 and Li3-xInxYb3-yInySe6/Li3-xInxYb3-y□ySe6. However, no long-range magnetic order is detected in thermomagnetic measurements above 1.8 K due to geometrical frustration. Thus, Li3-xGaxYb3Se6, Li3-xInxYb3-yInySe6/Li3-xInxYb3-y□ySe6, and the LiYbSe2 previously discovered by our group provide an ideal platform to understand the complex structure-magnetism correlations from 3D to 2D frustrated lattices.

The non-centrosymmetric layered compounds IrTe2I and RhTe2I.

The previously unreported layered compounds IrTe2I and RhTe2I were prepared by a high-pressure synthesis method. Single crystal X-ray and powder X-ray diffraction studies find that the compounds are isostructural, crystallizing in a layered orthorhombic structure in the non-centrosymmetric, non-symmorphic space group Pca21 (#29). Characterization reveals diamagnetic, high resistivity, semiconducting behavior for both compounds, consistent with the +3 chemical valence and d6 electronic configurations for both iridium and rhodium and the Te-Te dimers seen in the structural study. Electronic band structures are calculated for both compounds, showing good agreement with the experimental results.

Ferromagnetic Double Perovskite Semiconductors with Tunable Properties.

The authors successfully dope the magnetically silent double perovskite semiconductor Sr2 GaSbO6  to induce ferromagnetism and tune its bandgap, with Ga3+ partially substituted by the magnetic trivalent cation Mn3+ , in a rigid cation ordering with Sb5+ . The new ferromagnetic semiconducting Sr2 Ga1- x Mnx SbO6  double perovskite, which crystallizes in tetragonal symmetry (space group I4/m) and has tunable ferromagnetic ordering temperature and bandgap, suggests that magnetic ion doping of double perovskites is a productive avenue toward obtaining materials for application in next-generation oxide-based spintronic devices.

Honeycomb-Structure RuI3 , A New Quantum Material Related to α-RuCl3.

The layered honeycomb lattice material α-RuCl3 has emerged as a prime candidate for displaying the Kitaev quantum spin liquid state, and as such has attracted much research interest. Here a new layered honeycomb lattice polymorph of RuI3 , a material that is strongly chemically and structurally related to α-RuCl3 is described. The material is synthesized at moderately elevated pressures and is stable under ambient conditions. Preliminary characterization reveals that it is a metallic conductor, with the absence of long-range magnetic order down to 0.35 K and an unusually large T-linear contribution to the heat capacity. It is proposed that this phase, with a layered honeycomb lattice and strong spin-orbit coupling, provides a new route for the characterization of quantum materials.

K3Ir2O6 and K16.3Ir8O30, Low-Dimensional Iridates with Infinite IrO6 Chains.

A previously unreported 1D iridate, K3Ir2O6, has been grown by a flux method in O2-rich environment, and its crystal structure was determined via single crystal structural analysis. It exhibits straight chains of face-sharing [IrO6] octahedra, which are arranged along the crystallographic c axis, separated by nonmagnetic K ions. No magnetic transitions are observed during measured range, and the material is electrically insulating. Potentially interesting electronic behavior for K3Ir2O6 is supported by electronic structure calculations. A structurally related material, K16.3Ir8O30, which displays similar fundamental geometric units but in a different spatial arrangement-zigzag chains-based on edge and face sharing [IrO6] octahedra, is also reported. Both materials are of interest for probing the properties of a 1D system with strong spin-orbit coupling.

Reduction-induced CO dissociation by a [Mn(bpy)(CO)4][SbF6] complex and its relevance in electrocatalytic CO2 reduction.

[Mn(bpy)(CO)3Br] is recognized as a benchmark electrocatalyst for CO2 reduction to CO, with the doubly reduced [Mn(bpy)(CO)3]- proposed to be the active species in the catalytic mechanism. The reaction of this intermediate with CO2 and two protons is expected to produce the tetracarbonyl cation, [Mn(bpy)(CO)4]+, thereby closing the catalytic cycle. However, this species has not been experimentally observed. In this study, [Mn(bpy)(CO)4][SbF6] (1) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO2 to CO in the presence of H2O. Complex 1 was characterized using X-ray crystallography as well as IR and UV-Vis spectroscopy. The redox activity of 1 was determined using cyclic voltammetry and compared with that of benchmark manganese complexes, e.g., [Mn(bpy)(CO)3Br] (2) and [Mn(bpy)(CO)3(MeCN)][PF6] (3). Infrared spectroscopic analyses indicated that CO dissociation occurs after a single-electron reduction of complex 1, producing a [Mn(bpy)(CO)3(MeCN)]+ species. Complex 1 was experimentally verified as both a precatalyst and an on-cycle intermediate in homogeneous Mn-based electrocatalytic CO2 reduction.

VI3 -a New Layered Ferromagnetic Semiconductor.

2D materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here the discovery of ferromagnetism is reported in a layered van der Waals semiconductor, VI3 , which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI3 -type structure ( R 3 ¯ , No.148) at room temperature, and the experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical bandgap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.

Revisiting the optimized doping ratio in core/shell nanostructured upconversion particles.

The development of rare-earth doped upconversion nanoparticles (RE-UCNPs) in various applications is fuelling the demand for nanoparticles with highly enhanced upconversion luminescence (UCL). Although the core/shell structure is proved to enhance the UCL effectively, there is still plenty of room to further improve the UCL by optimizing the doping ratio of the materials. In this article, a general strategy is demonstrated to achieve highly-enhanced visible UCL in core/shell nanostructured NaREF4 by increasing the doping ratio of Yb3+ in the core region. The energy transfer from RE-UCNPs to surface quenching sites through Yb3+-Yb3+ energy migration is demonstrated to be the main reason for restricting the doping ratio of Yb3+. Notable UCL enhancement (ca. 15 times) of core/shell structured α-NaYF4:Yb,Er@CaF2 nanoparticles is observed by increasing the concentration of Yb3+ to 98 mol%. The highly-enhanced visible UCL signal is used to guide the lymphatic vessel resection with the naked eye.