Identification of high ionic conducting members (M(III) = Co, Ni) in the new lithium rich Li4.50M0.50TeO6 (M(III) = Co, Ni, In) series.

Many new lithium-excess compounds with rock-salt related structures have been extensively studied in recent years to discover high-capacity electrode materials for lithium-ion batteries. In the present work, lithium rich layered tellurates, Li4.50M0.50TeO6 (M(III) = Co, Ni, In), are added to the existing series of Li4.50M0.50TeO6 (M(III) = Cr, Mn, Fe, Al, and Ga) oxides. Structural investigations revealed their stabilization in the space group C2/m with a new cationic ordering. The structure consists of (Li1.50M0.50TeO6)3- honeycomb arrays along the ab plane by the edge sharing of TeO6 with (Li/M)O6 octahedra. The honeycomb arrays are separated by the intermediate layer of Li alone in Li4.50Co0.50TeO6. On the other hand, in the Ni and In analogues, the interlayer region consists of Li with Te, and Li with In ions, respectively. XPS studies confirmed the +3 oxidation state of Co and Ni ions. The appearance of a strong band at 680 nm resulting from LMCT (O → Co) in the UV-vis DRS data of the Li4.50Co0.50TeO6 sample further indicated the presence of Co3+ (d6, low spin) ions. The absence of characteristic Ni2+ bands at around ∼650 and 740 nm supported Ni3+ ions. Li4.50Co0.50TeO6 showed diamagnetic behaviour, while Li4.50Ni0.50TeO6 displayed paramagnetic nature. A negative θ (-14(2)) K has been obtained in the temperature region of 300-100 K for Li4.50Ni0.50TeO6 representing dominant antiferromagnetic interactions. At 2 K, Li4.50Ni0.50TeO6 unveiled a non-linear trend with no significant hysteresis and nearly saturation at 5 T field indicating the existence of additional interactions. Li4.50Co0.50TeO6 and Li4.50Ni0.50TeO6 exhibited significant conductivity values of 0.016 and 0.003 S cm-1, respectively, at 300 °C, thereby opening up further studies in this direction.

Evidence of Long-Range and Short-Range Magnetic Ordering in the Honeycomb Na3Mn2SbO6 Oxide.

We present a comprehensive study of the synthesis, structure, and magnetic properties of the honeycomb oxide Na3Mn2SbO6 supported by neutron diffraction, heat capacity, and magnetization measurements. The refinements of the neutron diffraction patterns (150, 50, and 45 K) using the Rietveld method confirm the monoclinic (S. G. C2/m) structure. Temperature-dependent magnetic susceptibilities measured at varying fields along with the heat capacity measurements demonstrate the coexistence of long-range ordering (∼42 K) and short-range ordering (∼65 K). The field-dependent isothermal magnetization measurements at 5 K indicate a spin-flop transition around 5 T. Rietveld refinements of the low-temperature (below 45 K) neutron diffraction data further confirm the long-range magnetic ordering. In addition, the temperature variation of the lattice parameters obtained from the neutron powder diffraction analysis exhibited a distinct anomaly near the antiferromagnetic transition temperature. The appearance of the concomitant broadened backgrounds in the neutron powder diffraction data collected at 80, 50, and 45 K supports the short-range ordering. The resultant magnetic structure consists of spins that are aligned antiparallel with the nearest neighbors and also with the spins of the adjacent honeycomb layers. The occurrence of a fully ordered magnetic ground state (Neel antiferromagnetic (AFM)) in Na3Mn2SbO6 consolidates the significance of fabricating new honeycomb oxides.

Synergistic Influence of d0 (Nb5+) and d10 (Cd2+) Cations in Stabilizing Noncentrosymmetric Dion-Jacobson Layered Perovskites, A'Cd2Nb3O10 (A' = Rb, Cs).

New Dion-Jacobson (n = 3) layered perovskites, A'Cd2Nb3O10 (A' = Rb, Cs), have been synthesized by a solid-state method. Powder X-ray diffraction measurements confirm the noncentrosymmetric orthorhombic (space group Ima2) structures for both rubidium- and cesium-containing layered oxides. The distorted octahedral coordination of the d0 metal cations (Nb5+) coupled with the increased covalency in the lattice by the introduction of d10 metal cations (Cd2+) is responsible for the acentric structures. The resulting second-harmonic-generation (SHG) efficiencies of the polycrystalline samples (size 45-63 µm) using 1064 nm radiation reveal comparable values for CsCd2Nb3O10 and nearly 5 times higher output values for RbCd2Nb3O10 with respect to potassium dihydrogen phosphate. These structures were further confirmed from transmission electron microscopy and Raman spectroscopy measurements. The optical characteristics show interesting variations to the expected photocatalytic activities. Ion-exchange reactions result in the synthesis of proton- and lithium-containing oxides, which are otherwise inaccessible by direct solid-state reactions. The mobilities of the interlayer ions have also been confirmed by ionic conductivity measurements.

Synthesis and Characterization of Sodium-Iron Antimonate Na2FeSbO5: One-Dimensional Antiferromagnetic Chain Compound with a Spin-Glass Ground State.

A new oxide, sodium-iron antimonate, Na2FeSbO5, was synthesized and structurally characterized, and its static and dynamic magnetic properties were comprehensively studied both experimentally by dc and ac magnetic susceptibility, magnetization, specific heat, electron spin resonance (ESR) and Mössbauer measurements, and theoretically by density functional calculations. The resulting single-crystal structure (a = 15.6991(9) Å; b = 5.3323 (4) Å; c = 10.8875(6) Å; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The 57Fe Mössbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in a distorted tetrahedral oxygen coordination. The magnetic susceptibility and specific heat data show the absence of a long-range magnetic ordering in Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates spin-glass-type behavior with a unique two-step freezing at Tf1 ≈ 80 K and Tf2 ≈ 35 K. Magnetic hyperfine splitting of 57Fe Mössbauer spectra was observed below T* ≈ 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T < T*) exhibit a relaxation behavior caused by critical spin fluctuations, indicating the existence of short-range correlations. The stochastic model of ionic spin relaxation was used to account for the shape of the Mössbauer spectra below the freezing temperature. A complex slow dynamics is further supported by ESR data revealing two different absorption modes presumably related to ordered and disordered segments of spin chains. The data imply a spin-cluster ground state for Na2FeSbO5.

Catalytic Application of Oxygen Vacancies Induced by Bi3+ Incorporation in ThO2 Samples Obtained by Solution Combustion Synthesis.

Recognizing immense advantages of solution-based combustion synthesis, its applicability to determine the extent of dissolution of Bi3+ in fluorite-structured thoria has been examined to generate high-surface-area samples with massive defects. Up to 50 mol % of thorium could be substituted with bismuth retaining fluorite structure beyond which phase separation occurred. The lattice parameters from Le-Bail refinements of their powder X-ray diffraction patterns showed marginal increase with increase in bismuth content, suggesting the competing effect between the size of the cation and the oxygen vacancy concentration. Energy-dispersive X-ray spectrometry analysis and high-resolution transmission electron microscopy measurements have also confirmed the composition and structure of the limiting composition. With progressive bismuth content, the band due to the fluorite (at 460 cm-1) diffused and a defect band in the region 570-600 cm-1 emerged in the Raman spectra. From these changes, the oxygen vacancy concentrations in these samples have been determined, which increased with increase in bismuth content. Absorbance in the visible region was noticed for bismuth-containing samples, and band gap values determined from the Kubelka-Munk function were in the range 2.34-3.24 eV. In addition to the blue emission from oxygen vacancies, 3P1 → 1S0 transition of Bi3+ was noticed in the photoluminescence spectrum. From Brunauer-Emmett-Teller measurements, the surface area of Th0.50Bi0.50O2-δ obtained by solution combustion synthesis was measured to be 265.74 m2 g-1, higher than the value (39.00 m2 g-1) for the sample prepared by solid-state synthesis. All of these factors combined with oxygen vacancies as defect centers have been found to play critical control over their use as catalyst for the reductive transformation of nitroaromatics and oxidative decolorization of organic dye molecules (methyl orange and xylenol orange). A nice correlation between oxygen vacancy concentration and pseudo first-order rate constants of these catalytic conversions has been arrived. The catalyst was found to retain its efficiency up to four cycles without undergoing any structural change during these experiments.

Precursor driven one pot synthesis of wurtzite and chalcopyrite CuFeS2.

A facile precursor dependent single step, one pot solution based synthesis of wurtzite and chalcopyrite polymorphs of CuFeS2 has been developed by reacting a Cu(I) thiourea complex with Fe2(SO4)3 and FeCl3 separately in ethylene glycol. The phases have been characterized by structural refinements, SEM-EDX, TEM-SAED, Raman, UV-Visible spectroscopy and TGA measurements.

Structural defects cause TiO2-based photocatalysts to be active in visible light.

Oxidation of TiO or Ti2O3 led to the formation of TiO2 with activity in visible light much higher than when TiN was used as a precursor, pointing out the importance of oxygen defects/vacancies for extension of activity of TiO2 into the visible region.