Stuffed Tridymite Structures: Synthesis, Structure, Second Harmonic Generation, Optical, and Multiferroic Properties.

The stuffed tridymite structure Ba(Zn/Co)1-x Si1-x M2x O4 (M=Al3+ and Fe3+ ) is explored for the possible multiferroic behavior and to develop new inorganic colored materials. The compounds were synthesized by employing conventional solid-state chemistry methods in the temperature range 1100-1175 °C for 24 h. The powder X-ray diffraction (PXRD) and Rietveld refinement studies indicate that the compounds stabilize in the P63 space group (no. 173). The refinement results were also rationalized by employing Raman spectroscopic studies. The compounds were found to be second harmonic generation (SHG) active and show weak ferroelectric behavior. The co-substitution of Co2+ and Fe3+ in the structure gives rise to a weak ferromagnetic behavior to the compound, BaCo0.75 Si0.75 Fe0.5 O4 , making it a multiferroic material. The optical studies on the prepared compounds exhibited blue color (Co2+ in Td geometry), purple color (Ni2+ in Td geometry), and simultaneous substitution of Co2+ and Fe3+ gives rise to blue-green color owing to metal-to-metal charge transfer (MMCT) effect.

New Members of SHG Active Dugganite Family, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V): Synthesis, Structure, and Materials Properties.

A family of compounds, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V), with the Dugganite structure was prepared employing traditional solid-state chemistry methods. PXRD and Rietveld refinement studies indicate that the compounds are stabilized in P321 space group (no. 150). The compounds are found to be SHG active with values ranging from 1.9 to 15.0 × KDP. The compounds exhibit high dielectric constants and low loss in our studies. The noncentrosymmetry related properties of the new Dugganites were understood by band structure calculations. We also explored the present Dugganite-structured oxides for the development of new inorganic colored materials by substituting Co2+, Ni2+, Cu2+, and Fe3+ in place of Zn2+. Thus, substitution of Co2+ and Fe3+ together tunes the blue color of the cobalt compound to blue-green color arising from metal-to-metal charge transfer (MMCT) of Fe3+ and Co2+ ions. The tetrahedrally coordinated Ni2+ in the Dugganite imparts a magenta color.

Stabilization of a Tetrahedral (Mn(5+)O4) Chromophore in Ternary Barium Oxides as a Strategy toward Development of New Turquoise/Green-Colored Pigments.

An experimental investigation of the stabilization of the turquoise-colored chromophore Mn(5+)O4 in various oxide hosts, viz., A3(VO4)2 (A = Ba, Sr, Ca), YVO4, and Ba2MO4 (M = Ti, Si), has been carried out. The results reveal that substitution of Mn(5+)O4 occurs in Ba3(VO4)2 forming the entire solid solution series Ba3(V1-xMnxO4)2 (0 < x ≤ 1.0), while with the corresponding strontium derivative, only up to about 10% of Mn(5+)O4 substitution is possible. Ca3(VO4)2 and YVO4 do not stabilize Mn(5+)O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1-xMn(5+)xO4+x/2 for x up to 0.15, that are turquoise-colored. We rationalize the results that a large stabilization of the O 2p-valence band states occurs in the presence of the electropositive barium that renders the Mn(5+) oxidation state accessible in oxoanion compounds containing PO4(3-), VO4(3-), etc. By way of proof-of-concept, we synthesized new turquoise-colored Mn(5+)O4 materials, Ba5(BO3)(MnO4)2Cl and Ba5(BO3)(PO4)(MnO4)Cl, based on the apatite-Ba5(PO4)3Cl-structure.

Visible-Light-Activated C-C Bond Cleavage and Aerobic Oxidation of Benzyl Alcohols Employing BiMXO5 (M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P).

The synthesis, structure, optical and photocatalytic studies of a family of compounds with the general formula, BiMXO5 ; M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P is presented. The compounds were prepared by regular solid-state reaction of constituents in the temperature range of 720-810 °C for 24 h. The compounds were characterized by powder X-ray diffraction (PXRD) methods. The Rietveld refinement of the PXRD patterns have been carried out to establish the structure. The optical absorption spectra along with the colors in daylight have been explained employing the allowed d-d transition. In addition, the observed colors of some of the V5+ containing compounds were explained using metal-to-metal charge transfer (MMCT) from the partially filled transition-metal 3d orbitals to the empty 3d orbitals of V5+ ions. The near IR (NIR) reflectivity studies indicate that many compounds exhibit good NIR reflectivity, suggesting that these compounds can be employed as 'cool pigments'. The experimentally determined band gaps of the prepared compounds were found to be suitable to exploit them for visible light activated photocatalysis. Photocatalytic C-C bond cleavage of alkenes and aerobic oxidation of alcohols were investigated employing visible light, which gave good yields and selectivity. The present study clearly demonstrated the versatility of the Paganoite family of compounds (BiMXO5 ) towards new colored inorganic materials, visible-light photocatalysts and 'cool pigments'.

La1.5Sr0.5NiMn0.5Ru0.5O6 Double Perovskite with Enhanced ORR/OER Bifunctional Catalytic Activity.

Perovskites (ABO3) with transition metals in active B sites are considered alternative catalysts for the water oxidation to oxygen through the oxygen evolution reaction (OER) and for the oxygen reduction through the oxygen reduction reaction (ORR) back to water. We have synthesized a double perovskite (A2BB'O6) with different cations in A, B, and B' sites, namely, (La1.5Sr0.5)A(Ni0.5Mn0.5)B(Ni0.5Ru0.5)B'O6 (LSNMR), which displays an outstanding OER/ORR bifunctional performance. The composition and structure of the oxide has been determined by powder X-ray diffraction, powder neutron diffraction, and transmission electron microscopy to be monoclinic with the space group P21/ n and with cationic ordering between the ions in the B and B' sites. X-ray absorption near-edge spectroscopy suggests that LSNMR presents a configuration of ∼Ni2+, ∼Mn4+, and ∼Ru5+. This bifunctional catalyst is endowed with high ORR and OER activities in alkaline media, with a remarkable bifunctional index value of ∼0.83 V (the difference between the potentials measured at -1 mA cm-2 for the ORR and +10 mA cm-2 for the OER). The ORR onset potential ( Eonset) of 0.94 V is among the best reported to date in alkaline media for ORR-active perovskites. The ORR mass activity of LSNMR is 1.1 A g-1 at 0.9 V and 7.3 A g-1 at 0.8 V. Furthermore, LSNMR is stable in a wide potential window down to 0.05 V. The OER potential to achieve a current density of 10 mA cm-2 is 1.66 V. Density functional theory calculations demonstrate that the high ORR/OER activity of LSNMR is related to the presence of active Mn sites for the ORR- and Ru-active sites for the OER by virtue of the high symmetry of the respective reaction steps on those sites. In addition, the material is stable to ORR cycling and also considerably stable to OER cycling.

Color Tuning in Garnet Oxides: The Role of Tetrahedral Coordination Geometry for 3 d Metal Ions and Ligand-Metal Charge Transfer (Band-Gap Manipulation).

We explored garnet-structured oxide materials containing 3d transition-metal ions (e.g., Co2+ , Ni2+ , Cu2+ , and Fe3+ ) for the development of new inorganic colored materials. For this purpose, we synthesized new garnets, Ca3 Sb2 Ga2 ZnO12 (I) and Ca3 Sb2 Fe2 ZnO12 (II), that were isostructural with Ca3 Te2 Zn3 O12 . Substitution of Co2+ , Ni2+ , and Cu2+ at the tetrahedral Zn2+ sites in I and II gave rise to brilliantly colored materials (different shades of blue, green, turquoise, and red). The materials were characterized by optical absorption spectroscopy and CIE chromaticity diagrams. The Fe3+ -containing oxides showed band-gap narrowing (owing to strong sp-d exchange interactions between Zn2+ and the transition-metal ion), and this tuned the color of these materials uniquely. We also characterized the color and optical absorption properties of Ca3 Te2 Zn3-x Cox O12 (0