Crystal structures and properties of BiMn1−xAlxO3 with x = 0.03 and 0.1

Crystal structures of BiMn_0.97Al_0.03O₃ (I) at 300 and 470 K and BiMn_0.9Al_0.1O₃ (II) at 90 and 300 K were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed at 300 K in I and associated with orbital order, disappeared at 470 K completely for one site and partially for the second site. The Mn/Al-O distances were very close to each other in I at 470 K and in II at 90 and 300 K indicating that orbital order did not appear in II even at 90 K. Magnetic properties of I and II were investigated with specific heat, high-temperature dc magnetic susceptibility, and ac magnetic susceptibility using different driving ac and applied dc magnetic fields and different ac magnetic field frequencies. The anomaly on the specific heat associated with a magnetic transition was strongly suppressed in II compared with that of I and BiMnO₃.     

Synthesis and properties of A6B2(OH)16Cl2·4H2O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) materials for environmental applications

Double layered hydroxide materials of composition A₆B₂(OH)₁₆Cl₂·4H₂O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) were synthesized by chemical precipitation at 60 °C. Different levels of crystallinity and ordering degree were observed depending upon the chemical environment or the combination between divalent and trivalent cations. The results from high-resolution transmission electron microscopy revealed that nanostructured layered samples were obtained with interplanar spacing compatible with previous literature. Raman scattering was employed to investigate the complex band structure observed, particularly the lattice vibrations at lower frequencies, which is intimately correlated to the cationic radius of both divalent and trivalent ions. The results showed that strongly coordinated water and chloride ions besides highly structured hydroxide layers have a direct influence on the stability of the hydrotalcites. It was observed that transition and decomposition temperatures varied largely for different chemical compositions.     

Synthesis, characterization and electrical conductivity studies on A3Bi2P3O12 (A = Na, K) materials

Crystalline Na₃Bi₂P₃O₁₂, K₃Bi₂P₃O₁₂ and glassy K₃Bi₂P₃O₁₂ compounds were prepared by solid-state reaction method. The prepared samples are characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline materials are found to be orthorhombic. The electrical conductivity measurements on the crystalline and glassy samples show that at ∼373 K, the σ_DC for crystalline K₃Bi₂P₃O₁₂ (0.81 × 10⁻⁸ S/cm) is about two orders of magnitude higher than the corresponding glassy phase (1.25 × 10⁻¹⁰ S/cm). The scaling results show that the conductivity relaxation mechanism is independent of temperature.     

Electrochemical behaviors of Li[Li(1−x)/3Mn(2−x)/3Nix/3Cox/3]O2 cathode series (0 < x < 1) synthesized by sucrose combustion process for high capacity lithium ion batteries

A mixed cathode material between Li₂MnO₃ and Li[Mn_1/3Ni_1/3Co_1/3]O₂ for high capacity lithium secondary batteries was introduced in this study. It was prepared using the sucrose combustion process because this is a simple process. The oxidation states of Mn, Co and Ni ions in the pristine Li[Li_(1−x)/3Mn_(2−x)/3Ni_x/3Co_x/3]O₂ compounds were confirmed to be tetravalent, trivalent and divalent, respectively, via XANES measurements. Electrochemical charge/discharge studies showed that the highest first discharge capacity of 224 mAh/g was obtained in composition of x = 0.5 at a 0.2 C rate. The oxidation state of the Co and Ni ions in the Li[Li_1/6Mn_1/2Ni_1/6Co_1/6]O₂ changed to higher oxidation states, but that of the Mn ions did not change.     

Catalytic behavior of AMoOx (A = Ba, Sr) in oxidation of 2-propanol

Perovskite-type oxides, BaMoO₃ and SrMoO₃, were prepared by reduction of scheelite-type oxides, BaMoO₄ and SrMoO₄, in H₂ flow at 873 K and characterized by XRD, TG, SEM, TPR, NH₃-TPD, UV-vis DRS and BET measurement. The catalytic activity of these alkaline-earth molybdenum oxide catalysts was tested for oxidation of 2-propanol with gaseous oxygen under atmospheric pressure. Dehydration to propylene was mainly promoted over the scheelite-type with Mo⁶⁺, while oxidative dehydrogenation to acetone was mainly promoted over the perovskite-type with Mo⁴⁺, and selectivity to acetone was much higher over BaMoO₃ than over SrMoO₃. Both perovskite-type oxide catalysts underwent oxidation to some degree during the catalytic reaction, so that they also contained some Mo⁶⁺. We concluded that the high selectivity to acetone resulting from oxidative dehydrogenation during 2-propanol conversion is related to the constantly changing oxidation state of the catalyst, resulting in coexistence of Mo⁶⁺ octahedra and Mo⁴⁺ octahedra on the AMoO₃ oxides.     

Transformation of Dion-Jacobson phase to Aurivillius phase: synthesis of (PbBiO2)MNb2O7 (M = La, Bi)

We describe transformations of the Dion-Jacobson (D-J) phases, KLaNb₂O₇ and RbBiNb₂O₇, to the Aurivillius (A) phases, (PbBiO₂)LaNb₂O₇ (1) and (PbBiO₂)BiNb₂O₇ (2), in a metathesis reaction with PbBiO₂Cl. Oxide 1 adopts centrosymmetric tetragonal structure (a = 3.905(1) Å, c = 25.66(1) Å), whereas oxide 2 crystallizes in a noncentrosymmetric orthorhombic (A2₁am) (a = 5.489(1) Å, b = 5.496(2) Å, c = 25.53(1) Å) structure. Oxide 2 shows a distinct SHG response towards 1064 nm laser radiation. The role of La³⁺ versus Bi³⁺ in the perovskite slabs for the occurrence of noncentrosymmetric structure/ferroic property in these materials is pointed out.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Inorganic phosphate crystal Na15−n[Al(PO4)2F2]3−n[Ti(PO4)2F2]n (0 ≤ n < 1): A novel cation exchanger with high exchange selectivity for Li and Pb ions

A series of inorganic phosphate crystals have been hydrothermally synthesized, which have high chemical stability and can keep their crystal structure after acid/base treatments. Its cation-exchange properties have been investigated and the results show that it is an excellent ion exchanger with high exchange capacities for H⁺, Li⁺ and Pb²⁺ ions (12.74, 6.98 and 3.92 mequiv./g, respectively). Selective extractions of Li⁺ and Pb²⁺ from the synthetic mixtures containing (Li⁺, Sr⁺, K⁺, Mg²⁺, Ca²⁺ and Ba²⁺) and (Pb²⁺, Ca²⁺, Ba²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Mg²⁺) have been observed. The reasons of the high exchange selection of NATP for Li⁺ and Pb²⁺ ions have been discussed.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

Preparation of nitrogen doped TiO2 photocatalyst by oxidation of titanium nitride with H2O2

Nitrogen doped anatase TiO₂ (N-TiO₂) were prepared by hydrothermally treating TiN with H₂O₂. The as-synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), UV-vis diffuse reflectance spectrum (DRS), Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) techniques. The results confirmed that the hydrothermal oxidation is an effective method to prepare N-doped TiO₂ anatase. The nitrogen concentration in TiO₂ could be controlled by the concentration of H₂O₂ solution. Photocatalytic degradation of methyl orange (MO) was carried out under visible light and UV-visible light irradiation, respectively. The as-prepared optimal N-TiO₂ showed higher photocatalytic activity than N-P25 and P25, and exhibited excellent reusability.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Fabrication of porous TiO2 nanofiber and its photocatalytic activity

Porous TiO₂-based nanofiber was fabricated via a combined electrospinning and alkali-dissolution method. TiO₂/SiO₂ composite nanofiber was firstly prepared by electrospinning and sintering, and then silica was leached out with alkaline solution from the bulk of TiO₂/SiO₂ composite nanofiber to produce porous microstructure. The thermal decomposition and phase structure of the composite nanofiber precursor was investigated with TG/DSC and XRD, and optimal sintering temperature was obtained. SEM-EDX and FT-IR characterization show that most silica can be dissolved out from the composite nanofiber and thus porous nanofiber with excellent microstructure can be spontaneously formed. The effect of composite nanofiber composition on porous microstructure was studied, and it is found that the composite nanofiber with 20wt% silica can produce better porous microstructure compared to those with 10wt% and 30wt% silica. Meanwhile, porous TiO₂ nanofiber with 20wt% silica shows higher degradation efficiency to Congo Red.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Structural and dielectric properties of A(Fe1/2Ta1/2)O3 [A = Ba, Sr, Ca]

The complex perovskite oxide barium iron tantalate (BFT), BaFe_1/2Ta_1/2O₃, strontium iron tantalate (SFT), SrFe_1/2Ta_1/2O₃ and calcium iron tantalate (CFT), CaFe_1/2Ta_1/2O₃ are synthesized by a solid-state reaction technique. Rietveld refinement of the X-ray diffraction data of the samples shows that BFT and SFT crystallize in cubic structure, with lattice parameter a = 4.06 Å for BFT and 3.959 Å for SFT, whereas CFT crystallizes in orthorhombic structure having lattice parameters a = 5.443 Å, b = 5.542 Å and c = 7.757 Å. Fourier transform infrared spectra show two primary phonon modes of the samples at around 450 cm⁻¹ and 620 cm⁻¹. The compounds show significant frequency dispersion in its dielectric properties. The complex impedance plane plots of the samples show that the relaxation (conduction) mechanism in these materials is purely a bulk effect arising from the semiconductive grains. The relaxation mechanism of the samples is modelled by Cole-Cole equation. The frequency dependent conductivity spectra are found to follow the power law.     

Controlled synthesis of Mn3O4 and MnCO3 in a solvothermal system

Controlled synthesis of Mn₃O₄ nanocrystals and MnCO₃ aggregates was achieved by a facile solvothermal method using different divalent manganese source in the solvent of N,N-dimethylformamide (DMF) with/without the introduction of poly(vinylpyrrolidone) (PVP). PVP was used as a co-reducing reagent in the controlled formation of MnCO₃ crystal. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED), Fourier transform infrared (FTIR) spectra, Raman spectrum and magnetic measurement. Higher process temperature and longer solvothermal time were favorable for the formation of MnCO₃ single phase using MnCl₂ as the manganese source. Mn₃O₄ nanocrystals were prepared at a relatively lower temperature. MnCO₃ aggregates consisted by small nanoparticles have a certain orientation, showing that the nanocrystals formed earlier through oriented aggregation. The size of Mn₃O₄ nanocrystals was 22.5 ± 7.3 nm and 7.3 ± 1.4 nm prepared using MnCl₂ and Mn(CH₃COO)₂, respectively, at 160 °C for 24 h. Raman spectra showed size-dependent characteristics. Smaller Mn₃O₄ nanoparticle resulted in a red-shift in Raman spectra. Magnetic property of the prepared Mn₃O₄ nanoparticle was influenced by the size distribution and crystallinity.     

Synthesis and photoluminescence of new phosphors M2(Mg, Zn)Si2O7:Mn (M = Ca, Sr, Ba)

New phosphors M₂(Mg, Zn)Si₂O₇:Mn²⁺ (M = Ca, Sr, Ba) were prepared by sol-gel process, and their luminescent properties in ultraviolet and vacuum ultraviolet region were investigated. The results showed that the (Ca, Sr, Ba)₂MgSi₂O₇:Mn²⁺ samples did not emit any visible light; the Sr₂ZnSi₂O₇:Mn²⁺ and Ca₂ZnSi₂O₇:Mn²⁺ samples showed green light. The Ba₂ZnSi₂O₇:Mn²⁺ sample mainly showed green light under 254 nm excitation and red light under 147 nm excitation. The different emission was due to the Mn²⁺ ions occupied different sites, which were excited selectively. Among the three phosphors Sr₂ZnSi₂O₇:Mn²⁺ showed the highest green emission intensity, and its decay time was shorter than that of Zn₂SiO₄:Mn²⁺ under 147 nm excitation.     

Crystal and magnetic structure and cation distribution of Mn2−xV1+xO4 spinels (x = 0, 1/3 and 1)

We synthesized the spinel-type compounds belonging to the Mn_2−xV_1+xO₄ series with x = 0, 1/3 and 1 as polycrystalline powders. Crystal and magnetic structures were refined using synchrotron X-ray and neutron powder diffraction. At 300 K all members crystallize in the cubic system, space group , and show a structural transition at low temperature, changing to a tetragonal symmetry (space group I4₁/amd). Cations distributions between octahedral and tetrahedral sites were refined from neutrons diffraction (ND) data and explained based on crystal field stabilization energies (CFSE) and ionic radii. The magnetic unit cell is the same as the crystallographic one, having identical symmetry relations. The magnetic structure was refined as an arrangement of collinear spins, antiferromagnetically ordered, parallel to the c-axis of the unit cell. The refined site magnetic moments are smaller than those obtained from hysteresis cycles of the M vs. H measurements, indicating that some non-collinear disordered component coexists with the ordered component along the c-axis.     

Solution calorimetric study of mixing enthalpy in 0.8[xB2O3-(1 − x)SiO2]-0.2K2O glasses at 298 K

The mixing enthalpies in the 0.8[xB₂O₃-(1 − x)SiO₂]-0.2K₂O glassy system with 0 ≤ x ≤ 1 have been deduced from accurate calorimetric measurements of solution enthalpies in acidic solvent at 298 K. The substitution of SiO₂ by B₂O₃ corresponds to a negative enthalpic effect and suggests the absence of some glassy miscibility gap. The mixing enthalpies are of the same order of magnitude as mixing enthalpies usually found in molten salts.     

Raman scattering characterization of bismuth based mixed oxides with Bi2O3 related structures

Orthorhombic Bi₁₇Ln₇O₃₆ (=Bi_0.708Ln_0.292O_1.5) and triclinic BiLnO₃ (=Bi_0.5Ln_0.5O_1.5) with Ln = Tm, Yb and Lu, as well as a cubic δ-type Bi_0.7Yb_0.3O_1.5, have been characterized at room temperature by Raman scattering spectrometry. The Raman spectrum deconvolution of the δ-Bi₂O₃ type cubic form allowed us to establish a correlation between the different phases of these mixed metallic oxides for the lattice modes as well as for the internal species. Using the empirical relation from Hardcastle, the BiO bond lengths have been estimated from the stretching frequency value observed at 635, 597 or 639 cm⁻¹ for cubic, orthorhombic or triclinic forms, respectively. A comparison with the X-ray results is discussed. The peritectoïdic transformation BiTmO₃ → δ + Tm₂O₃ has also been characterized by Raman spectroscopy.