Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

All-solid-state cells based on solid electrolyte systems Cu1−xAgxI-Ag2O-Y, where x = 0.05-0.25; Y = MoO3, B2O3, SeO2, V2O5 and CrO3

All-solid-state cells of the configuration (−)Ag + SE//SE//I₂-phenothiazine + C(+) using the best conducting compositions of the solid electrolyte systems, namely, Cu_1−xAg_xI-Ag₂O-Y where x = 0.05, 0.1, 0.15, 0.2 and 0.25, Y = MoO₃, B₂O₃, SeO₂, V₂O₅ and CrO₃, as the electrolytes were fabricated. Discharge, polarization and power characteristics of these cells were also evaluated. The open circuit voltage values of these cells were in the range 620-635 mV. The stability of these cells has been indicated by the constancy of their OCV over a period of 6 months. The polarization and discharge studies on these cells have shown that typical cells based on the electrolytes with Y = B₂O₃, SeO₂ and V₂O₅ would possess discharge capacities of 12.84, 3.76 and 5.05 mA h and specific energy of 6.55, 1.81 and 2.77 W h kg⁻¹, respectively. The solid electrolytes have good electrochemical stability and compatibility with the Ag/Phenothiazine-I₂ electrode couple thus offering their suitability of application in microwatt power sources.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Synthesis under high-oxygen pressure, magnetic and structural characterization from neutron powder diffraction data of YGa1−xMn1+xO5 (x = 0.23): A comparison with YMn2O5

A new material of nominal stoichiometry YGaMnO₅ has been prepared in polycrystalline form from citrate precursors followed by thermal treatments under high-oxygen pressure. This compound has been characterized from neutron powder diffraction (NPD) data and magnetic measurements. For comparison, the parent compound YMn₂O₅ has also been synthesized and its crystal structure refined by NPD data. The new oxide has an actual stoichiometry YGa_1−xMn_1+xO₅ (x = 0.23), determined by NPD, showing an important cationic disorder between both metal sites; it is orthorhombic, Pbam (SG), and its crystal structure contains chains of Mn⁴⁺O₆ edge-sharing octahedra, linked together by Ga³⁺O₅ pyramids and YO₈ units. With respect to YMn₂O₅, containing axially elongated MnO₅ pyramids due to the Jahn-Teller effect of Mn³⁺ cations, the GaO₅ pyramidal units in YGa_0.77Mn_1.23O₅ are substantially flattened. This compound has a paramagnetic behaviour with two weak anomalies at about 50 K and 350 K. The magnetic structures, studied at 1.4 K and 100 K show a ferromagnetic coupling along the chains of MnO₆ octahedra.     

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.     

X-ray Rietveld analysis and Fourier transform infrared spectra of the solid solutions [Eu2−xMx][Sn2−xMx]O7−3x/2 (M = Mg or Zn)

Two series of mixed oxides with formula [Eu_2−xM_x][Sn_2−xM_x]O_7−3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center -3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M²⁺ (Mg²⁺, Zn²⁺) substitute isomorphically for Eu³⁺ and Sn⁴⁺ ions producing vacancies in the anionic sublattice.     

Structural study by X-ray diffraction and transmission electron microscopy of the misfit compound (SbS1−xSex)1.16(Nb1.036S2)2

In the Sb-Nb-S-Se system, a new misfit layer compound (MSL) has been synthesized and its structure was determined by combining single crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It presents a composite crystal structure formed by (SbS_1−xSe_x) slabs stacking alternately with double NbS₂ layers and both can be treated as separate monoclinic subsystems. The (SbS_1−xSe_x) slabs comprise a distorted, two-atom-thick layer with NaCl-type structure formed by an array of {SbX₅} square pyramids joined by edges (X: S, Se); the NbS₂ layers consist of {NbS₆} trigonal prisms linked through edge-sharing to form sheets, just as in the 2H-NbS₂ structure type. Both sublattices have the same lattice parameters a = 5.7672(19) Å, c = 17.618(6) Å and β = 96.18(3)°, with incommensurability occurring along the b direction: b₁ = 3.3442(13) Å for the NbS₂ subsystem and b₂ = 2.8755(13) Å for the (SbS_1−xSe_x) subsystem. The occurrence of diffuse scattering intensity streaked along c^* indicates that the (SbS_1−xSe_x) subsystem is subjected to extended defects along the stacking direction.     

Glass formation tendency in the system SeO2-Ag2O-B2O3

The glass formation regions in the system SeO₂-Ag₂O-B₂O₃ have been determined using the melt quenching method of evacuated silica ampoules. The structural units forming the amorphous network have been established by IR spectroscopy. The presence of SeO₃ (ν = 820 cm⁻¹; 760-750 cm⁻¹), BO₃ (ν = 1340, 1270 cm⁻¹) and BO₄ (ν = 1050 cm⁻¹) units has been confirmed. Crystallization of Ag₂SeO₃ only has been observed in a wide concentrate region near the glass formation boundary. A model explaining the unsatisfactory glass formation ability in the system investigated has been developed. It has been suggested that Ag⁺ ions are predominantly located near the selenite units, which stimulates the formation of isolated SeO₃ groups. The transformation of BO₃ into BO₄ units is hindered by the absence of free Ag⁺ ions near the borate units.     

Luminescence characterization of (Ca1−xSrx)(S1−ySey):Eu,M (M = Sc and Y) for high color rendering white LED

The highly efficient red phosphors (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺,M³⁺ (M = Sc and Y) were prepared, starting from CaCO₃, SrCO₃, Eu₂O₃, Sc₂O₃, Y₂O₃, S, and SeO₂ with a flux, by a conventional solid-state reaction. The optimized red phosphors converted 11.8% (Sc³⁺) and 11.7% (Y³⁺) of the absorbed blue light into luminescence. These quantum values are much higher than Q = 3.0% of CaS:Eu²⁺. For the fabrication of light-emitting diodes (LEDs), the prepared phosphors were coated with MgO from non-aqueous solution to overcome their weakness against moisture. White LEDs were fabricated by pasting the prepared red phosphors and the yellow YAG:Ce³⁺ phosphor on an InGaN blue chip (λ_ems = 446.5 nm). The incorporation of the red phosphor to the YAG:Ce³⁺ phosphor resulted in an improved color rendering index (Ra) from 70 to 80.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.     

Supercritical hydrothermal synthesis of Cu2O(SeO3): Structural characterization, thermal, spectroscopic and magnetic studies

Cu₂O(SeO₃) has been synthesized in supercritical hydrothermal conditions, using an externally heated steel reactor with coupled hydraulic pump for the application of high pressure. The compound crystallizes in the P2₁3 cubic space group. The unit cell parameter is a = 9.930(1) Å with Z = 12. The crystal structure has been refined by the Rietveld method. The limit of thermal stability is, approximately, 490 °C. Above this temperature the compound decomposes to SeO₂(g) and CuO(s). The IR spectrum shows the characteristic bands of the (SeO₃)²⁻ oxoanion. In the diffuse reflectance spectrum two intense absorptions characteristic of the Cu(II) cations in five-coordination are observed. The ESR spectra are isotropic from room temperature to 5 K, with g = 2.11(2). The thermal evolution of the intensity and line width of the signals suggest a ferromagnetic transition in the 50-45 K range. Magnetic measurements, at low temperatures, confirm the existence of a ferromagnetic transition with a critical temperature of 55 K.     

A new organically templated monodimensional mixed valence (Fe/Fe) phosphite: (C4H12N2)[FeFe(HPO3)2F3]: Solvothermal synthesis, crystal structure, spectroscopic and magnetic properties

The organically templated (C₄H₁₂N₂)[Fe^IIFe^III(HPO₃)₂F₃] compound has been synthesized under mild solvothermal conditions. The crystal structure has been determined from X-ray single-crystal diffraction data. The compound crystallizes in the P2₁/n monoclinic space group, with the unit-cell parameters a = 12.935(1), b = 6.4476(7), c = 15.693(2) Å, β = 105.630(9)° and Z = 4. The crystal structure consists of [Fe^IIFe^III(HPO₃)₂F₃]²⁻ chains formed by a central chain built of [Fe(2)O₄F₂] edge-sharing octahedra, and two side chains formed by alternating [Fe(1)O₃F₃] octahedra and [HP(1)O₃] tetrahedra. The piperazinium cations are placed between the chains linked by ionic and hydrogen interactions. The IR and Raman spectra show the existence of two phosphite crystallographically independent. From the diffuse reflectance spectrum the D_q parameter for the iron(II) cations has been calculated (D_q = 820 cm⁻¹). The Mössbauer spectrum in the paramagnetic state shows the simultaneous presence of Fe²⁺ and Fe³⁺. The magnetic measurements indicate the existence of antiferromagnetic interactions.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.     

Electrochemical properties of (La1−xTix)0.67Mg0.33Ni2.75Co0.25 (x = 0-0.20, at%) hydrogen storage alloys

(La_1−xTi_x)_0.67Mg_0.33Ni_2.75Co_0.25 (x = 0, 0.05, 0.10, 0.15 and 0.20, at%) alloys are synthesized by arc-melting and subsequent heat solid-liquid diffusing techniques, and the crystalline structures and electrochemical properties of the alloys are investigated systematically. The structural analysis results show that all the alloys mainly consist of (La, Mg)Ni₃ phase with the rhombohedral PuNi₃-type structure and LaNi₅ phase with the hexagonal CaCu₅-type structure. However, when the Ti content is higher than 0.10, a little amount of TiNi₃ phase start to form. Electrochemical measurements show that the alloy electrodes could be activated to their maximum discharge capacity within four cycles, the maximum discharge capacity is around 321.9-384.6 mAh g⁻¹, both the cyclic stability and the high-rate discharge ability first increased and then decrease with increasing x. All the results show that a little amount of Ti substitution for La in AB₃-type hydrogen storage alloys is effective to the improvement of the overall electrochemical properties.     

Oxygen vacancy ordering in strontium doped rare earth cobaltate perovskites Ln1−xSrxCoO3−δ (Ln = La, Pr and Nd; x > 0.60)

A family of Sr-doped perovskite compounds Ln_1−xSr_xCoO_3−δ (Ln = La³⁺, Pr³⁺ and Nd³⁺; x > 0.60), were prepared by sol-gel chemistry and reaction at 1100 °C under 1 atm of oxygen. This structural family has been shown to be present only for rare earth ions larger than Sm³⁺ and an upper limit of Sr²⁺ solubility in these phases was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m) perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite unit along the crystallographic c-axis. No additional Ln³⁺/Sr²⁺ cation ordering was observed.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Formation, sintering, and electrical conductivity of (Nd1−xCax)CrO3 (0≤x≤0.25) using citric acid as a gelling agent

In Ca²⁺-substituted NdCrO₃, single-phase perovskite compounds (Nd_1−xCa_x)CrO₃, where x=0-0.25, have been formed by a citric acid processing. (Nd_1−xCa_x)CrO₃ powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700°C under atmospheric pressure. The relative densities, grain sizes, and electrical conductivities increase with increased Ca²⁺ content. (Nd_0.75Ca_0.25)CrO₃ materials show an excellent electrical conductivity of 1.9×10 S m⁻¹ at 1000°C.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Rietveld refinements of the solid solution Li(1−2x)NixTiO(PO4) (0 ≤ x ≤ 0.50)

Li_(1−2x)Ni_xTiO(PO₄) oxyphosphates with 0 ≤ x ≤ 0.10 crystallize in the orthorhombic system with the space group Pnma, those with 0.10 < x ≤ 0.25 crystallize in the monoclinic system with the space group P2₁/c and compositions with 0.25 < x < 0.50 present a mixture of the limit of the solid solution Li_0.50Ni_0.25TiO(PO₄) and Ni_0.50TiO(PO₄). The structure of the compositions 0 ≤ x ≤ 0.25 is based on a three-dimensional anionic framework constructed of chains of alternating TiO₆ octahedra and PO₄ tetrahedra, with the lithium and nickel atoms in the cavities in the framework. The dominant structural units in the compositions are chains of tilted corner-sharing TiO₆ octahedra running parallel to one of the axis. The oxygen atoms of the shared corners, not implied in (PO₄) tetrahedra, justify the oxyphosphate designation. Titanium atoms are displaced from the geometrical center of the octahedra resulting in alternating long (≈2.25 Å) and short (≈1.71 Å) TiO(1) bonds. The four remaining TiO bond distances have intermediate values ranging from 1.91 to 2.06 Å.