(Oxy)nitride materials have attractive properties directly related to the role played by nitrogen. A commonly used synthesis method consists of the thermal nitridation of an oxide precursor in flowing ammonia. As a consequence of the anionic N3-/O2- substitution results an increase in the covalent character, illustrated by a shift of the absorption edge towards higher wavelength values. Thus, oxynitrides offer potentialities as optical materials in the domain of colored pigments, UV absorbers and visible-light photocatalysts.
The structure of LiMn2O4, its phase transition at around 290 K, and the diffusion mechanism of Li are discussed based on the recent synchrotron X-ray diffraction and molecular dynamics simulation studies. The high-temperature modification (cubic Fd 3m) is essentially of spinel-type containing various types of disorder; (1) the Li atoms are not located exactly at the tetrahedral 8a sites, but mainly distributed among four positions 0.14 Å apart from 8a, (2) some portion of Li atoms are further displaced toward the 16c octahedral interstices with a notable accumulation at the positions 0.35 Å apart from 16c, and (3) the O atoms also show a statistical distribution around their ideal 32e sites. The low-temperature modification adopts a 3 × 3 × 1 superstructure (orthorhombic Fddd) with respect to the high-temperature modification. The bond-length fluctuation has been observed along the pseudo-tetragonal Jahn-Teller distortion direction parallel to the a axis in the heterocubane Mn24O94 cluster. The four Mn2 atoms in the heterocubane presumably shares three electrons in the e-parentage low-energy-level orbitals by the double-exchange Zener mechanism. The time-averaged oxidation state for Mn2 is estimated to be +3.25. The heterocubane Zener polarons are isolated with each other and embedded in an ordered way in the charge-ordered matrix containing Mn1III, Mn3III, Mn4IV and Mn5IV. Two kinds of diffusion mechanisms for Li atoms are proposed for the high-temperature modification from the molecular dynamics simulation. One is a microscopic version of the classical picture for diffusion based on the concentration difference in diffusion species. This mechanism requires an activation energy of ca. 0.25 eV to jump over a saddle point at the bottleneck. The other mechanism is a diffusion accompanied by the local lattice distortion coupled with the 3d electron transfer between a pair of nearby Mn atoms. This requires no activation energy for Li to pass through the bottleneck. The valence exchange between +3 and +4 in the neighboring Mn pair prompts the displacement of coordinating O atoms along the pseudo-tetragonal Jahn-Teller distortion direction, which presumably plays a principal role in opening the bottleneck of oxygen triangle along the diffusion pathway.
Both magnetization and coercivity should be controlled on BaFe12O19 for its usage in magnetic recording. Preparation of BaFe12O19 was studied in citrate route by changing the kinds of iron source. Stoichiometric mixtures of iron compounds with Ba(NO3)2 were dissolved in distilled water. They were mixed with citric acid aqueous solution and then condensed on a hot plate to their gelatinous products. Their prefired products below 450°C were pelletized and then fired in a temperature range between 750 and 900°C. The product from iron nitrate was a mixture of α-Fe2O3, BaFe2O4 and a small amount of BaFe12O19. The product from Fe(acac)3 was pure BaFe12O19 fine powder with a crystallite size of < 90 nm. The fine powdered product at 850°C had a saturation magnetization of 58.5 emu·g-1 and a coercivity of 5.5 kÖe.
Aluminum boron carbide (Al8B4C7) was synthesized by heating a mixed powder consisting of Al:B4C:C = 8:1:6 (molar ratio) at 1600°C for 1 h. The synthesized Al8B4C7 powder was ball milled for 24 h with alumina balls to prepare an Al8B4C7 powder with under 40 μm diameter. The fine Al8B4C7 powder was then put into a carbon die, pressed, and sintered using PECS at 1650°C for 10 min under an applied pressure of 60 MPa, resulting in an Al8B4C7 body with a relative density of 99.6%. The bending strength, Vickers hardness, and fracture toughness of the dense Al8B4C7 body were 500 MPa, 15.2 GPa and 2.3 MPa·m1/2, respectively. The bending strength of the dense Al8B4C7 body increased during heating to 510 MPa at 1000°C, but decreased to 440 MPa at 1200°C.
Two types of new quaternary carbide solid solutions, (ZrC)2Al3[Al1-xGex]C3 (x = 0.07) and (ZrC)3Al3[Al1-yGey]C3 (y = 0.04), have been synthesized and characterized using a laboratory X-ray powder diffractometer (XRPD) and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray analyzer (EDX). With the former carbide, the layered structure was imaged by TEM and the solubility of Ge in the crystal was confirmed by EDX. The crystal structures of both carbides were refined by the Rietveld analysis of the XRPD data. These carbides have been found to form a homologous series with the general formula (ZrC)mAl3[Al1-zGez]C3 (0 ≤ z ≤ 0.07), where m = 2 and 3. The crystal structures can be regarded as intergrowth structures, consisting of the NaCl-type [ZrmCm+1] slabs separated by the Al4C3-type [Al4-zGezC4] layers.
A ternary metal-rich telluride Pd2NiTe2 has been prepared by the solid-state reaction. This compound crystallizes in the orthorhombic K2ZnO2-type structure with space group Ibam. In this structure, the Ni atom is tetrahedrally coordinated and the NiTe4 tetrahedra share edges forming one-dimensional chains with short Ni-Ni bondings along the c-axis. Through magnetic susceptibility, specific heat, and electrical resistivity measurements, this compound shows Pauli paramagnetism and metallic electrical conductivity. The electronic structure calculation of Pd2NiTe2 suggests that its electrical conductivity is a three-dimensional one, which is caused by conduction bands originated from d orbitals of Pd and Ni and p orbitals of Te.
Influences of high-oxygen-pressure (350 MPa) annealing on the properties of leakage current, polarization, and strain were investigated for single crystals of (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 with P4mm tetragonal structure The oxygen annealing led to a marked increase in leakage current, showing that the oxidized crystals have a high concentration of electron hole (carrier) and a low concentration of oxygen vacancy compared with the crystals annealed in air. While the air-annealed crystals exhibited a giant strain up to 0.8% at an electric field of 40 kV/cm, the oxidized crystals showed a small strain less than 0.3%. The results can be explained by the interaction between ferroelectric domains and charged defects.
Two types of new quaternary carbide solid solutions, (ZrC)[Al1-xSix]8C6 with x = 0.06 and (ZrC)[Al1-ySiy]4C3 with y = 0.04, have been synthesized and characterized using a laboratory X-ray powder diffraction (Cu K α1), transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The average atom ratios [Al:Si] of both carbides were determined by EDX, and the crystal structures were refined using the Rietveld method. These carbides have been found to form a homologous series with the general formula (ZrC)m[Al1-zSiz]8C6 (0 ≤ z ≤ 0.07), where m = 1 and 2. The crystal structures can be regarded as intergrowth structures, consisting of the NaCl-type [ZrmCm+1] slabs separated by the Al4C3-type [(Al1-zSiz)8C7] layers.
We have synthesized a single-phase sample of the compound known as CaCo2O4 and investigated its crystal structure by means of a single crystal X-ray diffraction technique. The compound crystallizes in a triclinic system (space group P1) with lattice parameters of a = 492.4(2) pm, b = 568.3(3) pm, c = 568.5(3) pm, α = 75.400(9)°, β = 89.974(9)° and γ = 81.261(9)° at 293(2) K. The structure consists of an alternate stack of a CdI2-type CoO2 triangular sheet and a layer of Ca atoms, showing great similarity to that of β-NaxCoO2 (x∼ 0.67). However, the Ca atom has four crystallographic sites with different occupation probabilities, leading to the structure formula Ca1.87(5)Co4O8 (Ca0.47(1)CoO2). Due to a low bulk density as well as a small number of hole carriers, equivalent to the nominal valence state of Co ions close to 3.06, a polycrystalline sample of the Ca-phase exhibits much higher electrical resistivity ρ = 680 μΩ m, relative to that of the Na-phase ρ = 1.2 μΩ m, with a large positive Seebeck coefficient of S = 145 μV/K at 300 K.
TiN-coated Al2O3 particles of different size, shape and crystal structure were prepared by depositing nano-size TiO2 on the Al2O3 surfaces from Ti(O-i-C3H7)4 solution (TiO2 precursor) by controlled hydrolysis, then nitrided with NH3 at 1000°C. Irrespective of the source of Al2O3 particles, a homogeneous TiO2 coating was achieved by heating from 10 to 40°C an ultrasonic suspension of the Al2O3 particles containing the precursor with 1.0 vol% H2O. After nitridation, the Al2O3 grains were coated with 10-20 nm TiN particles. Spark plasma sintering of the TiN/Al2O3 particles at 1500°C and 1600°C yielded the composite ceramics with a relative density of 94-97%. Composite TiN/Al2O3 ceramics with an electrical resistivity of ∼10-2 Ω cm were obtained at 25 vol% TiN composition.
New Li-Ti-N solid-solution compounds with an anti-fluorite-type superstructure were synthesized by heating the mixture of Li3N and TiN at 800°C in Ar flow. The compositions of the new compounds seem to lie on the tie-line between the two well-known compounds, Li3N (hexagonal, P6/mmm) and Li5TiN3 (cubic, I21/a-3) by Li evaporation during heating in Ar gas flow, and nominally represented with Li5+yTi1-yN3-y. The solubility is large; the anti-fluorite-type superstructure of Li5TiN3 (y = 0) was retained up to y = ca. 0.9, which is close to the end member Li3N (y = 1) with a different layered structure. This finding opens up further exploring other new Li-M-N (M = transition metals, group XIII, or XIV elements) phases with some attractive functions.
High temperature synchrotron X-ray diffraction measurements have been performed on Ba2-xSrxIn2O5 and Ba2In2-xGaxO5, which exhibits two kinds of structural phase transition and high oxide ion conductivity at the high temperature phase. It has been revealed that the synchrotron X-ray diffraction with high sensitivity and resolution is necessary to detect any slight distortion from ideal cubic perovskite in cation substituted Ba2In2O5 system. By using synchrotron X-ray diffraction, the slight distortion from ideal cubic perovskite in Ba2In2-xGaxO5, which could not be observed by using Cu Kα X-ray diffraction, has been clarified. The larger and smaller distortions have been observed with an increase of Sr and Ga content, respectively, which can be explained from the viewpoint of variation of the tolerance factor.
Phase relationship of Ba2-xLaxIn2O5+δ has been investigated by using dilatometry, differential scanning calorimetry and synchrotron X-ray diffraction at high temperatures. Decrease of the temperature of the first order phase transition, from brownmillerite orthorhombic to tetragonal, was observed with an increase of La content in Ba2-xLaxIn2O5+δ for x = 0.0-0.2, whereas little variation was detected on temperature of the second order phase transition, from tetragonal to distorted cubic, in the specimens with x = 0.0-0.3. Ba2-xLaxIn2O5+δ with x larger than 0.4 showed no phase transition by temperature. Crystal structure of the specimens with 0.4 ≤ x ≤ 1.0, 1.2 ≤ x ≤ 1.7 and x = 2.0 were ideal cubic, tetragonal and orthorhombic distorted perovskite, respectively. Ba1.3La0.7In2O5+δ with ideal cubic perovskite structure showed higher oxide ion conductivity below 800°C than that of Ba2In2-xGaxO5 with distorted cubic structure, supporting that removal of the structural distortion should be effective to improve mobility of oxide ion, resulting in as high oxide ion conductivity as that of yttria stabilized zirconia.
(NaNbO3)1-x(BaTiO3)x (NN-xBT) thin films with low BaTiO3 (BT) concentrations x (x = 0.05 and 0.10) were fabricated on SrRuO3/(001)SrTiO3 (SRO/(001)STO) substrate by pulsed laser deposition (PLD). X-ray diffraction pattern (XRD) and transmission electron diffraction pattern (TED) showed that NN-0.10BT thin film was epitaxially grown on SRO/(001)STO substrate with a crystallographic relationship of NN-xBT||STO. From reciprocal space maps, the lattice parameters of the out-of-plane direction of NN-xBT thin films became larger with an increase in BT concentration, although the lattice parameter of the in-plane was hardly changed by the BT concentration. The value of relative dielectric constant εr of the NN-xBT thin films were increased with BT concentration. The εr and the dielectric loss tanδ of NN-0.10BT were 1220 and 0.02 at 1 kHz, respectively. The P-E hysteresis loops of the NN-xBT thin films showed clear ferroelectricity. Although the value of remanent polarization Pr decreased with the BT concentration, the behaviors of εr, Pr, and coercive electric field Ec of the NN-xBT thin films against the BT concentration accorded with those of NN-xBT ceramics, in which NN-0.10BT ceramics exhibited the largest piezoelectric property. Therefore, the NN-0.10BT thin film is expected to show high piezoelectricity.
A new intermetallic compound, MnGa2Sb2, has been synthesized by direct reaction from Mn and GaSb at 6 GPa and 500°C for 30 min using a belt-type high pressure apparatus. The compound crystallizes into an orthorhombic structure with the space group Iba2 (No. 45). The structure was refined by the Rietveld analysis of the powder X-ray diffraction data and the lattice constant was determined to be a = 1.180(7) nm, b = 0.5968(2) nm, c = 0.5858(2) nm. The structure consists of -Mn-Mn-Mn- linear chains parallel to the c-axis. The compound exhibits metallic conductivity and itinerant-electron ferromagnetic behavior with the Curie temperature of 310 K.
Three series of (La,Sr)TiO2N compounds, with either anion nonstoichiometry LaTiO2+xN1-x, cation nonstoichiometry La1-x Ti1+xO2N or an La1-xSrxTiO2N series, have been prepared in order to clarify, by XPS spectroscopy, the influence of composition on the valence state of titanium. Though the substitution of lanthanum by strontium was effective in the enhancement of reflectivity in the longer wavelength region above the absorption edge, the valence state of titanium did not change in either the La1-xTi1+xO2N or the La1-xSrxTiO2N series. However, it was found that the increase of oxygen content in the LaTiO2+xN1-x series causes oxidation of the titanium.
Magnetic properties were studied on polycrystalline composite mixtures of magnetoplumbite-type BaFe12O19 and W-type BaCo2Fe16O27 ferrites with α-Fe2O3. The respective ferrite mixtures were prepared with hematite in 0 to 70 mol% using solid state reaction. X-ray diffraction (XRD) analysis and magnetic measurement suggested the products were simple mixtures of the ferromagnetic BaFe12O19 or BaCo2Fe16O27 with α-Fe2O3 even after sintering at 1250°C. Their magnetization decreased with the increasing contents of α-Fe2O3. Their electrical resistivities of higher than 102 Ωm were too large to observe their magnetic field dependence, magnetoresistance (MR) effect. Electrical resistivity decreased to 0.36 Ωm by the post annealing of the as prepared BaCo2Fe16O27 with a small amount of α-Fe2O3 in Ar at 1000°C for 5 h. The improved electrical conductivity in 4 orders of magnitude showed the MR effect of 0.86% on the post annealed BaCo2Fe16O27 product.
Crystal structures and magnetic properties of the ternary sulfides BaLn2S4 (Ln = Sm, Gd-Lu) are reported. Their powder X-ray diffraction measurements and Rietveld analyses show that they adopt the CaFe2O4-type structure with space group Pnma. The lattice parameters linearly increase with the Ln3+ ionic radius. Magnetic susceptibility measurements were carried out from 1.8 to 300 K. The BaLn2S4 compounds for Ln = Gd-Yb show the Curie-Weiss paramagnetic behavior, and BaSm2S4 shows the van Vleck paramagnetism.
The crystal structure of Sr6Co5O14.3 has been analyzed by X-ray diffraction of a c-axis elongated prismatic single crystal grown by a flux method. Sr6Co5O14.3 is an oxygen deficient phase of Sr6Co5O15 that contains one-dimensional columns of Co-O octahedra and triangular prisms. Oxygen defects are preferentially introduced in an O site shared by the Co-O triangular prism and octahedron in the structure of Sr6Co5O15. The average structure of Sr6Co5O14.3 is refined by splitting the oxygen defect site with the rhombohedral space group R3 (a = 9.4334(6) Å and c = 12.5156(6)Å).
A polycrystalline, Ca0.88Al0.91Si1.09N2.85O0.15, was synthesized by a solid state reaction in the ternary system Ca3N2, AlN, and Si3N4, which was accompanied by a two-step heat treatment, 1600°C for 2 h and then 1800°C for 2 h under the nitrogen pressure of 0.92MPa. The space group of the polycrystalline was confirmed as orthorhombic Cmc21 (36) by means of a convergent-beam electron diffraction (CBED). The lattice parameters were refined using the Rietveld method to be a = 980.005(8), b = 564.928(4), and c = 506.241(3) pm with the corresponding weighted profile residual, Rwp = 0.092. Ca0.88Al0.91Si1.09N2.85O0.15 was formed due to the creation of cation vacancies at Ca2+ site in CaAlSiN3, which has the isomorphic structure with LiSi2N3 and NaSi2N3, by partial replacement of N3- by O2-.
Dense films of Mg doped lanthanum silicate were deposited on Fe, stainless steel and lanthanum chromite substrates by atmospheric DC plasma spraying with an input energy of 44-21 kW. Scanning electron microscopic observation showed that microstructure of the films on these substrates consists of densely piled up particles with a disk-like shape. The film deposited with the lowest input energy of 21 kW includes a larger amount of smaller particles. Bulk ionic conduction was observed in the film deposited on lanthanum chromite substrates with an input energy of 38 kW and annealed at above 1073 K. On annealing at above 1173 K the ionic conductivity measured at 973 K rapidly increased up to 2 mS cm-1, while the grain boundary resistance disappeared on annealing at above 1273 K and the electrode interface resistance gradually increased with the annealing temperature. A mixture of amorphous and apatite phase in the as-deposited films turned to a single apatite phase after the annealing at 1273 K.
A novel perovskite, PbFe1/2V1/2O3 was successfully synthesized under 7 GPa. PbFe1/2V1/2O3 exhibits a highly distorted tetragonal perovskite structure (c/a = 1.18) with the disordered arrangement of Fe and V at room temperature. The XPS reveals that the oxidation states of each ion are Pb2+, Fe3+, and V5+, respectively. The large tetragonal distortion is related to not only the presence of Pb2+ ion but also V5+ with an electronic configuration of d0. No structural phase transition up to 490 K above room temperature was observed by a high-temperature X-ray diffraction experiment. The temperature dependence of magnetic susceptibility exhibits non-Curie-Weiss behavior, which is attributed to the antiferromagnetic interaction between Fe3+-O2--Fe3+ originated from the disordered arrangement of Fe3+ and V5+ ions.
Calcium carbonate has been synthesized from Ca(OH)2 in ethanol-ethylene glycol solvent by carbonation and aging process. Carbonation in organic solvent caused Ca(OH)2 to form a white gelatinous CaCO3 product, which was then destroyed by aging process. The calcium carbonate precipitated and agglomerated in these steps was composed of stable calcite, and meta-stable aragonite and vaterite. These meta-stable particles were shaped either peanut-like or sphere. When a little amount of distilled water was added to organic solvent, the meta-stable phases remarkably decreased, whereas stable calcite increased. When Ca(OH)2 was carbonated with water addition increased to 2 vol% of the total organic solvent and aged at 40°C, only rhombic calcite was synthesized.
Tetrapod-shaped ZnO nanowires were synthesized by the directive oxidation process of Al-Zn mixtures containing Mg. Mg played a crucial role in the formation of the ZnO nanowires. X-ray diffraction data and SEM images of the nanowires revealed that the ZnO nanowires were single crystalline with wurtzite structure of hexagonal phase. With increasing Mg concentration in Al-Zn mixture from 2 mass% to 8 mass%, the tip morphology of the nanowires changed from spherical shape to needle-like shape, and went back to spherical shape. The quantity of the nanowires first increased and then decreased with Mg concentration. From Energy dispersive X-ray spectrum and SEM image taken for the tip of the ZnO nanowire, it is supposed that the growth of the nanowires proceeded in vapor-solid mechanism. The green emission centered at 510 nm was observed at room temperature, which resulted from high density of oxygen vacancies in the nanowires.
Strontium europium aluminum silicon nitride Sr0.99Eu0.01AlSiN3 was synthesized as a major phase by heating a single phase Sr1-xEux(Al0.5, Si0.5)2 intermetallic compound at 2173 K under a N2 gas pressure of 190 MPa, while mixture of SrSiN2, AlN, Sr2Si5N8, and trace amount of Sr0.99Eu0.01AlSiN3 was obtained from mixture of binary nitrides. From the intermetallic precursor, polycrystalline sample of Sr0.99Eu0.01AlSiN3 approximately 3 μm was obtained as major phase. Nitrogen pressure lower than 80 MPa resulted in the impurity phase, Sr2Si5N8 and AlN formation. The particle composition and shape of the purified sample were more homogeneous than those of the sample made from nitride mixture. The higher yield and homogeneity of the product were attributed to the higher reactivity and homogeneous distribution of the elements in the single phase intermetallic precursor. The structure of Sr0.99Eu0.01AlSiN3 was confirmed to be an isotypic structure of CaAlSiN3 in the orthorhombic space group Cmc21, analyzed by powder X-ray diffraction. The lattice parameters are a = 9.8096(1), b = 5.75856(9), c = 5.16880(7) Å, cell volume = 291.982(7) Å3. It shows an intence orange-red photoluminescence by 5d → 4f transition of Eu2+ at around 610 nm under excitation ranging from ultraviolet to 525 nm. The photoluminescence intensity was comparable or superior to those of SrAlSiN3:Eu2+ and CaAlSiN3:Eu2+ phosphor made from conventional nitride mixture method.
The effects of phase separation on the crystallization behavior of glasses were investigated in order to materialize a high-luminous glass-ceramic phosphor, which consists of fine phosphor crystals and a transparent glass matrix prepared due to phase separation. The glass ceramics in BaO-TiO2-SiO2 has the potential to show high photoluminescence intensity, because of multiple light scattering at the boundary between the phosphor crystal and the glass matrix. The crystallization behaviors of the glasses were examined using both the XRD analyses for an isothermal heat treatment process and the DTA measurements as a non-isothermal treatment process. The phase-separation in the glass significantly affected the subsequent crystallization behavior. In the non-phase-separated glasses, surface crystallization occurred first, and crystallization then proceeded inside the glasses. In the phase-separated glasses, the crystallization was considered to start at the boundary of two glass phases inside the glass and the crystals grew in BaO-TiO2-rich phases. This apparent bulk crystallization owing to the phase separation has the potential to materialize a high-performance phosphor composite.
A pulse electrophoresis method was designed for depositing TiO2 nanoparticles on a metal substrate inside an aqueous suspension. The suspension was prepared by mixing the commercially available nanometer-sized TiO2 powders (P25, Degussa) with an organic surfactant in water. A suspension with relatively high concentration (30 wt%) is stable for a few months; therefore it was not necessary to place an additional mixing during the deposition process. In the range of 2.5 to 40 Hz, the pulse direct current (PDC) charging type electrophoretic with a maximum applied voltage of 54 V (50% duty cycle) could narrow the particle size distribution or dispersity of TiO2 particles depositing on the surface of a stainless steel electrode. The morphology of the TiO2 nanoparticles layer deposited by PDC charging-mode was finer than those was deposited by a direct current (DC) charging mode.