铁和锰的化学状态对 LaFexMnyAl12-x-yO19 催化剂上 N2O 分解的影响

 以共沉淀法制备了 LaFexMnyAl12-x-yO19 六铝酸盐催化剂, 并用 X 射线衍射、扫描电镜、N2 吸附-脱附、紫外-可见漫反射光谱和穆斯堡尔谱对催化剂进行了表征, 考察了催化剂上高浓度 N2O 分解反应的性能. 结果表明, 在所考察的条件下, Mn 比 Fe 更有利于促进六铝酸盐晶相的形成. LaFexAl12-xO19 (x = 0.5, 1) 中 Fe 以 Fe3+位于六铝酸盐尖晶石结构中的四面体位和镜面层结构中的三角双锥位, 其中后者为 N2O 分解的主要活性中心. LaMnyAl12-yO19 (y = 0.5, 1) 中 Mn 优先以 Mn2+进入四面体位, 然后以 Mn3+进入尖晶石结构中的八面体位, 并成为 N2O 分解的主要活性中心.     

Synthesis,composition, and magnetic properties of the ferrimagnetic NdCu3−xMn4+xO12 perovskite-like phases

A magnetic oxide with composition close of NdCu₃Mn₄O₁₂ with a perovskite-related cubic structure ($\text{a ? 7.30}$ Å, space group Im3, Z = 2) has been synthesized by using either the high-pressure or the hydrothermal technique. The composition is strongly dependent on the synthesis conditions. A partial reduction of Mn⁴⁺ in the octahedral sites, resulting in a partial substitution of Cu²⁺ by Mn³⁺ in the Jahn-Teller sites, leads to the actual formula Nd(Cu²⁺_3?xMn³⁺_x)(Mn⁴⁺_3?xMn³⁺_1+x)O₁₂. For the compound synthesized at 650°C/2 kbar, the value of the substitution parameter x, as determined by neutron diffraction, is 0.32. For samples synthesized at higher temperatures, larger values of x are obtained. The compound is ferrimagnetic with Néel temperature of 390 K and a spontaneous magnetization of 93 emu/g at 4 K (52 emu/g at room temperature). For larger x values, magnetizations up to 118 emu/g at 4 K are obtained.     

Synthesis and solid state studies on Mb2Sb2O7 and (Mn1−xCdx)2Sb2O7 pyrochlores

Pyrochlore oxides of the type Mn₂Sb₂O₇ and (Mn_1?xCd_x)₂Sb₂O₇ have been synthesized by high-temperature solid state reactions and characterized by X-ray diffraction and chemical analysis. X-Ray diffraction studies showed that the compound Mn₂Sb₂O₇ has a rhombohedrally distorted pyrochlore structure. In the solid solutions (Mn_1?xCd_x)₂Sb₂O₇, the phases with x ≥ 0.6 are cubic. Magnetic and ¹²¹Sb Mössbauer studies indicate that all the Mn and Sb are present in the +2 and +5 state occupying A and B sites, respectively, in the pyrochlore structure. Electrical measurements indicate that the compounds are insulators or semiconductors exhibiting p-type behavior. The stoichiometry and probable cause of the rhombohedral distortion in Mn₂Sb₂O₇ and solid solutions are discussed.     

Optical and ESR investigations of lanthanum aluminates LaMg1−xMnxO19 single crystals with magnetoplumbite-like structure

New lanthanum aluminates LaMAl₁₁O₁₉ (M²⁺ = Ni, Co, Mn, Mg_1?xMn_x, 0 ≤ x ≤ 1), with magnetoplumbite-like structure have been obtained as single crystals. This paper is particularly devoted to the Mn²⁺ and $\text{Mg}{}^{\mathrm{2+}}\text{Mn}{}^{\mathrm{2+}}$ mixed compounds, which exhibit promising luminescent properties. Several characteristics of the crystals are given. The absorption spectra of the materials, as grown, are assigned to Mn²⁺ ions in tetrahedral sites. After annealing in air new absorptions attributed to octahedral Mn³⁺ ions, appear. The ESR spectra of Mn²⁺ in all these crystals exhibit axial symmetry. For x ≤ 0.25 they arise from isolated Mn²⁺ ions in slightly distorted tetrahedral sites and reveal a strong disorder effect. For x ≥0.5 the spectra consist of a single line, attributed to clusters of magnetically interacting Mn²⁺ ions.     

New transparent conductors with the M7O12 ordered oxygen-deficient fluorite structure: from In4Sn3O12 to In5.5Sb1.5O12

A new transparent conductor, containing pentavalent antimony, In_4+xSn_3−2xSb_xO₁₂, has been synthesized for 0?x?1.5. The latter exhibits an ordered oxygen-deficient fluorite structure with an ordered distribution of Sb⁵⁺ and In³⁺/Sn⁴⁺ species in the octahedral and seven-fold coordinated sites, respectively. More importantly, it is shown that the electronic conductivity of this transparent conducting oxide (TCO) at room temperature, is one order of magnitude larger for x=1 (In₅SnSbO₁₂) than for x=0 (In₄Sn₃O₁₂) and it turns to a semi-metallic behavior in contrast to In₄Sn₃O₁₂ which is a semi-conductor. The potential of this new material, as TCO, is also shown by its reflectance spectra, similar to In₄Sn₃O₁₂, involving only a small increase of the optical bandgap, by 0.15 eV.     

Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 °C). Phases containing Mn²⁺, Mn³⁺, and Mn⁴⁺ were all observed. Ternary compound formation was limited to pyrochlore (A₂B₂O₆O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi₂(Mn,Nb)₂O₇) suggesting that ≈14-30% of the A-sites are occupied by Mn (likely Mn²⁺). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi₂O₃:Mn₂O_3±x:Nb₂O₅ using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn²⁺ on A-sites and Mn³⁺ on B-sites (Bi_1.6Mn²⁺_0.4(Mn³⁺_0.8Nb_1.2)O₇, (?227), a=10.478(1) Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi_1.600Mn_1.200Nb_1.200O₇ was ≈128 with tan δ=0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.     

Crystal chemistry of Co-doped Zn7Sb2O12

Zn₇Sb₂O₁₂ forms a full range of Co-containing α solid solutions, Zn_7−xCo_xSb₂O₁₂, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co²⁺ to Co³⁺. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb₂O₅-CoO have been determined at 1100 °C for the compositions containing ?50% Sb₂O₅.     

ZnAl2O4:Mn材料的制备及发光性能

以共沉淀法与煅烧法联用,成功制备了一系列ZnAl₂O₄:xMn样品。通过扫描电镜和X射线粉末衍射测试研究了样品的形貌和物相特征,结果表明尖晶石结构的ZnAl₂O₄中[AlO₆]的八面体位可以有效被Mn⁴⁺替代。通过荧光激发和发射光谱研究了样品的发光性能,发现Mn⁴⁺在ZnAl₂O₄体系中掺杂可以显示出明亮的红色发光(发射峰值位于680 nm处)。比较不同Mn⁴⁺浓度(Mn与Al的物质的量之比)掺杂样品的发光强度时发现,Mn⁴⁺最佳掺杂浓度为0.06%。通过德克斯特公式分析了发光强度与浓度关系,探究浓度猝灭机制,结果表明最邻近离子之间能量传递造成Mn⁴⁺浓度猝灭的发生。为了提高Mn⁴⁺的发光强度,选择了7种金属离子(Li⁺、Na⁺、K⁺、Ca²⁺、Sr²⁺、Sn²⁺和Ga³⁺)与Mn⁴⁺共掺杂进入ZnAl₂O₄基质中,其中效果较突出的为Li⁺和Ga³⁺,其共掺杂使Mn⁴⁺发光强度分别增强0.6倍和1倍。     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.     

Synthesis and crystal structure of (ThCu3)(Mn3+2Mn4+2)O12, a new ferrimagnetic perovskite-like compound

Single crystals of [ThCu₃](Mn³⁺₂Mn⁴⁺₂]O₁₂, a ferrimagnetic perovskite-like compound, have been synthesized by hydrothermal conditions at 600°C and 2 kbar. They have been found to be cubic, of space group Im3, with a = 7.359 Å, and isostructural with [NaMn₃](Mn³⁺₂Mn⁴⁺₂)O₁₂. The crystal structure has been refined by single-crystal X-ray diffraction data. The Th⁴⁺ cations are surrounded by slightly distorted icosahedra; the ThO distance is 2.556 Å. The Cu²⁺ cations are also surrounded by 12 oxygens, which are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of CuO distances are 1.973, 2.800, and 3.238 Å. The octahedral MnO distance is 1.950 Å. A test based on neutron diffraction powder data indicated that the square sites are occupied by only the Cu²⁺ cations.     

Photon cascade luminescence in CaAl12O19:Pr, Cr

In this report, the Cr³⁺ ion was chosen as a co-dopant to modify the unpractical photon cascade emission properties of Pr³⁺-doped CaAl₁₂O₁₉ phosphors, which emit one near-UV photon from the ¹S₀ state followed by visible photons from the ³P₀ state, into phosphors that emits two visible photons through energy transfer. The photon cascade emission process via energy transfer and the transfer mechanisms were systemically investigated by luminescence spectra and dynamics using synchrotron radiation as one of the excitation sources. The internal visible quantum efficiency of CaAl₁₂O₁₉:Pr, Cr was estimated and compared with CaAl₁₂O₁₉:Pr and the drawback to obtain the visible quantum efficiency higher than unit for CaAl₁₂O₁₉:Pr, Cr as a practical VUV phosphor was also discussed.     

Aurivillius phases of PbBi4Ti4O15 doped with Mn synthesized by molten salt technique: Structure, dielectric, and magnetic properties

Doping of manganese (Mn³⁺/Mn⁴⁺) into the Aurivillius phase Pb_1−xBi_4+xTi_4−xMn_xO₁₅ was carried out using the molten salt technique for various Mn concentrations (x=0, 0.2, 0.4, 0.6, 0.8, and 1). Single phase samples could be obtained in the composition range with x up to 0.6 as confirmed by X-ray and neutron diffraction analysis. Dielectric measurements show a peak at 801, 803, 813 and 850 K for samples with x=0, 0.2, 0.4, and 0.6, respectively, related to the ferroelectric transition temperature (T_c). The main contribution of the in-plane polarization for x≤0.2 which was calculated from the atomic positions obtained by the structure analysis is the dipole moment in the Ti(1)O₆ layer; however, for x≥0.4 the polarization originates from the dipole moment in the Ti(2)O₆ layer. Mn doping in the Pb_1−xBi_4+xTi_4−xMn_xO₁₅ does not show any long range magnetic ordering.     

An investigation of the Cr1−xMnxO2 series and the characterization of orthorhombic CrMnO4

Several members of the Cr_1?xMn_xO₂ series were prepared in the tetrahedral anvil press by the reaction of CrO₂ with MnO₂. The tetragonal, rutile-type products were single-phase and have been characterized by crystallographic and magnetic measurements. The results are consistent with the formulations Cr⁴⁺_1?2xCr³⁺ Mn⁵⁺O₂ for 0 ? x ? 0.5. At low manganese concentration, x < 0.20, the magnetic moments are consistent with ferromagnetic contribution from Mn⁵⁺. A two-phase product was noted at the composition x = 0.90. The CrMnO₄ composition was found to have a powder pattern similar to that of orthorhombic PtO₂.     

Mn environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[Mg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic Li[Li_1/3Mn_2/3]O₂ oxides (containing Mn⁴⁺ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni²⁺ and Mn⁴⁺ are paramagnetic. For LiMg_0.5−xNi_xMn_0.5O₂ and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂, an EPR response from Mn⁴⁺ ions only was detected, while the Ni²⁺ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg_0.25Ni_0.25Mn_0.5O₂ and Li[Li_0.10Ni_0.35Mn_0.55]O₂, two types of Mn⁴⁺ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn⁴⁺ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1[Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2[Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li_1/3Mn_2/3]O₂.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl2O4 phosphors

MgAl₂O₄:Mn phosphors have been prepared at 500 °C by combustion route. Powder X-ray diffraction (XRD) indicated the presence of mono-MgAl₂O₄ phase. Scanning electron microscopy showed that the powder particle crystallites are mostly angular. Fourier transform infrared spectroscopy confirmed the presence of AlO₆ group which makes up the MgAl₂O₄ spinel. Photoluminescence studies showed green/red emission indicating that two independent luminescence channels in this phosphor. The green emission at 518 nm is due to ⁴T₁ → ⁶A₁ transition of Mn²⁺ ions. The emission at 650 nm is due to the charge-transfer deexcitation associated with the Mn ion. EPR spectrum exhibits allowed and forbidden hyperfine structure at g=2.003. The g≈2.00 is due to Mn²⁺ ion in an environment close to tetrahedral symmetry. It is observed that N and χ increase with decrease of temperature obeying the Boltzmann law. The variation of zero-field splitting parameter (D) with temperature is evaluated and discussed.     

Magnetic properties in the system BaCo1−xMnxO3 and SrCo1−xMnxO3 (0 ≦ x ≦ 1)

The compounds in the systems of BaCo_1?xMn_xO₃ (0 ≦ x ≦ 1) and SrCo_1?xMn_xO₃ (0 ≦ x ≦ 1) were prepared at an oxygen pressure of 1400 bars. The former had a two-layer hexagonal structure and that of the latter was cubic perovskite type. From the variation of the unit-cell parameters and of the magnetic properties, it is found that the Co⁴⁺ ions change from the low-spin to the high-spin state. In the system of SrCo_1?xMn_xO₃, the change of magnetic property from ferromagnet to antiferromagnet is related to the spin state of Co⁴⁺ ions located at the octahedral sites.     

The synthesis and crystal structure of CaCu3Mn4O12: A new ferromagnetic-perovskite-like compound

Single crystals of CaCu₃Mn₄O₁₂, a new ferromagnetic perovskite-like compound (T_c ? 160°C), have been synthesized at 50 kbar and 1000°C. By X-ray analysis it was found to be cubic (a = 7.241Å), space groupIm3 with two molecules per unit cell. The two Ca²⁺ and six Cu²⁺ cations occupy theA sites of the ideal perovskite structure, while the eight Mn⁴⁺ cations occupy theB sites. In theIm3 space group the sites occupied by the calcium and copper cations have different point symmetry and therefore the 12-oxygen polyhedra have different distortions. The Ca cations are surrounded by slightly distorted icosahedra, the CaO distance is 2.562Å. The twelve oxygens around the copper cations are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of Cu0 distances are 1.942, 2.707, and 3.181Å, respectively. The octahedral Mn0 distance is 1.915Å. This arrangement is similar to that found in NaMn₃Mn₄O₁₂. A comparison between the two structures and a discussion of their thermal data are given.     

Synthesis and phase transition of Li-Mn-O spinels with high Li/Mn ratio by thermo-decomposition of LiMnC2O4(Ac)

LiMnC₂O₄(Ac) precursor in which Li⁺ and Mn²⁺ were amalgamated in one molecule was prepared by solid-state reaction at room-temperature using manganese acetate, lithium hydroxide and oxalic acid as raw materials. By thermo-decomposition of LiMnC₂O₄(Ac) at various temperatures, a series of Li_1+y[Mn_2−xLi_x]_16dO₄ spinels were prepared with Li₂MnO₃ as impurities. The structure and phase transition of these spinels were investigated by XRD, TG/DTA, average oxidation state of Mn and cyclic voltammeric techniques. Results revealed that the Li-Mn-O spinels with high Li/Mn ratio were unstable at high temperature, and the phase transition was associated with the transfer of Li⁺ from octahedral 16c sites to 16d sites. With the sintering temperature increasing from 450 to 850 °C, the phase structure varied from lithiated-spinel Li₂Mn₂O₄ to Li₄Mn₅O₁₂-like to LiMn₂O₄-like and finally to rock-salt LiMnO₂-like. A way of determining x with average oxidation state of Mn and the content of Li₂MnO₃ was also demonstrated.