磁铅石型复合氧化物CaNiyAl12-yO19-δ的制备和性能表征

制备了一系列磁铅石型复合氧化物ＣａＮｉｙＡｌ１２－ｙＯ１９－δ（ｙ＝０．３,０．６,０．９,１．０）,并用ＸＲＤ、ＸＰＳ、ＴＰＲ和ＴＧＡ技术对其结构和性能进行了表征。结果表明,该系列复合氧化物具有相同的晶体结构和相似的还原稳定性。当Ｎｉ调变量既０〈ｙ≤１,可以获得磁铅石型复合氧化物ＣａＮｉｙＡｌ１２－ｙＯ１９－δ晶相。在７８０℃二氧化碳重整甲烷制合成气反应过程中,还原态复合氧化物ＣａＮｉｙＡｌ１２     

Structural phase transitions in BaV6O11

BaV₆O₁₁ was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. [V(1)O₆]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal [0 0 1] direction. The layers are connected in the direction of c via trigonal [V(3)O₅]-bipyramids and [V(2)O₆]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P6₃/mmc, at lower temperature the symmetry reduces to P6₃mc. For the refinements in P6₃mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the [V(1)O₆]-octahedra and [V(3)O₅]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.     

Crucial influence of crystal site disorder on dynamical spectral response in artificial magnetoplumbites

High quality PbFe₁₂O₁₉ single crystals were grown by a solution-melt crystallization technique and characterized by synchrotron radiation diffraction structural studies, Mössbauer, Raman and infrared spectral measurements. In the terahertz and sub-terahertz range two absorption bands are observed, which are associated with the dynamics of structurally disordered lead and iron ions. We develop a structural model that comprises Fe²⁺ ions at the (1/2 × 4e) (0, 0, z) position with bipyramidal oxygen environment, and Pb ions at the (24l) (x, y, z) position with generic strongly distorted 12 vertex polyhedron of oxygen. The atomic displacement ellipsoids of the oxygen surrounding the lead ion are stretched towards the Pb ion within the plane perpendicular to the hexagonal axis. The suggested model is in full agreement with the local symmetry of the Fe2 and Pb ions and with the chemical composition of the sample determined by X-ray analysis.     

VUV-UV luminescence of magnetoplumbite: (Sr0.96−xBa0.04)Al12−yMgyO19:Tbx

The title compounds (Sr_0.96−xBa_0.04)Al_12−yMg_yO₁₉:Tb_x (0<x<0.4; 0<y<0.18) are single-phase magnetoplumbite determined by X-ray powder diffraction analysis. The characteristic emission lines of ⁵D₃→⁷F_j (j=2, 3, 4, 5) and ⁵D₄→⁷F_j (j=4, 5, 6) of Tb³⁺ are recorded under the VUV excitation. The intensive luminescence mainly comes from ⁵D₃→⁷F_j transition when the concentration of Tb³⁺ is low. However, when the concentration of Tb³⁺ starts to increase from very low concentration, ⁵D₄→⁷F_j transition is becoming dominant. Three broad excitation bands at 165, 193 and 233 nm have been observed. The band at 165 nm originates from the overlap between the host absorption and the charge transfer of Tb³⁺-O²⁻. The other two broad bands are the first spin-allowed and the spin-forbidden of 4f-5d transition, respectively. The experimental observation of the 4f-5d transition of Tb³⁺ is consistent well with the theoretical expectations.     

磁铅石型复合氧化物CaNiyAl12-yO19-δ的制备和性能表征

制备了一系列磁铅石型复合氧化物CaNiyAl12-Yo19-δy=0.3,0.6,0.9,1.0),并用XRD、XPS、TPR和TGA技术对其结构和性能进行了表征。结果表明,该系列复合氧化物具有相同的晶体结构和相似的还原稳定性。当Ni调变量既0相似文献   

BaSn6Co6O19—A novel frustrated antiferromagnet with the magnetoplumbite type structure

Single crystals of the novel compound BaSn₆Co₆O₁₉ with maximum width 1 mm and thickness around 0.05 mm were grown from a barium chloride flux. The composition was determined from refinements of single crystal X-ray diffraction data and microprobe analysis. BaSn₆Co₆O₁₉ crystallizes in the magnetoplumbite type structure (hexagonal, space group P6₃/mmc, a=6.0940(1) Å, c=23.9633(5) Å, V=770.69 Å³, Z=2). A significant disorder is generated by random occupation of two octahedrally coordinated crystallographic sites with Co²⁺ and Sn⁴⁺ ions, while further sites are exclusively occupied by either Co²⁺ (tetrahedrally coordinated) or Sn⁴⁺ (octahedrally coordinated). One site with mixed occupation realizes the topology of a kagome net. The temperature dependence of the magnetic susceptibility for a single crystal BaSn₆Co₆O₁₉ reveals a low temperature antiferromagnetic order at T_N=14 K. A relatively large value of frustration factor f_//=|Θ_W//|/T_N≈26 and f_⊥=|Θ_W⊥|/T_N≈12 implies a frustrated antiferromagnetism.     

Crystal structure of Eu-doped magnetoplumbite-type lanthanum aluminum oxynitride with emission site splitting

Eu-doped lanthanum aluminum oxynitride (LaAl₁₂(O,N)₁₉) with magnetoplumbite structure was prepared by nitridation of the oxide precursor obtained from aluminum glycine gel and subsequent post-annealing. Eu-doped lanthanum aluminum oxynitride exhibited blue light emission at 440 nm with a shoulder at 464 nm under excitation at 254 nm. Isostructural Eu-doped calcium aluminum oxide (CaAl₁₂O₁₉) exhibited a single emission peak at 415 nm. Structural refinement using neutron powder diffraction indicated that the lanthanum site occupied partially by Eu²⁺ splits into 2d and 6h sites in the aluminum oxynitride. The longer emission and the shoulder peak in the former aluminum oxynitride were observed in relation to the increasing covalency as well as crystal field splitting around doped Eu²⁺ induced by site splitting involved with the two kinds of anions.     

High pressure synthesis and structure of a new magnetoplumbite-type cobalt oxide SrCo12O19

A new magnetoplumbite-type hexagonal cobalt oxide, SrCo₁₂O₁₉, has been synthesized at a high pressure of 3 GPa and its structural and magnetic properties have been studied using single crystalline samples. Bond-valence calculations for five crystallographically independent Co sites show a rather wide charge distribution from +2.15 to +3.50. Magnetic measurements have revealed anisotropic magnetic behavior below a magnetic ordering temperature of 80 K. Comparative discussions on the structure-property relationship for SrCo₁₂O₁₉ and related compounds suggest the presence of localized spins with a uniaxial anisotropy at the trigonal bipyramidal site.