Refined cell parameters of the Ln2WO6-type rare earth tungstates

Rare earth tungstates of the stoichiometry Ln₂WO₆ where Ln = Ce to Lu have been prepared and their lattice parameters were refined by a least-squares method. They have two structures: the $\text{C}\text{2}\text{c}$ symmetry for the compositions Ce₂WO₆ through Ho₂WO₆, and a monoclinic group from Er₂WO₆ to Lu₂WO₆ crystallizing most likely in the $\text{P}\text{2}\text{m}$ or $\text{P2}{}_1\text{m}$ structure. High pressure modifications are described for Dy₂WO₆ and Ho₂WO₆.     

Vibrational spectra of compounds Ln2MoO6 and Ln2WO6

The vibrational spectra of compounds Ln₂WO₆ and Ln₂MoO₆ (Ln = lanthanide, including Y and Bi) are reported. Neglecting details it is possible to assign the internal vibrations of the molybdate and tungstate group. The results are in agreement with the structural data known up till now and suggest further that tungsten in the unknown Y₂WO₆ structure is in six-coordination, that Bi₂MoO₆ shows still another modification than koechlinite and that vibrational spectroscopy may yield additional structural information for compounds like those under consideration.     

Influence of rare earth ions on CO oxidation over Ln2NiO4 oxides (Ln=La, Pr or Nd)

CO oxidation was studied over Ln₂NiO₄ compounds in an all-glass static recirculatory system. The reactions were performed at an initial total pressure of 50±2 Torr, in the temperature interval 340–450°C. The apparent activation energy was found to decrease from La to Nd. A correlation between activity and binding energy of oxygen in the compound as well as with the tolerance factor is observed. The role of rare earth ions in these catalysts is discussed.

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CO Ln₂NiO₄ , . 50±2 340–450°C. La Nd. , . .

     

Influence of rare earth ions on the properties of Pd/SiO2 in CO hydrogenation

Systematic studies of the effect of the lanthanoid (Ln) nature on the activity and selectivity of (Pd+Ln)/SiO₂ catalysts obtained by decomposition of anchored Ln and Pd complexes in methanol synthesis from CO and H₂ have been carried out. It has been established that the highest promoting effect is observed for Pr and Ho ions.

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(Ln) (Pd+Ln)/SiO₂ , Ln Pd, CO H₂. Pr Ho.

     

Effect of alkali metal ions on the catalytic of Pd/SiO2?AlPO4 systems

The effect of different alkali metal hydroxides on the metallic dispersity and catalytic activity of SiO₂–AlPO₄ (8020, by mass) supported palladium catalysts has been studied. The reduction by hydrogen transfer of benzylideneacetone and styrene (using cyclohexene and n-octanol, respectively, as hydrogen donors) and the liquid phase reduction of cyclohexene at low pressure (5 bar) in a Parr type reactor have been used as test reactions to follow changes in the catalytic activity.

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(SiO₂–AlPO₄/8020 ). ( n-, , , (5 ) .

     

Synthesis and properties of the double perovskites La2NiVO6, La2CoVO6, and La2CoTiO6

The double perovskites La₂CoVO₆, La₂CoTiO₆, and La₂NiVO₆, are described. Rietveld fitting of neutron and powder X-ray diffraction data show La₂NiVO₆ and La₂CoVO₆ to have a disordered arrangement of B-cations whereas La₂CoTiO₆ shows ordering of the B-cations (with ∼5% Co/Ti inversion). Curie-Weiss fits to the linear region of the 1/χ plots reveal Weiss temperatures of −107, −34.8, and 16.3 K for La₂CoVO₆, La₂CoTiO₆, and La₂NiVO₆, respectively, and magnetic transitions are observed. La₂CoTiO₆ prepared by our method differs from material prepared by lower-temperature routes. A simple antiferromagnetic spin model is consistent with the data for La₂CoTiO₆. These compounds are semiconductors with bandgaps of 0.41 (La₂CoVO₆), 1.02 (La₂CoTiO₆) and 0.45 eV (La₂NiVO₆).     

Studies on magnetic and calorimetric properties of double perovskites Ba2HoRuO6 and Ba2HoIrO6

Magnetic properties of double perovskite compounds Ba₂HoRuO₆ and Ba₂HoIrO₆ have been reported. Powder X-ray and neutron diffraction measurements show that these compounds have a cubic perovskite-type structure with the space group and the 1:1 ordered arrangement of Ho³⁺ and Ru⁵⁺ (or Ir⁵⁺) over the 6-coordinate B sites. Results of the magnetic susceptibility and specific heat measurements show that Ba₂HoRuO₆ exhibits two magnetic anomalies at 22 and 50 K. Analysis of the temperature dependence of magnetic specific heat indicates that the anomaly at 50 K is due to the antiferromagnetic ordering of Ru⁵⁺ ions and that the anomaly at 22 K is ascribable to the magnetic interaction between Ho³⁺ ions. Neutron diffraction data collected at 10 and 35 K show that the Ba₂HoRuO₆ has a long range antiferromagnetic ordering involving both Ho³⁺ and Ru⁵⁺ ions. Each of their magnetic moments orders in a Type I arrangement and these magnetic moments are anti-parallel in the ab-plane with each other. The magnetic moments are aligned along the c-direction. On the other hand, Ba₂HoIrO₆ is paramagnetic down to 1.8 K.     

Effect of conversion of NO2 into no on the catalytic reduction of NO2 with C3H6 in excess O2

During the reduction of NO₂ by C₃H₆ in O₂ over alumina-supported Au, Rh and Pt it was found that three parallel reactions take place,i.e., reduction of NO₂ to N₂ and N₂O, partial decomposition of NO₂ to NO and oxidation of C₃H₆ to CO and CO₂. In the absence of C₃H₆, the NO₂→NO+O₂ reaction reaches a fast equilibrium on Rh and Pt but not on Au and γ-Al₂O₃. Addition of C₃H₆ to the NO₂+O₂ mixture leads to the formation of NO above equilibrium conversion levels.     

Effect of addition of intermetallic componds on the catalytic activity of La2CuO4

Decomposition of nitrous oxide has been studied on pure La₂CuO₄, La₂CuO₄ with 5 wt.% CaNi₅ and La₂CuO₄ with 5 wt.% NiO in the temperature range 200–480°C at 200 Torr initial pressure of reactant gas. The addition of 5 wt.% CaNi₅ decreases the energy of activation drastically in comparison with La₂CuO₄. The results are explained on the basis of the dispersity of Ni/NiO on La₂CuO₄.

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La₂CuO₄ 5.% CaNi₅ 5 .% NiO , 200–480°C 200 . 5 . % CaNi₅ . Ni/NiO La₂CuO₄.

     

稀土-天冬氨酸-咪唑配合物Sm2(Asp)2(Im)6(ClO4)6.6H2O的特殊低温相变和热力学性质

合成了标题配合物,测定了其晶体在80~385 K温度范围的等压摩尔热容,低温区间的绝热量热和差示扫描量热均发现配合物在220K和245K附近存在固-固相转变,推测其机理可能是配合物中高氯酸根的重取向运动不同阶段所造成;根据实验热容数据和热力学公式,计算出配合物在80~385 K温度区域内相对于298.15K的标准热力学函数[HT-H298.15]和[ST-S298.15],根据热容测定数据计算出该相变的焓变和熵变。用热重法检测了配合物的热稳定性并推测其热分解机理。这两个低温区相变过程的发现,使开发此类配合物作为新低温相变材料成为可能。     

Effect of transition element doping on crystal structure of rare earth borosilicides REB44Si2

On a previous study on samples of doped-YB₄₄Si₂, an improvement of thermoelectric properties has been achieved. Regarding the interesting effect of the doping of transition elements on the thermoelectric properties, a single crystal study has been carried out on Zn doped, Rh doped and Ni doped samples to assess how the transition element doping affects the crystal structure. Refinements were carried out based on the structural model solution of YB₄₄Si₂ reported in a previous study. Variations in the silicon contents were found in the doped single crystals. Splitting of partially occupied sites has also been detected for some of the doped samples. In this paper we present differences in the partial occupations of boron and silicon sites. Possibility of transition elements insertions based on the differences in crystal structures will be presented.     

Crystal growth, structures, and optical properties of the cubic double perovskites Ba2MgWO6 and Ba2ZnWO6

Single crystals of the tungstates Ba₂MgWO₆ and Ba₂ZnWO₆ have been grown for the first time. The crystals were prepared with molten potassium carbonate acting as a flux. According to the single-crystal X-ray diffraction structure determination, the compounds crystallize in space group Fm3¯m of the cubic system with a double perovskite structure, A₂BB′O₆. These structural findings were confirmed with neutron diffraction on polycrystalline samples synthesized by a high-temperature solid-state route. Both sets of diffraction data reveal that the M²⁺ and W⁶⁺ cations are fully ordered on the B and B′ sites. Ba₂MgWO₆ and Ba₂ZnWO₆ exhibit room-temperature luminescence with green and yellow emissions, respectively.     

Valence and environment of rare earth ions in CaAl2O4:Eu,R persistent luminescence materials

The existence of the different R²⁺/R³⁺/R^IV (R: rare earth) ions as well as the modifications in the structural environment around the dopant and co-dopants in CaAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials was studied by L_III edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements at Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY) (Hamburg, Germany). The measurements were carried out at 10 and 296 K for selected rare earth (co-)dopants (Eu²⁺; Ce³⁺, Nd³⁺, Sm³⁺, and Yb³⁺).     

Magnetic and structural properties of NaLnMnWO6 and NaLnMgWO6 perovskites

We have prepared 14 new AA′BB′O₆ perovskites which possess a rock salt ordering of the B-site cations and a layered ordering of the A-site cations. The compositions obtained are NaLnMnWO₆ (Ln=Ce, Pr, Sm, Gd, Dy, and Ho) and NaLnMgWO₆ (Ln=Ce, Pr, Sm, Eu, Gd, Tb, Dy, and Ho). The samples were structurally characterized by powder X-ray diffraction which has revealed metrically tetragonal lattice parameters for compositions with Ln=Ce, Pr and monoclinic symmetry for compositions with smaller lanthanides. Magnetic susceptibility vs. temperature measurements have found that all six NaLnMnWO₆ compounds undergo antiferromagnetic ordering at temperatures between 10 and 13 K. Several compounds show signs of a second magnetic phase transition. One sample, NaPrMnWO₆, appears to pass through at least three magnetic phase transitions within a narrow temperature range. All eight NaLnMgWO₆ compounds remain paramagnetic down to 2 K revealing that the ordering of the Ln³⁺ cations in the NaLnMnWO₆ compounds is induced by the ordering of the Mn²⁺ sub-lattice.     

Determination of rare earth elements in Gd2O3 by ICP-OES

Summary Rare earth impurities in high-purity Gd₂O₃ have been determined by ICP-OES. HREE except Ho were analysed by directly nebulising 1 mg/ml of the matrix solution and the LREE were analysed after separation on an anionexchange Dowex 1×8 resin. The recovery studies show nearly 100% separation. The determination of 30 ppm of HREE in gadolinium oxide is possible.     

Some A6B5O18 cation-deficient perovskites in the BaO-La2O3-TiO2-Nb2O5 system

Some dielectric oxides have been synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, Ba₅LaTi₂Nb₃O₁₈ and Ba₄La₂Ti₃Nb₂O₁₈ are identified as the A_nB_n−1O_3n (n=6) type cation-deficient perovskites with space group and lattice constants , and for Ba₅LaTi₂Nb₃O₁₈; , and for Ba₄La₂Ti₃Nb₂O₁₈, respectively. Their ceramics exhibit high dielectric constant up to 57 and high quality factors (Qf) up to 21,273 GHz. The temperature coefficient of resonant frequency (τ_f) of these ceramics is decreased with the increase of B-site bond valence.     

Ln2Zr2O7 compounds (Ln=La,Pr, Sm,Y) with varied rare earth A sites for low temperature oxidative coupling of methane

The replacement of Ln site with different rare earth cations alters the crystalline phases of Ln₂Zr₂O₇, which in turn influences the surface active oxygen and alkaline sites significantly. The abundance and the interaction of the two types of sites determine the OCM reaction performance.     

La2O3助剂对Rh/SiO2催化CO加氢反应性能的影响

采用溶胶凝胶法制备了SiO_2和La_2O_3-SiO_2载体,再通过浸渍法分别引入Rh-La和Rh组分,研究考察了La引入方式对Rh/SiO_2催化CO加氢反应性能的影响。结果表明,La的添加有利于提高Rh的分散度,促进Rh+中心数的增加,有效地抑制产物中CO2的生成,提高含氧化合物选择性。此外,La的引入方式会影响La与Rh间的相互作用强弱,Rh和La共浸渍制得的2Rh-5La_2O_3/SiO_2催化剂中Rh-La相互作用较强,削弱的Rh-CO键有利于反应过程中CO的插入反应,使得产物以C_(2+)含氧化合物为主。而La以助剂形式掺入SiO_2制得的2Rh/5La_2O_3-SiO_2催化剂具有较弱的La-Rh相互作用,其产物则以甲醇、乙醇等低碳醇为主。     

Effect of Mo content in MoO3/g-Al2O3 on the catalytic activity for transesterification of dimethyl oxalate with phenol

The influence of molybdenum content on the catalytic performance in the transesterification of dimethyl oxalate (DMO) with phenol to methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) was investigated. The results indicated that the MoO₃/Al₂O₃ catalyst with 14 wt% Mo content gave maximal DPO yield with 6.1% and 75.1% DMO conversion. The component, structure and phase of MoO₃/Al₂O₃ catalysts were characterized by means of X-ray diffraction (XRD), BET specific surface area, temperature-programmed desorption of ammonia (NH₃-TPD), and FTIR analysis of adsorbed pyridine. This revised version was published online in June 2006 with corrections to the Cover Date.     

Electrical transport properties of a nonstoichiometric rare earth sulfide,EuGd2S4

The electrical transport properties of nonstoichiometric EuGd₂S₄ prepared by heating under a high vacuum have been investigated. The samples heated were classified into two groups on the basis of their electrical transport behavior. One group comprised semiconducting materials heated at 1500 and 1600°C, for which the transport mechanism was found to be via electron hopping with activation energies ranging from 0.013 to 0.027 eV. Another group comprised metallic materials heated at 1700 and 1800°C. Their electrical transport was carried out through ordinary band conduction over the measured temperature range except at temperatures lower than 120°K, where hopping with a very small activation energy (～0.0035 eV) occurred predominantly.