The flux growth of some new rare earth and iron group complex oxides

The flux growth of crystals of a number of new rare-earth and transition metal compounds is reported. The following empirical formulae were in good agreement with electron probe microanalysis (EPMA): (Ba, Gd)TiO₃, R₄Ba₆(BO₃)₉(R = Ho to La), Ni₃Nb₂O₈, Ni₈NbB₃O₁₅, Ni₂V₂PbO₈, Ta₂Co₄O₉ and PbCr_2.3Ti_2.3O₉. X-ray powder pattern data are given. The growth of crystals of GdVO₃, Pb₂CrO₅ and Ni₃V₂O₈, which have previously been prepared only as powders, is also reported.     

Crystal growth and magnetic susceptibility of some rare-earth compounds

Magnetic susceptibility measurements have been carried out on five series of rare-earth compounds using an ac mutual inductance technique. The compounds satisfy the general formulae RNbO₄, R₃NbO₇, R₂TiO₅, R₂Ti₂O₇, or RAIO₃ where R is a rare-earth ion. The measurements were mainly carried out in the helium range 1 to 4.2° K, and the results analysed in terms of the general Van Vleck formula = C/(T+) +. Many of the compounds were observed to undergo antiferromagnetic transitions, and in some cases showed evidence of extremely anisotropic behaviour above the Néel temperature.     

Improved flux growth of rare-earth arsenates,RAsO4

The tetragonal rare-earth arsenates (R = Lu to Sm) have previously been grown as small crystals from Pb₂As₂O₇ as flux.Attempts to prepare larger crystals by modifying the flux and controlling evaporation by means of a crystalline seal are described. The crystal size was increased by 1 to 2 orders of magnitude.The preparation of the monoclinic arsenates (R = Nd to La) is also described.     

Wetting of rare-earth element oxides by metallic melts

The results of studies on the wettability and contact interaction of rare-earth (REE) oxides and aluminium oxide (for comparison) with metallic melts are presented. Proceeding from analysis of the data obtained, a mechanism of interaction of metallic melts with rare-earth element oxides is proposed, based on the interaction of the metallic melt components with the cationic and the anionic sublattice of the oxides. The necessity to take into account the interaction of the melt metal with the cationic sublattice of oxide is determined by a high chemical affinity of the REE for quite a number of metals and by specific features of the structure of the surface of the oxides.     

On the high-temperature plasticity of cubic-C rare-earth oxides

High-temperature plasticity of cubic-C rare-earth oxides is studied through a study of the behaviour of two different dislocations. One is largely dissociated by glide; the other dissociates by climb. In both case the stacking fault is the same and is characteristic of the cubic-C structure. In the first part, a non-stoichiometric climb model of extended dislocation is suggested. Such a model may lead to a recovery process through an unpinning mechanism of the dislocations. It has been shown that, depending upon the size of an initial cluster, a super-jog may be obtained without any activation energy. It is suggested that such a climb mechanism may be particularly likely in oxides containing a large amount of impurities. In the second part, the climb dissociation is considered. The influence of the surrounding network of dislocation has been studied. The results show an increase in the climb dissociation width due to the nearest dislocations. As a consequence the dislocation is sessile and therefore plays an important role in the high-temperature plasticity of these oxides.     

The behavior of iron oxides in reducing atmospheres

Comparison of unpromoted α-Fe₂O₃, K, and Tl promoted α-Fe₂O₃ in atmospheres of H₂, CO, 1:1 $\text{H}{}_2\text{CO}$, and CO following an H₂ prereduction showed that K or Tl promoters do not significantly modify the reduction behavior of α-Fe₂O₃, but can either increase (K) or decrease (Tl) the rate of growth of surface carbon in CO-containing atmospheres.     

Flux growth of some complex oxide materials

This paper describes the preparation of the following complex oxide crystals which have not previously been grown by the flux method: FeNbO₄, MnWO₄, CoWO₄, NiWO₄, RMn₂O₅ (R = Er to Sm, and Y), RMnO₃ (R = Er to Gd), LaOCl, La₂Ti₂O₇₁, Bi₂Sn₂O₇, and PbTiP₂O₈. Single crystal X-ray diffraction data are reported for the latter material, the preparation of which has not previously been reported, and for La₂Ti₂O₇ and Bi₂Sn₂O₇₁, for which only powder data were previously available. Improved methods for the growth of Mn₃O₄, LaCoO₃, RPO₄ (R = Yb to Gd), RCrO₃ (R = Lu, Yb), ErOF and NaNbO₃ are also reported.     

X-ray and spectrographic investigations of phase transitions in refractory oxides (ZrO2 and rare-earth oxides)

The investigation of phase transformations in rare-earth oxides and polymorphism of zirconium dioxide has shown that only a complex application of x-ray methods (at room and high temperatures), molecular spectroscopy, thermal analysis, thermogravimetric analysis and also phase-shift chemical analysis allows one to determine the mechanism and kinetics of these physiochemical processes.     

The origin of highly efficient selective emission in rare-earth oxides for thermophotovoltaic applications

Rare-earth oxide materials emit thermal radiation in a narrow spectral region, and can be used for a variety of different high-temperature applications, such as the generation of electricity by thermophotovoltaic conversion of thermal radiation. However, because a detailed understanding of the mechanism of selective emission from rare-earth atoms has so far been missing, attempts to engineer selective emitters have relied mainly on empirical approaches. In this work, we present a new quantum thermodynamic model to describe the mechanisms of thermal pumping and radiative de-excitation in rare-earth oxide materials. By evaluating the effects of the local crystal-field symmetry around a rare-earth ion, this model clearly explains how and why only some of the room-temperature absorption peaks give rise to highly efficient emission bands at high temperature (1,000-1,500 degrees C). High-temperature emissivity measurements along with photoluminescence and cathodoluminescence results confirm the predictions of the theory.     

X-ray photoelectron and x-ray absorption spectroscopic study of rare-earth molybdenum oxides

The chemical state of atoms in rare-earth molybdenum oxides has been studied by x-ray photoelectron and x-ray absorption (XANES) spectroscopies. The photoelectron spectra has been analyzed in the Mo 3d, Mo 3p, O 1s, Ce 3d, Y 3d, Er 4d, and Yb 4d regions. The results indicate that the oxidation state of the molybdenum in the compounds studied is 6+. The measured Mo K-edge spectra and XANES calculations are used to develop a model of a molybdenum-oxygen cluster in the rare-earth molybdenum oxides.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

Characterisation of high purity rare-earth oxides by x-ray fluorescence methods

X-ray fluorescence methods for the determination of individual rare earth impurities in high purity rare earth oxidesviz., Y₂O₃, La₂O₃, CeO₂, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃ and Er₂O₃ have been developed in our laboratory. The samples are converted to oxalate form and double-layer pellets are prepared using boric acid as binding material. A semiautomatic x-ray spectrometer is used for the analyses. Choice of instrumental parameters and analysis lines is discussed. The determination range varies from 0.005% to 1.0% for most elements with good precision and adequate accuracy.     

掺杂稀土氧化物的ZnO-Bi2O3系压敏陶瓷中的"软心"

研究了掺杂Ce2O3和Gd2O3对ZnO—Bi2O3系压敏陶瓷电气性能的影响，发现大尺寸试样的内层电位梯度明显低于表层，即表现出“软心”特征．“软心”导致试样平均电位梯度随着试样几何尺寸的增大而下降．沿轴向晶粒尺寸没有明显的差异．根据沿轴向截取的等厚薄片的电流与温度的关系，发现表层的晶界势垒高度比内层的高．这种晶界势垒高度的非均匀分布可能与添加稀土氧化物后造成的氧元素分布不均匀有关．