Substitution Of Trivalent Cations Research Articles

High concentration of water in stishovite in the MORB system

Stishovite synthesized from natural basalt showed the highest concentration of hydrogen ever reported. The water concentration obtained from the polished single crystal of stishovite in this study was up to a maximum of 5628 ± 291 atoms of hydrogen per 106 silicon atoms (844 ± 44 ppm water). The concentration of hydrogen increased with increasing substitution of trivalent cations, which is almost aluminum, and temperature has a significant effect on the incorporation of hydrogen into stishovite. The result of this study suggests that stishovite in subducted basaltic crust could be an important water‐containing phase and contribute to the transport of hydrogen to the lower mantle.

高圧地球科学のフロンティア アルミニウムを含む下部マントル物質の鉱物化学

Recent high-pressure and high-temperature experiments strongly imply the possibility of non-stoichiometry for Al, Fe3+ containing MgSiO3 perovskite and the related Al-phases. Therefore, the non-stoichiometric behavior of perovskites in the MgSiO3-MgAlO2.5 system and also in the multi-component system which are similar to natural mantle minerals and rocks was discussed by considering the trivalent cation substitution mechanism: 2VI Si4+ ⇔ 2VI (Al3+, Fe3+) + VO••. Since the behavior of Al coupled with iron in the MgSiO3 perovskite is much more complicated because of the issue of fO2 conditions, it is still under discussion. For dealing with the chemical behavior of Fe3+ in the perovskite, the electron energy-loss spectroscopy (EELS) in transmission electron microscopy (TEM) is becoming a powerful tool by which we can determine the Fe3+ / ΣFe ratio of coexisting phases separately on a nanometer order of resolution. It is expected that the EELS method combined with TEM techniques will be very useful to characterize synthetic high-pressure mantle minerals.

Thermoelectric Properties of Cd3−xAxTeO6 (A = In3+, La3+ and Bi3+) Ceramics

ABSTRACTPerovskite oxide Cd3TeO6 was electron-doped by the introduction of oxygen vacancies and substitution of trivalent cations, In3+, La3+ and Bi3+. Their electric properties were investigated and compared with that of undoped Cd3TeO6. Negative temperature dependence of resistivity was observed in undoped, air-sintered Cd3TeO6. The resis tivity of Cd3−xAxTeO6 (A = In3+, La3+ and Bi3+) showed a metallic behavior with very slight temperature dependence. Indium-doped samples gave a low resistivity, which were decreased by more than three orders of magnitudes than that of air-sintered, undoped Cd3TeO6. The negative Seebeck coefficient and Hall coefficient obtained from all samples indicate that electrons are the charge carriers. The absolute Seebeck coefficients values of doped samples are decreased by 5 ∼ 10 times than that observed in undoped Cd3TeO6. Fortunately, the resistivity of indium-doped samples is low enough to provide a good thermoelectric power factor, and the optimum value of Cd2.97In0.03TeO6 was calculated as 1.35×10−4 Wm−1K−2. This result is close to that of the current best n-type perovskite thermoelectric material Ba0.4Sr0.6PbO3.

Synthesis and characterization of (Bi, AE)MnO 3 (AE=Ca, Sr) system

The effects of electron lone pairs of trivalent bismuth ions on the crystal structure and magnetic properties were examined in Bi 1− x AE x MnO 3 (AE=Ca, Sr). Polycrystalline perovskite manganites Bi 1− x AE x MnO 3 were synthesized by using a conventional ceramic technique and a high-pressure technique (4 GPa). Perovskites with triclinic, monoclinic, and tetragonal distortions were found in Bi 1− x Sr x MnO 3 from X-ray powder diffraction. In particular superlattice reflections were found in BiMnO 3 by electron diffraction which revealed two kinds of unit cell, i.e. polymorphism. The bismuth lone pair character was reflected in these structures. Both Bi 1− x Ca x MnO 3 and La 1− x Ca x MnO 3 showed the same crystal symmetry and weak ferromagnetism in the Ca-rich region. However, the magnetic moment of Bi 1− x Ca x MnO 3 was larger than that of La 1− x Ca x MnO 3 with the same x value. Their ferromagnetism was suppressed by charge ordering with further substitution of trivalent cations, although the charge ordering temperature of the La-system is higher than that of the Bi system. The difference is probably associated with the Bi lone pair.

Modification of mordenite acidity by isomorphous substitution of trivalent cations in the framework sites using the atom-planting method

Aluminum, gallium and antimony cations were incorporated into the framework of highly siliceous mordenite to modify the acid strength of Bronsted acid sites by the ‘atom-planting method’ with the corresponding metal chloride vapors at elevated temperatures. The framework incorporation of metal cations was confirmed by IR, MAS NMR and catalytic reactions. The optimum temperature to achieve a maximum incorporation of metal cations into the mordenite framework was demonstrated to be 873 K. Tetrahedral Ga cations exhibited an IR band owing to acidic bridging Si(OH)Ga groups at 3620cm −1 and a characteristic band at 159 ppm in the 71Ga MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb groups was observed at 3663 cm −1. The acid strength of Si(OH)Me (Me = Al, Ga and Sb) groups estimated from their IR frequency, IR spectra of adsorbed pyridine and the activity for toluene disproportionation and cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga> > Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactive for the toluene disproportionation, showed a significant activity for the cumene alkylation with higher para-selectivity.

Analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field

A synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3% enstatite was deformed at high temperature and high pressure such that the α → β olivine phase boundary was traversed. Both residual and new phases, their grain sizes, their deformation‐induced lattice defects, and their compositions were then characterized by analytical electron microscopy. Owing to large heterogeneities in temperature, pressure, and deformation conditions, contrasting microstructures were observed. Some areas consist essentially of untransformed olivine, while the β polymorph is the main constituent of other areas. Crystals of the β polymorph are generally free of dislocations, except at the grain boundaries of the larger grains where slip bands were nucleated (probably induced by stress concentrations). The activated glide systems are {021} [100]. The orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component. Compared to olivine, crystals of the β phase contain significantly higher concentrations of Fe and trivalent cations. All β crystals are free of inclusions, confirming the conclusion from previous infrared spectroscopic analysis that the high OH concentration in this specimen is dissolved in the β phase. Coupled with these previously measured OH concentrations, our new measurements suggest that incorporation of hydrogen into the β phase and substitution of trivalent cations into the octahedral sites may be coupled. Such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the β structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone.

Influence of the Partial Substitution of Small Rare Earths for Ca in the Superconductive Bi-Sr-Ca-Cu-O Compound

AbstractSuperconductive 2:2:1:2 phases of the type Bi2Sr2Ca1-yLnyCu2O8+x+y/2, where Ln stands for Yb or Er, have been synthesized and characterised by X-ray and electron diffraction. The small rare earth atoms prove to substitute preferentially in the Ca site. The electrical properties as well as the structural characteristics of these compounds are affected by the rare earth substitution rate. It is shown, in particular, that the long axis parameter of the subcell and the incommensurate period of the modulation decrease with increasing y. The substitution of trivalent cations for Ca requires the presence of additional oxygen atoms in the structure, the location of which is discussed.

Ferroelectric properties of lead germanate derivatives: Pb5-xMxGe3O11, m = (Cs½Bi½) and (Nd2/3□1/3

Abstract The substitution of (Cs1+,Bi3+) for Pb2+ in ferroelectric Pb5-x(Cs, Bi)xGe3O11 mixed crystals lowers Tc considerably. Improved pyroelectric coefficients of 2. 3⊃10-8 C cm-2K-1 for x = 0. 2 (Tc 376 K) suggest a gain in sensitivity by using the material for pyroelectric thermal detectors. By the substitution of trivalent cations like Nd3+ (or Bi3+) vacancies (□) are formed according to Pb5-x(Nd2/3□1/3 xGe3O11. Although this is only possible to about x = 0. 03 even this small content lowers Tc extremely to 425 K and raises the DC resistivity to around 1015OMcm.