Rare Earth Silicates Research Articles

Bulk and grain boundary ionic conduction in lithium rare earth-silicates “LiLnSiO 4” (Ln = La, Nd, Sm, Eu, Gd, Dy)

The lithium rare earth silicates of nominal composition “LiLnSiO 4” (Ln = La, Nd, Sm, Eu, Gd, Dy) have been synthesized. The lithium ion conductivity of their sintered samples was found to consist of two contributions; one is the grain boundary composed possibly of lithium silicate amorphous phases and the other the bulk phase with an apatite structure. Although the conductivity of the bulk phase was smaller than that of the grain boundary, it was apparently increased by several orders of magnitude than that of stoichiometric apatites of Li x Ln 10 − x Si 6O 24O 3 − x with x = 2 and x = 3. From the structure refinement of the bulk phase, this enhancement in the conductivity of the apatite phase in LiLnSiO 4 was reasonably interpreted by the model where lithium ions and vacancies were both partially introduced to the 4 f site of the apatite structure.

The Identity of Monoclinic La2O3 and Monoclinic Pr2O3 with La9.33(SiO4)6O2 and Pr9.33(SiO4)6O2, Respectively

A critical comparison of X-ray powder diffraction data, combined with experimental work, shows that "monoclinic B-type" La2O3 and Pr2O3 described in the literature are in fact rare earth silicates of the hexagonal apatite-type structure. A comparison of X-ray powder data reveals that both compounds are identical with La9.33(SiO4)6O2 and Pr9.33(SiO4)6O2, respectively. This confirms the currently acknowledged stability diagram of the rare earth sesquioxides.

Microstructure of silicon nitride ceramics sintered with rare-earth oxides

Microstructures of Si 3N 4 with crystallized grain boundaries of rare-earth silicates were investigated. β-Si 3N 4 grains in samples sintered with Yb 2O 3 were more elongated than those sintered with Dy 2O 3. Amorphous thin layers between crystallized grain boundaries of silicate and Si 3N 4 grains were recognized for both specimens sintered with Dy 2O 3 and with Yb 2O 3. In the sample sintered with Dy 2O 3, very clean grain boundaries between Si 3N 4 grains which did not contain the heavy elements were frequently recognized, while samples with Yb 2O 3 had an amorphous grain boundary phase containing Yb. Unusual structures with distorted lattice images in regions rich in Si and O were discovered in crystalline Dy-silicate phases.

ORE-BEARING METASOMATITES AT THE BAY AN OBO RARE EARTH DEPOSIT, INNER MONGOLIA, CHINA

The rare earth elements that make Bayan Obo the world's largest rare earth deposit are contained mostly in fluorocarbonate and phosphate minerals in apparently metasomatized sedimentary strata. The paper describes less well known rare earth silicate- and silicophosphate-bearing metasomatites near the contact of a Permian (Hercynian) granitoid intrusive at the eastern end of the ore field. Ontoyev believes the occurrence of these supports the hypothesis of mineralization by Hercynian magmatogenic fluids for the origin of the entire Bayan Obo deposit.

Phase separation in rare-earth-doped SiO 2 glasses

Phase separation in the rare-earth silicates, which is closely related to the phenomenon of concentration quenching in laser glasses, is discussed in terms of the theory of corresponding states in charged particle systems. This theory has been previously shown to apply to related systems such as the alkali and alkaline-earth silicates. Literature values of the critical temperatures and concentrations of the rare-earth silicates are found to increase with decreasing rare-earth ion radius in agreement with the corresponding states theory. Reduced values of the critical parameters are in excellent agreement with those of other charged particle systems if it is assumed that the rare-earth ions are effectively divalent. This assumption is consistent with the high degree of association observed for charged particle systems at their critical points. The theory of corresponding states is used to estimate the Er 2O 3SiO 2 immiscibility boundary by rescaling data from the Nd 2O 3 SiO 2 system.

Conductivity and crystallography of new alkali rare-earth silicates synthesized as possible fast-ion conductors

Eight potassium neodymium silicates have been synthesized as possible fast-ion conductors. Hydrothermal growth experiments in the high-silica region of the SiO 2·H 2O·Nd 2O 3·K 2O system yielded crystals of the following phases (five of which are previously unreported) :K 3NdSi 6O 15, K 8Nd 3Si 12O 32OH, K 10Nd 4Si 14O 39, K 4Nd 2Si 8O 21, K 3NdSi 18O 1 9, K 12Nd 2Si 18O 45, K 5Nd 3Si 20O 47, and KNd 9(SiO 4) 6O 2. The compositions and crystallographic data were determined using electron microprobe measurements and precession X-ray photographs, respectively. Single-crystal intensity data for phases K 3NdSi 6O 15 and K 8Nd 3Si 12O 32OH were obtained with a four-circle diffractometer. Of these eight phases, six have been obtained as crystals large enough to permit conductivity measurements along at least one crystallographic axis. Conductivities were measured from 300 to 900 °C in an air atmosphere using either complex impedance techniques or single-frequency ac methods. Our results show that activation energies range from 0.3 to 2.2 eV, that the conduction process does not strictly follow the Arrhenius equation, and that these silicates are significantly anisotropic with respect to conductivity. In the K 3NdSi 6O 15 phase this anisotropy corresponds well to expectations based on structural considerations.

Silicon Nitride Containing Rare Earth Silicate Intergranular Phases

ABSTRACTSi3N4 ceramics prepared with refractory grain boundary phases to improve high temperature properties are difficult to densify by conventional sintering methods. Gas-pressure sintering may be used to promote densification and development of acicular grains for improved fracture toughness. The current study examined rare earth silicate sintering aids with the composition M2Si2O7, where M is a trivalent cation (Y, La, Nd). M2O3 and Si02 additions were varied to develop a number of compositions in the Si3N4—Si2N2O—M2Si2O7 ternary phase field. Pressureless sintering and gas-pressure sintering were used to densify the samples. Densification, microstructure development, oxidation resistance, and mechanical properties were evaluated and compared with respect to compositional variations and processing conditions.

Phase separation during isothermal sintering of (1 − x)SiO 2- xLn 2O 3 aerogels

Rare earth silicate aerogels (1 − x )SiO 2 - x Ln 2 O 3 , with Ln = Dy or Er, have been prepared by the sol-gel process, starting from Dy or Er nitrates and TMOS. After an isothermal treatment of the aerogels, transparent glasses resulted when starting from aerogels with an Ln 2 O 3 mole fraction x x )SiO 2 - x Dy 2 O 3 and (1 − x )SiO 2 - x Er 2 O 3 glasses after the isothermal treatment. The (1 − x )SiO 2 - x Dy 2 O 3 sample shows a residual porosity of about 10%.

Effects of Rare Earth Oxide Addition on the Sintering of Mullite

Effects of various kinds of rare earth oxides (Y2O3, Er2O3, Sm2O3, La2O3 and CeO2) addition on the sintering of mullite and the precipitation behavior of secondary phases in the mullite sintered bodies were investigated. The sintering of the mullite compacts, to which a small amount of these rare earth oxides was added, was performed at 1500°-1600°C. The morphologies of the rare earth silicates precipitated and remained on the grain boundaries of the sintered bodies could be classified into three categories, i.e., crystalline, glassy and decomposed substances, depending upon the kinds of the rare earth oxides added. The microstructures of the mullite sintered bodies changed as the amount of the additives varied. The three point bending strength of the sintered mullite increased with the addition of 0.5wt% of Y2O3 or CeO2. On the other hand, excessive amount of these additives would result in the decrease of the three point bending strength. It was found that the maximum value of three point bending strength was evidently related to the melting point of the secondary phases existed in the sintered bodies of muilite.

Hydronium-ion conductors based on β/β″-alumina and rare-earth silicates

H 3O +-β/β″-alumina and H 3O + rare-earth silicates (HNRS where R = rare earth ion) hydronium-ion conductors were prepared by ion exchanging alkali-ion precursors. Conductivity measurements show the HNRS to be superior hydronium-ion conductors but physically less stable. The hydronium-ion conductors investigated suffer reversible degradation between 120 and 400 °C.

Alpha irradiation effects in Ca 2Nd 8(SiO 4) 6O 2

The effects of alpha irradiation on the rare-earth silicate, Ca 2Nd 8(SiO 4) 6O 2, was investigated by X-ray diffraction. X-ray absorption near-edge spectroscopy (XANES), and extended X-ray fine structure (EXAFS) techniques. Irradiation with alpha particles emitted isotropically from a 238PuO 2 source to a cumulative dose of 2.25 × 10 21 alphas/ m 2 (0.36 dpa) produced measurable anisotropic lattice expansions, with no tendency toward the amorphization that was previously reported due to self-radiation damage in Cm-doped samples at similar displacement-damage (0.25 dpa) levels. The unit-cell volume expanded exponentially with dose to a value corresponding to 2.5% volume expansion at saturation. The fluorescence XANES data of irradiated and unirradiated surfaces indicate a rearrangement of the near-neighbor ions about Nd. The EXAFS results indicate an irradiation-induced increase in structural disorder that becomes readily apparent at distances of several coordination spheres away from Nd.

Growth of Nd-doped rare earth silicates by the laser floating zone method

The laser floating zone (LFZ) technique has been used to grow single crystals of several rare earth silicates from polycrystalline precursors. These polycrystalline ceramics were prepared using both solid state and solution synthesis routes. An oxyorthosilicate (Gd 1− x Nd x ) 2(SiO 4)O, and a hexagonal cation-deficient oxyapatite, La 1− x Nd x ) 9.33□0.6 7(SiO 4) 6O 2, have been characterized by absorption and fluorescene spectroscopy. Several precursors and crystals have been characterized by elemental analysis and X-ray diffraction tecniques, since the crystal optical quality obtained was found to be dependent on the precursor synthesis route. Cryogenic (18 K) and room-temperature absorption and fluorescence spectra are presented for single crystals of high optical quality. These results are interpreted with respect to the active dopant crystallographic sites; applications in diode-laser-pumped solid-state lasers are also discussed.

ChemInform Abstract: Luminescence of Tb3+‐ and Ce3+‐Activated Rare Earth Silicates.

ChemInformVolume 18, Issue 48 Physical Inorganic Chemistry ChemInform Abstract: Luminescence of Tb3+- and Ce3+-Activated Rare Earth Silicates. M. J. J. LAMMERS, M. J. J. LAMMERS Phys. Lab., State Univ. Utrecht, 3508 TA Utrecht, Neth.Search for more papers by this authorG. BLASSE, G. BLASSE Phys. Lab., State Univ. Utrecht, 3508 TA Utrecht, Neth.Search for more papers by this author M. J. J. LAMMERS, M. J. J. LAMMERS Phys. Lab., State Univ. Utrecht, 3508 TA Utrecht, Neth.Search for more papers by this authorG. BLASSE, G. BLASSE Phys. Lab., State Univ. Utrecht, 3508 TA Utrecht, Neth.Search for more papers by this author First published: December 1, 1987 https://doi.org/10.1002/chin.198748006Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume18, Issue48December 1, 1987 RelatedInformation

Luminescence of Tb3+‐ and Ce3+‐Activated Rare Earth Silicates

The luminescence properties of Ce3+‐ and Tb3+‐activated and of Tb3+‐activated are reported. It is shown that activated with Tb3+ is an excellent phosphor both under UV and x‐ray excitation. In both types of compounds there are two different Ln3+ sites available for the activator ions, Ce3+ and Tb3+. The results show that the activator ions are incorporated on both sites. The differences in the luminescence properties due to the two types of activator sites are ascribed to the presence of oxygen ions which are coordinated to Ln3+ ions only and do not belong to any silicate group. The excitation spectra of the Tb3+ emission of these compounds show excitation bands due to spin‐forbidden and spin‐allowed Tb3+ 4f‐5d transitions. The positions of these bands can be predicted using the data of the Ce3+‐activated samples.

Sol-gel preparation of rare-earth silicate glasses

Rare-earth silicate glasses have been obtained by a sol-gel method starting with the rare earth carbonates of Pr, Dy and Er and TEOS (tetraethoxysilane). Expressed in the form xLn2O3(1 − x)SiO2, the glasses have compositions in the range x = 0.009 to 0.052, which corresponds to 0.9 to 5.2 mol% or up to 23 wt% rare earth oxide as determined by electron microprobe. The glasses were produced by densification at 800 ° C. Infrared and visible spectra and magnetic susceptibilities are reported.

Kinetics of ion-exchange in sodium rare-earth silicates

The mobile Na +-ions (42/90 of the total) in Na 5GdSi 4O 12 have been replaced by K + ions in K-NaCl melts at 800°C. K x Na 3− x GdSi 3O 9 (Na 3GdSi 3O 9-type structure) was formed on the surface during immersion, and its quantity was found to be dependent on the (K +)/(Na +) ratio in the melt. Hydronium rare earth silicate was subsequently obtained by means of field-assisted ion exchange in acid. Dc resistivities for the hydronium rare earth silicates ((H 3ONa) 5GdSi 4O 12 and (H 3ONa) 5YSi 4O 12) were estimated as 1.8×10 7 and 7.3×10 7 Ω cm , respectively, at room temperature with activation energies of 0.49 eV and 0.77 eV between room temperature to 90°C.

Spray-drying and fabrication of superionic conducting sodium rare-earth silicates

Fine precursor powders of particle diameter of 0.50–7 μm and high surface area (9 m 2/g) for superionic conducting Na 5RESi 4O 12 (RE=Gd, Y) were successfully produced by spray-drying and an optimised calcination procedure. Using these powders, dense sinters (>98% of theoretical) with ƒ(NGS) or ƒ( NGS)=1.0 (the proposed parameter for purity) were obtained under normal sintering conditions. 300°C ionic resistivities were calculated from ac impedance and had values of 5.6 Ω cm with an activation energy for conduction of 4.5 kcal/mol for Na 5GdSi 4O 12 and 7.7 Ω cm and 5.9 kcal/mol for Na 5YSi 4O 12. These values were dependent on the ƒ(NGS) or ƒ(NYS).

Crystallization of sodium rare earth silicate and germanate crystals from flux

Crystallization of sodium rare earth silicate and germanate crystals from flux

Rare-earth silicates

Binary systems of the type Ln 2O 3SiO 2 were studied. Three types of compounds were established for them: Ln 2O, SiO 4 (1:1) diorthosilicate, Ln 2Si 2O 7 (1:2), and the apatite-like silicate, Ln 4.67O [SiO 4] 3 (7:9), the latter being stable for the large cations. Systematization of the physical-chemical properties of rare-earth silicates was made. Based on the model of ideal solutions the liquidus curves were calculated for several Ln 2O 3  Ln 2O 3SiO 2 systems. The solid solution limits were determined for complex systems formed by rare-earth and alkaline-earth silicates. A series of rare-earth single crystals of simple and complex composition were grown by crystallization from molten solutions. Classification of the same type compounds into structural groups (morphotropic series) was made and partecipation of the 4f-orbitals in bondings was considered. The spectral-luminiscence characteristics of the rare-earth silicates are given.

Experimental and computed HREM images of the structure and defects in gadolinium silicate

Monoclinic Gd2SiO5 has been investigated by high-resolution electron microscopy (HREM) at 100 kV. Structure images are observed in the [100] and [001] projections and calculations of the image contrast using the multislice approximation are carried out to interpret the observations. For thin samples the image fit is improved by Fourier filtering of the observed images. For thicker samples the fit is found to be very sensitive to small tilt deviations of the zone axis to the electron beam. For defects observed in a fast-cooled specimen, a model in which Gd atoms replace Si atoms in segments along the (100) plane is proposed and tested by contrast calculations. These defects may be one of the reasons for deviations from stoichiometry as frequently observed in rare-earth silicates.