Rare-earth Disilicates Research Articles

Rare-earth disilicate formation under Deep Geological Repository approach conditions

The Deep Geological Repository (DGR) concept involves the placement of long-lived radioactive waste in rooms excavated deep. The major responsibility of the disposal safety falls on the Engineered Barrier System (EBS). The main constituent of EBS is bentonite that prevents the release of radiactive nuclei by physical and chemical mechanisms. The physical mechanism is expected to fault with the weathering of the bentonite while the chemical mechanisms have been only proved at 300 °C. It is the aim of this paper to explore the feasibility of the chemical mechanism at temperatures closer to the DGP conditions and to shed light on the mechanism of transformation of the argillaceous materials of the EBS in rare-earth disilicate phases. Saponite was submitted to hydrothermal reaction at 175 °C and 150 °C with different solutions of REE 3+ cations (REE = Sc, Lu, Y, Sm, Nd and La). The products were analyzed by XRD, NMR and electron microscopy. At conditions close to the DGP, the saponite was able to form rare-earth silicates. The formation of the disilicate phase, as final product, needs a set of stages and oxyorthosilicate as precursor.

First-principles investigation on the corrosion resistance of rare earth disilicates in water vapor

Rare earth disilicates are used as environmental barrier coatings to prevent the non-oxide silicon-based ceramics from water vapor corrosion at high temperatures. However, existing experimental results about the water vapor corrosion behavior of these materials are scarce and sometimes contradictory. In this paper, we report theoretical studies on the water vapor corrosion resistance of rare earth disilicates using Mulliken analysis based on first-principles. The Mulliken populations of Si–O bonds in Y 2Si 2O 7, Sc 2Si 2O 7, Yb 2Si 2O 7, Lu 2Si 2O 7, and substituted Y 2Si 2O 7 are calculated. The relative water vapor corrosion resistance of these materials is predicted by comparing the values of Mulliken population. The predictions are consistent with the reported experimental data. This work suggests a computational method for design and selection of rare earth disilicate materials for better water vapor corrosion resistance.

Chemical reactivity of argillaceous material in engineered barrier: Rare earth disilicate formation under subcritical conditions

The Deep Geological Repository (DGR) is the most internationally accepted option for the storage of high radioactive wastes. This confinement is based on the multibarrier concept where the engineered barrier is crucial for the safety of the repository. Nowadays, bentonite is accepted as the best argillaceous material in the engineered barrier of DGR. Additionally to its well-known physical role, a chemical interaction between lutetium, as actinide simulator, and the smectite has been demonstrated. The aim of this paper is the study of the hydrothermal reaction of smectite and a wide set of rare earth cations as trivalent actinide simulators. Saponite was submitted to hydrothermal treatment at 300 °C with a solution of REE 3+ cations (REE = Sc, Lu, Y, Sm, Nd and La). The products were analyzed by XRD, NMR and electron microscopy. Under hydrothermal conditions, saponite formed rare earth silicates. The final products and the reactivity of saponite depend on, among other factors, the nature of those cations.

Structures and anisotropic thermal expansion of the α, β, γ, and δ polymorphs of Y2Si2O7

The α, β, γ, and δ polymorphs of Y2Si2O7 were synthesized using sol-gel and solid-state methods. The structures of the α and γ polymorphs were determined by identification of isostructural rare-earth disilicates, and the structures were refined using Rietveld analysis of X-ray powder diffraction data. The α polymorph crystallizes in space group P1, with a=6.5872(6) Å, b=6.6387(7) Å, c=12.032(1) Å, α=94.501(7)°, β=90.984(8)°, γ=91.771(7)°, and volume=524.16(9) Å3. The γ form is described by space group P21/c, a=4.68824(5) Å, b=10.84072(9) Å, c=5.58219(6) Å, and γ=96.0325(3)°. The anisotropic thermal expansion of each phase was measured using high temperature diffraction up to 1200 or 1400 °C, depending on the stability of the polymorph. The thermal expansion is highly anisotropic for all polymorphs, with the low-expansion direction normal to the long axis of the corner-shared SiO4 tetrahedra.

Influence of Impurities on the High-Temperature Water-Vapour Corrosion of Environmental Barrier Rare-Earth Silicates

Corrosion experiments on a number of rare earth di-silicates containing Y, Yb and Lu have been conducted in flowing air with 30 vol.% H2O at 1500°C. Nominally the corrosion rate is slower by a factor 5 to 10 compared to silica. However, alumina impurity incorporation has a profound influence on the process of corrosion as it masks not only the gravimetric results but also changes the surface phase assemblages to contain rare earth garnets and liquid phases. It is presumed that silica loss occurs under those conditions via the liquid phase and does not necessarily follow the same kinetics as direct silicate corrosion. Furthermore, atmospheric silicon hydroxide saturation effects contribute to the corrosion process. Currently all determined corrosion rates of rare earth silicates are viewed as system specific only.

High temperature water vapour corrosion of rare earth disilicates (Y,Yb,Lu) 2Si 2O 7 in the presence of Al(OH) 3 impurities

Potential environmental barrier silicates based on rare earth disilicates containing Y, Yb and Lu have been investigated for their corrosive behavior in a gas stream containing water at 1500 °C. No currently used test method is unambiguous: silica or silica-forming tubing cause high internal P Si ( OH ) 4 , which should artificially slow down corrosion rates and alumina tubing cause alumina contamination via P Al ( OH ) 3 . We used the latter and report on the details of the interaction. In Y 2Si 2O 7 and a number of solid solutions with a Lu or Yb content up to 50% this contamination resulted in the formation of a melt. Depending on further impurities, particularly Ca, melt formation is accompanied by oxyapatite or monosilicate crystallisation. On cooling rare earth garnets crystallize from the melt. The melt ± oxyapatite/monosilicate formation does not create a protective effect. The corrosion kinetics is linear; the rates are slower than those of pure silica, but only at a level reflecting reduced silica activity due to dilution by a factor of 2–5. Porosity causes fast initial mass losses. The formation of secondary phases inside the material induces crack formation. A disilicate layer in those systems is unlikely to become an effective environmental barrier for non-oxide systems. For Yb and Lu silicates there are indications for the formation of a rare earth garnet layer during the corrosion process at high temperature, which has protective power. This causes a logarithmic law for the corrosion kinetics: for extended times the mass loss drops asymptotically. Under the assumption that the garnet formation also removes the sink for external alumina, the protection for Si-removal may even become perfect. The total mass change is a balance between Al-input and silica loss, which makes it currently impossible to formulate a quantitative equation for the time dependence of the process.

High-pressure rare earth silicates: Lanthanum silicate with barium phosphate structure, holmium silicate apatite, and lutetium disilicate type X

The phase relations of a wide selection of rare earth disilicates have been investigated up to 10 GPa and 1700 °C using piston cylinder and multi-anvil equipment. Single-crystal X-ray structures have been obtained for the following high-pressure phases: (1) La 2.67(SiO 4) 2: monoclinic, space group C2/ m, Z=2, a=9.419(2), b=5.445(1), c=7.214(1) Å, β=115.71(3)°, R=0.042; disordered Ba 3(PO 4) 2 structure type, with 3× b and 7× b superstructures identified. (2) Ho 8.67(SiO 4) 6(OH) 2: hexagonal, P6 3/ m, Z=1, a=9.3221(4), c=6.7347(2) Å, R=0.026; silicate hydroxyapatite. (3) Lu 2Si 2O 7: tetragonal, P4 12 12, Z=4, a=6.5620(2), c=11.9535(4) Å, R=0.023; type X diorthosilicate structure, and the silicate analogue of tetragonal Er 2Ge 2O 7.

Interaction Between Lu Cations and 2:1 Aluminosilicates under Hydrothermal Treatment

AbstractSmectites are considered to be an important component in backfill barriers due to their marked swelling and high cation exchange capacity. Both properties are affected considerably when these clays transform under natural conditions. However, we have recently described a chemical interaction between high-activity radionuclide simulators and smectites which could prove to be effective at immobilizing radionuclides definitively. Investigating the efficiency of this mechanism, independent of bentonite ageing, is a challenge. For this purpose, the reactivity shown by a non-expandable layered aluminosilicate, muscovite, has been compared to that shown by an expandable one, beidellite. Both samples were treated hydrothermally with a solution of lutetium nitrate, and the transformations were studied by X-ray diffraction, nuclear magnetic resonance and scanning electron microscopy/energy dispersive X-ray analysis. Lutetium cations react with the silicon framework of both 2:1 layered aluminosilicates under hydrothermal conditions, and new phases, lutetium disilicate, kaolinite, boehmite and natrosilite are generated. The results demonstrate that the efficiency of the chemical mechanism is not determined by the swelling and the cation exchange capacity of 2:1 layered aluminosilicates. Thus, the rare earth disilicate formation might account for the success of the clay barrier, once bentonite has lost its swelling and cation exchange capacity.

Sol–gel synthesis and coating of nanocrystalline Lu 2Si 2O 7 on Si 3N 4 substrate

In this present work, we report sol–gel synthesis of nanocrystalline α-Lu 2Si 2O 7, a candidate rare-earth disilicate phase being used recently as environmental barrier coating (EBC) layer for Si 3N 4 ceramics in high temperature water vapor corrosion environment. The method consists of the reaction of Lu(NO 3) 3 and TEOS in the adequate molar ratio to form a complex gel composed by the two cations, Si and Lu, which yields the stoichiometric Lu 2Si 2O 7 after calcining. Thermal history and phase identification of the synthesized phase were examined using TG–DTA and XRD analyses, respectively. High dense coating layer is obtained by pressureless sintering at relatively low temperature.

Additive Effects on Si3N4 Oxidation/Volatilization in Water Vapor

Two commercially available additive‐containing silicon nitride materials were exposed in four environments which ranged in severity from dry oxygen at 1 atm pressure, and low gas velocity, to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066° to 1400°C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare‐earth silicate phases in combustion environments when compared with the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare‐earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments. It was concluded that externally applied environmental barrier coatings are needed to protect additive‐containing silicon nitride materials from volatilization reactions in combustion environments.

High-Pressure Rare Earth Disilicates REE2Si2O7(REE=Nd, Sm, Eu, Gd): Type K

A new structure type (K) is reported for the disilicates of Nd, Sm, Eu, and Gd made at high pressure. Crystals of type K were synthesized at 10 GPa, 1600–1700°C in an MA6/8 superpress and used for single-crystal X-ray structure study by Kappa CCD diffractometry at room temperature. Crystal data are: monoclinic, space group P21/n, Z=4; Nd2Si2O7−a=6.6658(2), b=6.7234(3), c=12.3975(6) Å, β=102.147(3)°, V=543.2 Å3, R=0.029, and Dx=5.584 g/cm3; Sm2Si2O7−a=6.6039(3), b=6.6849(3), c=12.3069(5) Å, β=102.489(3)°, V=530.4 Å3, R=0.038, and Dx=5.871 g/cm3; Eu2Si2O7−a=6.5777(3), b=6.6652(4), c=12.2668(8) Å, β=102.671(4)°, V=524.7 Å3, R=0.030, and Dx=5.976 g/cm3; Gd2Si2O7−a=6.5558(4), b=6.6469(4), c=12.2394(6) Å, β=102.844(3)°, V=520.0 Å3, R=0.026, and Dx=6.166 g/cm3. The type K structure is built from a diorthosilicate group [Si2O7] interconnected by REE3+ cations in eightfold coordination with oxygen. The bridging oxygen (Si–O–Si) bond angle of the diorthosilicate group of rare earth disilicates stable at 1 bar (types A to G) is related to spatial accommodation of the REE3+ cation and ranges from 130 to 135° in the light rare earth disilicates (types A, F, G) to 180° in Lu2Si2O7 (type C). Volume reduction in the high-pressure type K structure is achieved largely by closure of the Si–O–Si bond angle to 122.7–124.4°, through rigid body rotation of the two SiO4 tetrahedra. This also permits a marginal increase in the average coordination of the REE3+ cation.

Investigation by the use of profile refinement of neutron powder diffraction data of the geometry of the [Si2O7]6− ions in the high temperature phases of rare earth disilicates prepared from the melt in crucible-free synthesis

Abstract Powders and X-ray size single crystals of the rare earth disilicates RE2Si2O7 (RE = La to Nd and Gd to Lu and Y) were synthesized from the melts in a fast crucible-free process. The purity and unit cell parameters of the disilicates were determined from Guinier photographs. Previously reported structure types of the high temperature phases of RE2Si2O7 (RE = La, Ce, Ho, Er, Yb, and Y) were confirmed by profile refinement of neutron powder diffraction data. The high temperature modifications of La2Si2O7 and Ce2Si2O7 have the type G-structure. They are monoclinic, P21/c with Z = 4 and a = 5.4109(6), b = 8.796(8), c = 14.287(1) Å, β = 112.74(1)° for La2Si2O7, and a = 5.401(1), b = 8.723(1), c = 14.169(2) Å, β = 112.61(2)° for Ce2Si2O7, respectively. The high temperature modifications of Ho2Si2O7 and Y2Si2O7 have the type E-structure. They are orthorhombic, Pna21 with Z = 4 and a = 13.681(4), b = 4.997(3), c = 8.187(2) Å for Ho2Si2O7, and a = 13.6650(1), b = 5.0166(1), c = 8.1494(1) Å for Y2Si2O7, respectively. Er2Si2O7 and Yb2Si2O7 have the type C-structure at high as well as at low temperatures. They are monoclinic, C2/m with Z = 4 and a = 6.8491(9), b = 8.9395(10), c = 4.7222(5) Å, β = 108.810(9)° for Er2Si2O7, and a = 6.7988(3), b = 8.8759(4), c = 4.7101(3) Å, β = 101.982(4) for Yb2Si2O7, respectively.

Luminescence of the Europium(III) Doped High Temperature and Pressure Modification of Rare Earth Disilicates

Luminescence of the Europium(III) Doped High Temperature and Pressure Modification of Rare Earth Disilicates

Preparation of rare-earth disilicates and their X-ray diffraction studies

Preparation of rare-earth disilicates and their X-ray diffraction studies

Determination of Ho2O3 solubility in (Bi2O3+V2O5)-SiO2 flux by the thermogravimetric method, and the application of solubility data to the growth of rare-earth disilicates

Determination of Ho2O3 solubility in (Bi2O3+V2O5)-SiO2 flux by the thermogravimetric method, and the application of solubility data to the growth of rare-earth disilicates

Flux growth of polymorphic rare-earth disilicates, R 2Si 2O 7 (R = Tm, Er, Ho, Dy)

Optically clear single crystals of the polymorphic rare-earth disilicates R 2Si 2O 7 (R = Tm, Er, Ho, Dy) have been produced by the flux method. The single crystal growth of R 2Si 2O 7 (R = Tm, Ho, Dy) has not been reported previously. Wanklyn's generalised pseudoternary composition model was used successfully for the prediction of favourable starting compositions. Crystals of Er 2Si 2O 7 and Ho 2Si 2O 7 of both C- and D-types were obtained. Indexed X-ray powder pattern data is given for C-type R 2Si 2O 7 (R = Er, Ho) and E-type Dy 2Si 2O 7, and the antiferromagnetic transition temperatures of C- and D-Er 2Si 2O 7 are reported. Substitutional flux impurity levels have been determined by electron probe micro-analysis (EPMA).

Les disilicates de terres rares Ln 2Si 2O 7: Precisions sur leur nouvelle forme cristalline X et leur polymorphisme sous pression

The new X form of rare earth disilicates Ln 2Si 2O 7 (Ln=Lu, Yb, Tm) is described by means of X-ray powder data. The X-ray indexation is made using single microcristal electronic diffraction. The investigation of structural relation between Ln 2Si 2O 7 and M 2A 2O 7 compounds as also the X-ray powder data of X-Ln 2Si 2O 7 show that this X form is isostructural with tetragonal Er 2Ge 2O 7. The rare earth coordination polyhedra and the (Si 2O 7) 6− configuration are compared with those actually known in neighbouring phases and discussed in terms of pressure effect.

High-pressure silicate pyrochlores, Sc 2Si 2O 7 and In 2Si 2O 7

The silicate compounds Sc 2Si 2O 7 and In 2Si 2O 7 have been converted from thortveitite type to pyrochlore type at 1000°C, 120 kbar, with resulting cell constants of 9.287(3) and 9.413(3) Å, respectively. Invariant reflection intensities in the X-ray powder diffraction patterns allowed precise absorption corrections to be made, and refinement of thermal parameters and of the single structural parameter x gave values of 0.4313(21) and 0.4272(15), respectively. The corresponding six-coordinate SiO distances were 1.761(7) and 1.800(5) Å, and the average eight-coordinate distances for ScO 8 and InO 8 were 2.267 and 2.275 Å. Values of structure-refined bond lengths for compounds containing six-coordinate silicon are surveyed, and overall weighted average octahedral distances of 1.782(14) Å for SiO and 2.520(18) Å for OO are derived. Pyrochlore phases were not produced from rare-earth disilicate or monosilicate phases subjected to the same reaction conditions as the Sc and In compounds.

The crystal structures of the dimorphic rare earth disilicate, Pr2Si2O7*

The crystal structures of the dimorphic rare earth disilicate, Pr₂Si₂O₇*

Polymorphism and crystal data of the rare-earth disilicates of type R.E. 2Si 2O 7

The polymorphism of 14 rare-earth disilicate compounds, in the temperature range 900 °–1800 °C, is described by means of single-crystal and powder X-ray data. The thermal stability areas of the 7 observed structure types at normal pressure, and the transition characteristics of the polymorphs are discussed in terms of structural aspects. A description of the rare-earth-oxygen coordination polyhedra and of the [Si 2O 7] 6− configurations are given approximately by isostructural relations to other disilicate or diphosphate compounds. Complete crystal data and cell dimensions for 30 polymorphic forms, and d-spacings of one representative of each structure type, are provided.