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Multi-components and equimolar rare earth monosilicates, (Y 1/3 Dy 1/3 Er 1/3) 2 SiO 5 , (Y 1/3 Dy 1/3 Lu 1/3) 2 SiO 5 , (Y 1/4 Dy 1/4 Ho 1/4 Er 1/4) 2 SiO 5 and (Yb 1/4 Dy 1/4 Ho 1/4 Er 1/4) 2 SiO 5 , were prepared by solid-state reactions and the following hot-pressing. Dense microstructures with uniform elemental distributions were obtained for all samples. These investigated multi-components monosilicates exhibit low thermal conductivities and similar coefficients of thermal expansion with SiC. Moreover, they exhibit high corrosion resistances in 1400 °C water vapor, especially, four-components (Y 1/4 Dy 1/4 Ho 1/4 Er 1/4) 2 SiO 5 and (Yb 1/4 Dy 1/4 Ho 1/4 Er 1/4) 2 SiO 5 experienced almost invariable weights after small weight losses during the initial 0.5 h. All those results indicate that multi-components rare earth monosilicates are promising candidates of environmental barrier coatings for SiC/SiC composites. [ABSTRACT FROM AUTHOR]

Rare‐earth oxides are intrinsically hydrophobic, and this characteristic opens new horizons for their design and diverse applications as robust hydrophobic surfaces. Here, we discover some rare‐earth (RE) monosilicates (RE2SiO5) that are hydrophobic under ambient conditions. Their hydrophobicity is positively correlated with their hot‐steam corrosion at 1400°C. RE3+ in RE2SiO5 is further found to be a hydration prohibitor, and its species can regulate hydration and volatilization at very high temperatures. These results may provide basis for innovative designs of RE silicates in harsh hot‐steam environments for robust environmental barrier coatings (EBC) or as ceramic components. [ABSTRACT FROM AUTHOR]

Rare earth monosilicates RE 2 SiO 5 have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE 2 SiO 5 (RE = Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials. [ABSTRACT FROM AUTHOR]

A series of new lanthanide (Ln) silicates have been synthesized using high temperature hydrothermal techniques, and structurally characterized using single crystal and powder X-ray diffraction. The compounds have the general formula NaBa 3 Ln 3 Si 6 O 20 (Ln = Y, Nd, Sm, Eu, Gd), and crystallize in the space group Ama2 (No.40). As a representative example, the unit cell parameters of NaBa 3 Gd 3 Si 6 O 20 are a = 14.731(3) Å, b = 23.864(5) Å, c = 5.5449(11) Å and Z = 4. The title compounds adopt a three dimensional polar acentric framework made of Ln–O–Si bonding. The framework is comprised of LnO 8 and LnO 7 units forming edge-sharing infinite chains along the c -axis. These oxy-bridged infinite chains are also linked by [Si 4 O 13 ] tetrasilicate and [Si 2 O 7 ] disilicate units to form the three-dimensional framework structure, with Ba 2+ and Na + cations residing inside channels of the framework. The polarity in the structure is imparted by the unusual tetrasilicate arrangement. The luminescence and magnetic properties were investigated on selected compounds. The temperature dependent magnetic susceptibility measurements on the Nd, Sm, and Gd derivatives reveal a Curie–Weiss behavior with an antiferromagnetic coupling parameter. For the Eu-derivative, the temperature dependent magnetic susceptibility deviates significantly from Curie–Weiss behavior. Luminescence properties of NaBa 3 Eu 3 Si 6 O 20 and NaBa 3 Sm 3 Si 6 O 20 compounds exhibited the characteristic transitions of Eu 3+ ( 5 D 0 → 7 F J , J = 0–4) and Sm 3+ ( 4 G 5/2 → 6 H J , J = 5/2, 7/2), respectively, leading to strong visible red and orange emissions, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Polar crystals provide an interesting avenue for materials research both in the structures they form and the properties they possess. This work describes the hydrothermal synthesis and structural characterization of three novel silicate fluorides. Compound (1), LiY3(SiO4)2F2 crystallizes in space group C2/c, with a=17.651(4)Å, b=4.8868(10)Å, c=11.625(2)Å and β=131.13(3)°. BaY2(Si2O7)F2 (2) crystallizes in space group P−1, with a=5.1576(10)Å, b=6.8389(14)Å, c=11.786(2)Å, α=93.02(3)°, β=102.05(3)° and γ=111.55(3)°. Finally, the structure of Ba2Y3(SiO4)2F5 (3) was determined in the polar orthorhombic space group Pba2, having unit cell parameters a=8.8864(18)Å, b=12.764(3)Å and c=5.0843(10)Å. The structures are compared based on their building blocks and long range polarities. Aligned silicate tetrahedra segregated into a single layer in (3) impart the observed polarity. [Copyright &y& Elsevier]

Abstract: The paper presents the results of capacitance–voltage, conductance-frequency and current–voltage characterization in the wide temperature range (140–300K) as well as results of low temperature (5–20K) thermally stimulated currents (TSC) measurements of metal–oxide-semiconductor (MOS) structures with a high-κ LaSiO x dielectric deposited on p- and n-type Si(100) substrate. Interface states (D it) distribution determined by several techniques show consistent result and demonstrates the adequacy of techniques used. Typical maxima of interface states density were found as 4.6×1011 eV−1cm−2 at 0.2eV and 7.9×1011 eV−1cm−2 at 0.77eV from the silicon valence band. The result of admittance spectroscopy showed the presence of local states in bandgap with activation energy Ea =0.38eV from silicon conductance band, which is in accord with interface states profile acquired by conductance method. Low-temperature TSC spectra show the presence of shallow traps at the interface with activation energies ranging from 15 to 32meV. The charge carrier transport through the dielectric film was found to occur via Poole–Frenkel mechanism at forward bias. [Copyright &y& Elsevier]

Abstract: Rare earth silicates with the structure of apatite are attracting considerable interest since they show oxygen ion conductivities higher than that of yttria-stabilized zirconia (YSZ) at moderate temperature. Based on the hydrothermal synthesis we presented a simple one step process for the direct preparation of the pure and the high crystalline nanosized rare earth silicates with the structure of apatite under a mild condition (230°C). Since the preparation of the high crystalline silicon based rare earth apatites is performed at high temperature previously and accompanied by subsequent process of grinding, results of this work provide a promising alternative of the existing methodology. Furthermore, due to the relatively low temperature of the preparation of these materials, high doping of monovalent cation can be done, which was not achieved before. [ABSTRACT FROM AUTHOR]