Twinning modes of Nd2Si2O7

Theory of deformation twinning by Bilby and Crocker [B.A. Bilby, A.G. Crocker, Proc. R. Soc. 288 (1965) 241] is applied to calculate the twinning elements for all possible low index twinning modes in tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning modes for tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. These predictions of the theory are compared with the available experimental information for analogue oxides and may act as guideline for future experimental work.     

Preparation and characterization of bioactive sol-gel-derived Na<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Si<Subscript>3</Subscript>O<Subscript>9</Subscript>

In this study, pure Na₂Ca₂Si₃O₉ was synthesized by a sol-gel method, and Na₂Ca₂Si₃O₉ cuboids and disks were prepared by uniaxial pressing and calcining at 700 °C. The porosity and mechanical strength of the Na₂Ca₂Si₃O₉ cuboids were measured, and the results showed that the Na₂Ca₂Si₃O₉ cuboids were porous with an average porosity of 44%, and the 3-point bending strength of the cuboids was 6.08 MPa. The in vitro bioactivity of Na₂Ca₂Si₃O₉ was carried out by soaking Na₂Ca₂Si₃O₉ disks in simulated body fluid (SBF). The results showed that hydroxyapatite (HA) formed on the surface of Na₂Ca₂Si₃O₉ samples after soaking for 1 day, which indicated good bioactivity of Na₂Ca₂Si₃O₉.     

Spectroscopic studies of TeO<Subscript>2</Subscript>–ZnO–Er<Subscript>2</Subscript>O<Subscript>3</Subscript> glass system

Series of glass based on the (80 − x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ≤ x ≤ 2.5 mol%) has successfully been made by melt quenching technique. The optical properties of glass have been investigated by means of IR and Raman spectroscopy. It is observed that as the Er₂O₃ content is being increased, the sharp IR absorption peaks are consistently shifted from 650 to 672 cm⁻¹ while the Raman shift intensity around 640–670 cm⁻¹ is decreases but increases around 720–740 cm⁻¹. It is found out that both phenomenons are related to the structural changes between the stretching vibration mode of TeO₄ tbp and TeO₃ tp, and bending vibration mode of Te–O bonds in the glass linkages.     

Luminescence of europium-doped BaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

We have prepared europium-doped BaO-Bi2O₃-B2O₃ glasses and investigated the doping effect on the main physicochemical properties and local structure of the glasses. Using Judd-Ofelt analysis, we calculated intensity parameters (Ω₂, Ω₄, and Ω₆), spontaneous emission probabilities, the radiative lifetime, luminescence branching factors, the quantum yield of luminescence, and the stimulated emission cross sections for ⁵ D ₀ → ⁷ F _J transitions.     

Photoluminescence properties and charge compensation investigations of novel orange-emitting Eu<Superscript>3+</Superscript> doped Na<Subscript>4</Subscript>Ca<Subscript>4</Subscript>(Si<Subscript>6</Subscript>O<Subscript>18</Subscript>) phosphors

A series of polycrystalline Na₄Ca₄(Si₆O₁₈):Eu³⁺ orange emitting phosphors were synthesized by a conventional high-temperature solid-state reaction. The phase formation was confirmed by X-ray power diffraction analysis. The excitation spectra show a strong host absorption indicating an efficient energy transfer process from O^2? to Eu³⁺ ions. Upon NUV radiation, the phosphors showed strong red emission around 610 nm (⁵D₀ → ⁷F₂) and orange emission around 591 nm (⁵D₀ → ⁷F₁), but the ⁵D_1,2,3 emission nearly can not be seen. Compared with the luminescence properties of Li⁺, Na⁺, and K⁺ co-doped samples, we deduced that Na⁺ ions probably prefer to dope into the intrinsic Na vacancies rather than Ca²⁺ ions vacancies in Na₄Ca₄(Si₆O₁₈) crystal. Thermal stability properties, quantum efficiency and chromaticity coordinates of the phosphors have been investigated for the potential application in white LEDs.     

CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-B<Subscript>2</Subscript>Al<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-H<Subscript>2</Subscript>O (B = Na,K) zeolites

The structures of gismondine and amicite are analyzed in comparison with one another and with those of the zeolites whose compositions lie in the hypothetical plane “CaAl₂Si₃O₁₀”-Na₂Al₂Si₃O₁₀-H₂O. It is shown that the structures are similar to each other and may undergo mutual transformations.     

Structural and electrical properties of Ba(Sb<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramic

Lead-free perovskite Ba(Sb_1/2Nb_1/2)O₃ was prepared by conventional ceramic fabrication technique at 1200 °C/5 h in air atmosphere. The crystal symmetry, space group and unit cell dimensions were determined from the experimental results using FullProf software whereas crystallite size and lattice strain were estimated from Williamson–Hall approach. XRD analysis of the compound indicated the formation of a single-phase monoclinic structure with the space group P2/m. EDAX and SEM studies were carried out to evaluate the quality and purity of the compound. Dielectric study revealed the frequency-dependent dielectric anomaly. To find a correlation between the response of the real system and idealized model circuit composed of discrete electrical components, the model fittings were presented using the impedance data. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in Ba(Sb_1/2Nb_1/2)O₃. The ac conductivity data were used to evaluate the density of states at Fermi level, minimum hopping length and apparent activation energy of the compound.     

Thermoanalytical study of the polymorphism and melting behavior of Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

We have developed a procedure for the synthesis of phase-pure α- and β-Cu₂V₂O₇. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu₂V₂O₇ (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu₂V₂O₇, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu₂V₂O₇, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu₂V₂O₇, CuVO₃, and other compounds.     

Low-temperature sintering behavior and properties of <Emphasis Type="Italic">monoclinic</Emphasis>-SrAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics prepared via an aqueous suspension milling process

In this work, by an aqueous suspension milling process, boric acid (H₃BO₃), calcium hydroxide [Ca(OH)₂], strontium carbonate (SrCO₃) and barium hydroxide octahydrate [Ba (OH)₂·8H₂O] are mixed with strontium carbonate (SrCO₃) and kaolin (Al₂O₃·2SiO₂·2H₂O) to prepare SrAl₂Si₂O₈ ceramics with a sintering temperature of 950 °C. According to chemical compositions of flux agents B₂O₃, CaO·2B₂O₃, SrO·2B₂O₃ and BaO·2B₂O₃, raw materials boric acid, calcium hydroxide, strontium carbonate and barium hydroxide octahydrate were introduced to the suspension slurries of strontium carbonate and kaolin to decrease the densification sintering temperature of SrAl₂Si₂O₈ ceramics. In addition, the Sr element in SrAl₂Si₂O₈ ceramics are partly substituted with Ba and Ca elements, respectively, to investigate the low-temperature sintering behavior of partly substituted SrAl₂Si₂O₈ ceramics. The results indicated that the addition of flux agents to SrAl₂Si₂O₈ ceramics can availably achieve the densification sintering of SrAl₂Si₂O₈ ceramics at 950 °C, whereas the substitution of Sr with Ca or Ba have a great effect on sintering behaviors and dielectric properties of SrAl₂Si₂O₈ ceramics. Additionally, main crystal phases of the SrAl₂Si₂O₈ ceramics are monoclinic- SrAl₂Si₂O₈ and small quartz, but the evolution of crystal phases also depend on flux agents.     

Studies of the EPR Parameters for Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Cu<Subscript>2</Subscript>O<Subscript>8</Subscript>:Er<Superscript>3+</Superscript>

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors g _x , g _y , g _z , and the hyperfine structure constants A _x , A _y , and A _z ) of the two orthorhombic Er³⁺ centers in Bi₂Sr₂ YCu₂O₈ are theoretically studied from the perturbation formulas of these parameters for a 4f¹¹ ion in orthorhombic symmetry. In these formulas, the contributions due to the admixtures of various states are taken into account, and the orthorhombic field parameters are determined from the superposition model and the local geometry of Bi³⁺ site in Bi₂Sr₂ YCu₂O₈. The calculated EPR parameters show reasonable agreement with the observed values. The anisotropy g _z >g _x (g _y ) for the g factors may be attributed to the compression of the ligand octahedra in the Er³⁺ centers.     

Magnetic properties of Er<Subscript>2</Subscript>Mn<Subscript>2/3</Subscript>Re<Subscript>4/3</Subscript>O<Subscript>7</Subscript>, a new zirkelite-structure oxide

We have studied the magnetic properties of the new compound Er₂Mn_2/3Re_4/3O₇ prepared by reacting Er₃ReO₈, ReO₂, MnO, and metallic Re at 1020°C in silica tubes sealed off under vacuum. The compound is shown to have the zirkelite structure with hexagonal cell parameters a = 7.3174(6) Å and c = 17.365(1) Å (sp. gr. P3₁21₁, Z = 6). Magnetization data obtained in the range 2–300 K demonstrate that, above ～150 K, its magnetic susceptibility exhibits Curie-Weiss behavior with an effective magnetic moment of 9.50μ_B. Dynamic magnetic susceptibility measurements point to spin-glass behavior of this compound at low temperatures.     

Multifunctional anti-stokes Y<Subscript>2</Subscript>O<Subscript>2</Subscript>S-based white phosphors

We have established the key trends in the variation of the intensity of the visible and IR luminescence of (Y_{1 − x − y − z} Yb _x Tm _y Er _z )₂O₂S solid solutions in relation to their composition under laser excitation at λ = 0.940 μm. The results obtained have been used to develop multifunctional anti-Stokes white phosphors with various tinges and a predetermined relative intensities of visible and IR emission bands.     

Er-Doped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

LiMn_2-x Er_xO₄ (x ≤ 0.02) materials were synthesized by a rheological phase reaction method. The thermal behavior of the materials was examined by thermogravimetric and differential scanning calorimetry. X-ray diffraction showed that the samples (x ≤ 0.02 ) exhibited the same phase as the pure spinel. The lattice parameter of the Er-doped spinel was smaller than that of the undoped one and decreased with increasing doping level. Cyclic voltammograms showed two reversible processes corresponding to the typical response of spinel LiMn₂O₄ and revealed an insertion-extraction reaction occurring at two stages in the 4-V region. The electrochemical performances of the samples were studied and displayed a better reversibility and cyclability.__________From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 740–743.Original English Text Copyright © 2005 by Haowen Liu, Li Song, Kelli Zhang.This article was submitted by the authors in English.     

Nano-domain structure of Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript> spinel

XRD-pure Li₄Mn₅O₁₂ spinels are obtained below 600 °C from oxalate and acetate precursors. The morphology consists of nanometric particles (about 25 nm) with a narrow particle size distribution. HRTEM and electron paramagnetic resonance (EPR) spectroscopy of Mn⁴⁺ are employed for local structure analysis. The HRTEM images recorded on nano-domains in Li₄Mn₅O₁₂ reveal its complex structure. HRTEM shows one-dimensional structure images, which are compatible with the (111) plane of the cubic spinel structure and the (001) plane of monoclinic Li₂MnO₃. For Li₄Mn₅O₁₂ compositions annealed between 400 and 800 °C, EPR spectroscopy shows the appearance of two types of Mn⁴⁺ ions having different metal environments: (i) Mn⁴⁺ ions surrounded by Li⁺ and Mn⁴⁺ and (ii) Mn⁴⁺ ions in Mn⁴⁺-rich environment. The composition of the Li⁺, Mn⁴⁺-shell around Mn⁴⁺ mimics the local environment of Mn⁴⁺ in monoclinic Li₂MnO₃, while the Mn⁴⁺-rich environment is related with that of the spinel phase. The structure of XRD-pure Li₄Mn₅O₁₂ comprises nano-domains with a Li₂MnO₃-like and a Li_4/3−x Mn_5/3+x O₄ composition rather than a single spinel phase with Li in tetrahedral and Li_1/3Mn_5/3 in octahedral spinel sites. The annealing of Li₄Mn₅O₁₂ at temperature higher than 600 °C leads to its decomposition into monoclinic Li₂MnO₃ and spinel Li_4/3−x Mn_5/3+x O₄.     

Structural and optical characterization of RE (Eu<Superscript>2+</Superscript>, Ce<Superscript>3+</Superscript>) doped SrMg<Subscript>2</Subscript>Al<Subscript>6</Subscript>Si<Subscript>9</Subscript>O<Subscript>30</Subscript> nanocrystalline phosphor

This article present the reports on optical study of Eu²⁺ and Ce³⁺ doped SrMg₂Al₆Si₉O₃₀ phosphors, which has been synthesized by combustion method at 550 °C. Here SrMg₂Al₆Si₉O₃₀:Eu²⁺ emission band observed at 425 nm by keeping the excitation wavelength constant at 342 nm, whereas SrMg₂Al₆Si₉O₃₀:Ce³⁺ ions shows the broad emission band at 383 nm, under 321 nm excitation wavelength, both the emission bands are assigned due to 5d–4f transition respectively. Further, phase purity, morphology and crystallite size are confirmed by XRD, SEM and TEM analysis. However, the TGA analysis is carried out to know the amount of weight lost during the thermal processing. The CIE coordinates of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor is observed at x?=?0.160, y?=?0.102 respectively, which may be used as a blue component for NUV-WLEDs. The critical distance of energy transfer between Ce³⁺ ions and host lattice is found to be 10.65 Å.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Hydrothermal synthesis and photoluminescence properties of Y<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>:Tb<Superscript>3+</Superscript> phosphors

This article presents the synthesis and photoluminescence (PL) properties of Y₂Zr₂O₇:Tb³⁺. The Tb³⁺-doped Y₂Zr₂O₇ zirconates were successfully synthesized by a hydrothermal process at 200 °C for 20 h. X-ray diffractometer (XRD) patterns revealed that all of the products were phase-pure with the fluorite structure. PL study showed that the Y₂Zr₂O₇:Tb³⁺ phosphors exhibited obvious PL emission peaks which located at 490, 545, 585, and 623 nm; the dominant emission located at 545 nm is assigned to ⁵D₄ → ⁷F₅ transition. Furthermore, Tb³⁺-doping concentration strongly affected the PL properties, and the quenching concentration is 5 at.%.     

Conductivity of Sm<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Sm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The oxygen-ion conductivity of porous materials, the coarse-grained pyrochlore-like Sm₂Ti₂O₇ and fine-grained Sm₂TiO₅ compounds, produced by mechanical activation of initial oxides is studied at 400–1000 °C. The Sm₂TiO₅ samples contain ～15 wt % of the nanosized pyrochlore-like Sm₂TiO₅ phase in addition to the rhombic phase. As determined by impedance spectroscopy, the ionic conductivities of Sm₂TiO₅ and Sm₂Ti₂O₇ at 1000°C are 1.3 × 10^?3 and 1.8 × 10^?4 S cm^?1, and the activation energies of the bulk and grainboundary conductivities of the materials are 1.04 and 1.24 eV for Sm₂TiO₅ and 1.69 and 1.80 eV for Sm₂Ti₂O₇.     

Synthesis,tunable luminescence and thermal stability of Mg<Subscript>2</Subscript>Al<Subscript>4</Subscript>Si<Subscript>5</Subscript>O<Subscript>18</Subscript>:Ce<Superscript>3+</Superscript>/Mn<Superscript>2+</Superscript> phosphors

Ce³⁺/Mn²⁺ singly doped and codoped Mg₂Al₄Si₅O₁₈ phosphors were synthesized by a solid state reaction. The phase, luminescent properties and thermal stability of the synthesized phosphors were investigated. Ce³⁺ and Mn²⁺ singly doped Mg₂Al₄Si₅O₁₈ phosphors show emission bands locating in blue and yellow–red regions, respectively. In Ce³⁺ and Mn²⁺ codoped Mg₂Al₄Si₅O₁₈, tunable luminescence was obtained because of the energy transfer from Ce³⁺ to Mn²⁺. In Mg₂Al₄Si₅O₁₈:Ce³⁺/Mn²⁺ phosphors with a fixed Ce³⁺ concentration, energy transfer efficiency increases with the increasing Mn²⁺ concentration, which is confirmed by the continually decreasing intensity and shortening decay time of Ce³⁺ emission. Moreover, the luminescent properties and thermal stability provide a great significance on the applications in the field of light emitting diodes.     

Synthesis and high-temperature electrical conductivity of Ln<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and LnYTi<Subscript>2</Subscript>O<Subscript>7</Subscript> (Ln = Dy,Ho)

Ho₂Ti₂O₇ and LnYTi₂O₇ (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho₂Ti₂O₇ is 4 × 10^?4 S/cm, and that of HoYTi₂O₇ is 1 × 10^?3 S/cm, with activation energies E _a = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi₂O₇ (3 × 10^?4 S/cm at 740°C, E _a = 1.09 eV) is almost one order of magnitude lower than that of HoYTi₂O₇ (1 × 10^?3 S/cm at 740°C, E _a = 1.17 eV).