The system Na2CO3–MgCO3 at 3 GPa

It was suggested that Na–Mg carbonates might play a substantial role in mantle metasomatic processes through lowering melting temperatures of mantle peridotites. Taking into account that natrite, Na₂CO₃, eitelite, Na₂Mg(CO₃)₂, and magnesite, MgCO₃, have been recently reported from xenoliths of shallow mantle (110–115?km) origin, we performed experiments on phase relations in the system Na₂CO₃–MgCO₃ at 3?GPa and 800–1250°C. We found that the subsolidus assemblages comprise the stability fields of Na-carbonate?+?eitelite and eitelite?+?magnesite with the transition boundary at 50?mol% Na₂CO₃. The Na-carbonate–eitelite eutectic was established at 900°C and 69?mol% Na₂CO₃. Eitelite melts incongruently to magnesite and a liquid containing about 55?mol% Na₂CO₃ at 925?±?25 °C. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 86–88?mol% Na₂CO₃. Melting point of Na₂CO₃ was established at 1175?±?25 °C. The Na₂CO₃ content in the liquid coexisting with magnesite decreases to 31?mol% as temperature increases to 1250°C. According to our data, the Na- and Mg-rich carbonate melt, which is more alkaline than eitelite, can be stable at the P–T conditions of the shallow lithospheric mantle with thermal gradient of 45?mW/m² corresponding to temperature of 900 °C at 3?GPa.     

The system Na2CO3–CaCO3–MgCO3 at 6 GPa and 900–1250°C and its relation to the partial melting of carbonated mantle

In order to constrain the Na₂CO₃–CaCO₃–MgCO₃ T–X diagram at 6?GPa in addition to the binary and pseudo-binary systems we conducted experiments along the Na₂CO₃–Ca_0.5Mg_0.5CO₃ join. At 900–1000°C, melting does not occur and isothermal sections are presented by one-, two- and three-phase regions containing Ca-bearing magnesite, aragonite, Na₂CO₃ (Na₂) and Na₂(Ca_1–0.9Mg_0-0.1)_3-4(CO₃)_4-5 (Na₂Ca_3-4), Na₄(Ca_1–0.6Mg_0–0.4)(CO₃)₃ (Na₄Ca), Na₂(Ca_0-0.08Mg_1–0.92)(CO₃)₂ (Na₂Mg) phases with intermediate compositions. The minimum melting point locates between 1000°C and 1100°C. This point would resemble that of three eutectics: Mgs–Na₂Ca₃–Na₂Mg, Na₂Mg–Na₂Ca₃–Na₄Ca or Na₂Mg–Na₄Ca–Na₂, in the compositional interval of [45Na₂CO₃·55(Ca_0.6Mg_0.4)CO₃]–[60Na₂CO₃·40Ca_0.6Mg_0.4CO₃]. The liquidus projection has seven primary solidification phase regions for Mgs, Dol, Arg, Na₂Ca₃, Na₄Ca, Na₂ and Na₂Mg. The results suggest that extraction of Na and Ca from silicate to carbonate components has to decrease minimum melting temperature of carbonated mantle rocks to 1000–1100°C at 6?GPa and yields Na-rich dolomitic melt with a Na# (Na₂O/(Na₂O?+?CaO?+?MgO))?≥?28?mol%.     

Solid state reduction of chromium (VI) pollution for Al2O3–Cr metal ceramics application

Reduction of chromium (VI) from Na₂CrO₄ through aluminothermic reaction and fabrication of metal-ceramic materials from the reduction products have been investigated in this study. Na₂CrO₄ could be successfully reduced into micrometer-sized Cr particles in a flowing Ar atmosphere in presence of Al powder. The conversion ratio of Na₂CrO₄ to metallic Cr attained 96.16% efficiency. Al₂O₃–Cr metal-ceramic with different Cr content (5 wt%, 10 wt%, 15 wt%, 20 wt%) were further prepared from the reduction product Al₂O₃–Cr composite powder, and aluminum oxide nanopowder via pressure-less sintering. The phase composition, microstructure and mechanical properties of metal-ceramic composites were characterized to ensure the potential of the Al₂O₃–Cr composite powder to form ceramic materials. The highest relative density and bending strength can reach 93.4% and 205 MP, respectively. The results indicated that aluminothermic reduction of chromium (VI) for metal-ceramics application is a potential approach to remove chromium (VI) pollutant from the environment.     

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO₂ have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P4₂/mnm), CaCl₂-type (Pnnm), pyrite-type (Pā3), and CaF₂-type (Fm-3m) phases of CrO₂ have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO₂ from the rutile phase to CaCl₂ phase is 10.9?GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type?→?CaCl₂-type?→?pyrite-type?→?CaF₂-type with the phase-transition pressures 10.9, 23.9, and 144.5?GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO₂.     

Spectrométrie électronique (ESCA) appliquée à la caractérisation de dérivés oxygénés de chrome et de sodium,NaxCryOz.

A series of compounds of chromium and sodium (Na₂CrO₄, Na₃CrO₄, Na₄CrO₄, NaCrO₂) and for comparison Cr₂O₃ and chromium metal were examined by ESCA. The binding energy values for Cr 2p, Cr 3s, Cr 3p levels increase with the oxidation number of chromium for the Na_xCr_yO_z derivatives. The separation, Δ, between Cr 2p $\text{1}\text{2}$ and Cr 2p $\text{3}\text{2}$ increases with the number of unpaired 3d electrons. The splitting of the Cr 3s peak is observed for NaCrO₂ and Cr₂O₃. For the O 1s, Na 2s and Na 2p levels, the binding energies vary little in the series. Chromium is in a tetrahedral environment of oxygens in all the compounds Na_xCrO₄ (x = 2, 3, 4), as shown by IR absorption spectroscopy.     

Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

ABSTRACT

The potential-energy surfaces of the amino radical (NH₂) with IO reaction have been studied at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level. Two kinds of pathways are revealed, namely H-abstraction and addition/elimination. Rice–Ramsperger–Kassel–Marcus theory and transition state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that, at atmospheric pressure with N2 as bath gas, the formation of P1 (HI?+?HNO) is the dominant pathways at 200–700?K, while the direct H-abstraction leading to P3 (³NH?+?HOI) takes over the reaction at a temperature above 700?K. At the high-pressure limit, IM1 [IONH₂] formed by collisional stabilisation is dominant at 200–700?K; the direct H-abstraction resulting in P3 (³NH?+?HOI) plays an important role at higher temperatures. However, the total rate constants are independence on the pressure; however, the individual rate constants are sensitive to pressure. The atmospheric lifetime of NH₂ in IO is around one week. TD-DFT computations imply that IM1 [IONH₂], IM1A [IONH₂′], IM2 [IN(H₂)O], IM3 [OINH₂], IM4 [HOINH], tra-IM5 [tra-HON(H)I] and cis-IM5 [cis-HON(H)I] will photolyze under the sunlight.     

Luminescence characteristics of Na21(SO4)7F6Cl:Cu+ phosphor

Copper-doped Na₂₁(SO₄)₇F₆Cl phosphor was synthesized via the conventional wet chemical method. The synthesis was carried using CuCl₂ and Cu (NO₃)₂·3H₂O as dopants in two different steps successively. The formation and phase purity of the compound were revealed by the X-ray diffraction pattern. Functional groups of the prepared phosphor were observed in the FT–IR spectrum. The emission along with excitation spectra were followed to explore the luminescence attributes. Photoluminescence (PL) emission spectrum of the material synthesized using CuCl₂ as the dopant was observed at 358?nm due to 3d^l0?3d⁹4s transitions when excited around 247?nm for various copper concentrations. Efficient blue emissions were obtained at peaks 423 and 469?nm for materials synthesized using Cu (NO₃)₂·3H₂O as the dopant, when monitored at 357?nm excitation. The Commission Internationale de I’Eclairage chromaticity coordinates for different copper concentrations were calculated for the emission around 423?nm. TL glow curves of Na₂₁(SO₄)₇F₆Cl:Cu phosphor for different dopant concentrations, irradiated with 100?Gy gamma dose, were studied and hence the trap parameters, namely order of kinetics (b), activation energy (E) and frequency factor (s) associated with the most intensive glow peak of Na₂₁(SO₄)₇F₆Cl:Cu phosphor were determined by using Chen’s Peak shape method. The results indicate that Na₂₁(SO₄)₇F₆Cl:Cu⁺ is a potential novel blue-emitting lamp phosphor and may be quite suitable for use in dosimetry of ionizing radiations.     

Synthesis,refinement of the structure,and AC conductivity behavior of sodium lithium orthonavadates

The Na _x Li_1-x CdVO₄ (x = 0.5, 1) orthovanadates were prepared using a solid-state reaction method. The x-ray diffraction patterns (XRDP) of both materials reveal the formation of the Na₂CrO₄ structure. Vibrational study confirms the existence of [VO₄]^3? group. Electrical measurements of our compounds have been investigated using complex impedance spectroscopy (CIS) in the frequency and temperature range 209 Hz–1 MHz and 589–703 K, respectively. Nyquist plots reveal the presence of tow contributions, an equivalent circuit was proposed. DC conductivity shows electrical conduction in the material as a thermally activated process. The AC conductivity is explained using the non-overlapping small polaron tunneling (NSPT) conduction mechanism. A relationship between crystal structure and ionic conductivity was established and discussed.     

Research on the defect structure and g factors for the tetragonal (CrO4)3− impurity center in ZrSiO4: Cr5+ crystal

ABSTRACT

The EPR g factors g// and g⊥ for the tetragonal (CrO₄)^3? impurity center in ZrSiO4: Cr⁵⁺ crystal are studied from the high-order perturbation formulas based on the two-mechanism (the crystal field and charge-transfer mechanisms) model. The studies indicate that differing from the tetragonally-elongated host (SiO4)^4- tetrahedron, the dominant defect structure of the substitutional (CrO₄)^3? tetrahedron is tetragonally- compressed with the ground state |d_z² due to the Jahn-Teller distortion. Furthermore, the agreement of g factors between calculation and experiment requires a small admixture of the first excited state |d_x²_?y² to the ground state |d_z² due to the vibrational motion of ligands, which leads a compressed (CrO₄)^3? tetrahedron to become a twinkling elongated one, These results are discussed.     

The mystery of abnormally large volume of PbCrO<Subscript>3</Subscript> with a structurally consistent Hubbard U from first-principles

PbCrO₃ is found experimentally to be a cubic perovskite with an abnormally large lattice constant of about 4.00 Å. To date, no successful first principle calculations have been carried out for cubic PbCrO₃ perovskite. Combining the generalized-gradient approximations and a structurally consistent Hubbard U approach, the crystal, electronic and magnetic structures of PbCrO₃ are calculated to explore the mystery of abnormally large volume of PbCrO₃ in cubic structure. The crystal structure, magnetic order and half-metallic properties of PbCrO₃ are determined. For the first time, a structurally consistent Hubbard U _eff of about 8.28 eV for PbCrO₃ is calculated based on linear-response approach. With the structurally consistent U obtained by first principles, a \(Pm\bar 3m\) cubic structure with a lattice constant of about 3.98 Å for PbCrO₃ is obtained successfully. Moreover PbCrO₃ is found to be half-metallic ferromagnet with an integral magnet moment of 2.00μ _B per unit cell. The anomalously large volume of PbCrO₃ in cubic structure results from the strongly correlated electron interaction. The results obtained agree well with experimental data.     

A quantum chemical study on •Cl-initiated atmospheric degradation of CH2BrO2•

The singlet and triplet potential energy surfaces for the CH₂BrO₂??+??Cl reaction have been researched theoretically. All of the probable reaction routes were investigated by using B3LYP and G3(MP2) models. Addition/elimination and S_N2 displacement exist on the singlet potential energy surfaces (PES), and the foremost approach process of CH₂BrO₂??+??Cl is generating IM1 (CH₂BrOOCl) with no barrier, followed by the O-O bond breaking accompanied by an H-migrate resulting in the most abundant product P1 (CHBrO?+?HClO). One direct H-abstraction and three S_N2 displacement reaction pathways exist on the triplet PES, and direct H-abstraction is the foremost pathway. RRKM-TST theory was employed to predict product distribution of the CH₂BrO₂??+??Cl reaction. At atmospheric pressure, the production of P1 (CHBrO?+?HClO) by addition/elimination dominants the reaction at T?≤?800?K, while the direct H-abstraction takes over the reaction at T?>?800?K. The total rate constants are insensitive to pressure, and the branching rate constants are just the opposite. The lifetime of CH₂BrO₂? in the presence of ?Cl was predicted to 3.2?d. Moreover, time-dependent density functional theory (TDDFT) calculations suggest that IM1 (CH₂BrOOCl), IM2 (CH₂BrOClO) and IM3 (CH₂(OBr)OCl) will photolyze under the sunlight.     

绿宝石晶体中(CrO6)9－团簇局域极化现象的理论研究

基于配位场理论,建立d³组态离子在三角晶场中的完全能量矩阵,采用完全能量矩阵的对角化计算方法,研究了绿宝石晶体Cr³⁺：Be₃Al₂(SiO₃)₆的光谱和EPR谱,理论值与实验值符合得很好.通过分析绿宝石晶体中(CrO₆)^9-团簇的光谱和EPR谱,研究了配体在络合物中的极化现象.结果表明由于周围配位环境的影响,绿宝石晶体中(CrO< 关键词： 3+：Be₃Al₂(SiO₃)₆体系')" href="#">Cr³⁺：Be₃Al₂(SiO₃)₆体系 光谱 EPR谱 局域结构极化     

SrB4O7:Sm2+: an optical sensor reflecting non-hydrostatic pressure at high-temperature and/or high pressure in a diamond anvil cell

A systematic investigation on the fluorescent spectra of SrB₄O₇:Sm²⁺ was performed in detail at high-temperature up to 623?K and/or high pressure up to 23.2?GPa with different pressure-transmitting media (PTMs), respectively. Combined with experiment data of previous research, the change of the ⁷D₀–⁵F₀ line (0–0 line) full width at half maximum (FWHM) of SrB₄O₇:Sm²⁺ under different pressure environments was specifically discussed. The results indicate that the FWHM of 0–0 line is sensitive to the non-hydrostatic pressure environment in 2-propanol, and methanol and ethanol mixture (ME) PTMs at ambient temperature. The first-order and the second-order derivation of the temperature dependence of 0–0 line FWHM at ambient pressure are 1.48(±0.21)?×?10^?4?nm/K and 9.63(±0.63)?×?10^?7?nm²/K² below 623?K. The 0–0 line FWHM is also sensitive to the non-hydrostatic pressure environment in ME at high-temperature and high pressure simultaneous, the non-hydrostatic transition pressures are 9.6?GPa at 323?K, 11.0?GPa at 373?K, 14.4?GPa at 423?K, respectively. SrB₄O₇:Sm²⁺ is recommended as an optical sensor to reflect the change of pressure environment in liquid media at high-temperature and/or high pressure.     

Synthesis of NaxMoO3 crystals through high pressure solid-state reaction

We report growth of high-purity single crystals of the sodium molybdenum bronzes Na_xMoO₃ through high pressure solid-state reaction routes. Our results demonstrate that the crystallite sizes of Na_xMoO₃ can rapidly grow to ～25?μm under a moderate pressure of 4.0?GPa in just 10?min. This discovery provides a starting point for the growth of large single crystals of alkali metal molybdenum bronzes under high pressure.     

Crystal chemical and dielectric study of niobochromates with the tungsten bronze structure

Abstract

A crystal chemical study has allowed us to identify new niobochromates crystallizing with the tungsten bronze structure: Ba₂LnNb₄CrO₁₅ (Ln = Bi, La), La₂ANb₄CrO₁₅ (A = Li, Na, K) and Ba₂Na_1-x La _x Nb_5-x Cr _x O₁₅ (0 ? x ? 1). A dielectric study has shown that the T _c (x) curve goes through a minimum at x ? 0.5 along the solid solution Ba₂Na_1-x La _x Nb_5-x Cr _x O₁₅, while the ferroelastic transition T_c changes very slightly in the same range of composition.     

Phase transitions and energy storage properties of some compositions in the (1−x)Li2SO4–xNa2SO4 system

In the binary system (1?x)Li₂SO₄–xNa₂SO₄, the solid–solid phase transitions and energy storage properties of Li₂SO₄, Na₂SO₄, the binary compound LiNaSO₄ and two eutectoids (E₁: 0.726Li₂SO₄–0.274Na₂SO₄; E₂: 0.03Li₂SO₄–0.97Na₂SO₄) were investigated by X-ray diffraction and differential scanning calorimetry. Li₂SO₄ has a solid–solid phase transition at 578 °C with the transition enthalpy 252 J g^?1. The binary compound LiNaSO₄ gives a slightly lower enthalpy value, 214 J g^?1 and its transition temperature is clearly reduced to 514 °C. The transition enthalpy of the eutectoid E₁ is maintained to 177 J g^?1 and its transition temperature is further reduced to 474 °C. Li₂SO₄, LiNaSO₄ and the eutectoid E₁ are applicable phase transition materials because of their large transition enthalpies. The enthalpies of Na₂SO₄ and the eutectoid E₂ are not very high (～45 J g^?1), but their transition temperatures are quite low (～250 °C); thus their transition properties may be applied at such low temperatures.     

Enhanced magnetoresistance and surface state of CrO2 particles improved by chemical process

The paper presents an approach to enhance a magnetoresistance (MR) effect in CrO₂ powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO₄) was used to react with the CrO₂ particles coated naturally with Cr₂O₃ layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr₂O₃ layer, and thereby change the surface state of the CrO₂ particles. Structural and magnetic properties for the improved CrO₂ particles have been characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and SQUID magnetometer. The results exhibit that the magnetotransport behavior of CrO₂ particles depends sensitively on the chemical reaction time. An optimal reaction process yields an obvious increase up to −33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO₂ powder, compared to MR=-27% for the original CrO₂ powder. The mechanism of magnetotransport is assumed to originate from the spin-dependent tunneling in the granular system, which is consistent with our experimental results. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.     

Manifestation of phase transformations in optical spectra of Na0.5Bi0.5TiO3 crystals between 25°C and 350°C

The pseudo dielectric function ?ε? (E) = ?ε ₁? (E) + i?ε ₂? (E) of sodium bismuth titanate Na_0.5Bi_0.5TiO₃, has been obtained in the spectral range of electronic excitations (2–9.5 eV) by spectroscopic ellipsometry using the synchrotron radiation source BESSY II. The spectrum contains a broad absorption band at 4.2–4.5 eV. The temperature dependence in the range of 25–350°C is presented by the related susceptibilities ?χ ₁? (T) and ?χ ₂? (T) at 5 eV. A clear peculiarity in the temperature behavior of ?χ ₁? (T) and ?χ ₂? (T) between 180 and 320°C has been revealed and discussed. In this range an extremum-like temperature dependence of intensity of the light reflected from the sample surface, has been revealed and can be explained by the diffraction of light on the grid of the elastic domain structure of the sample.     

Excited states in X-ray Lα emission spectra of transition metals in oxygen compounds

Experimental X-ray emission spectra for the metal and oxygen in Na₃VO₄, K₂CrO₄, V₂O₅ and YVO₄ are considered together with the results of MS-Xα and MO-LCAO cluster calculations. A new interpretation of the short-wave sub-bands in transition-metal Lα emission spectra of the compounds is given which connects their existence with excited states.     

Microstructure and mechanical properties of nano-Y2O3 dispersed ferritic steel synthesized by mechanical alloying and consolidated by pulse plasma sintering

Ferritic steel with compositions 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B), 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) (all in wt%) each with a 1.0?wt% nano-Y₂O₃ dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000°C using a 75-MPa uniaxial pressure applied for 5?min and a 70-kA pulse current at 3?Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been used to characterize the microstructural and phase evolution of all the alloys at different stages of mechano-chemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using a micro/nano-indenter and universal testing machine. All ferritic alloys recorded very high levels of compressive strength (850–2850?MPa), yield strength (500–1556?MPa), Young's modulus (175–250?GPa) and nanoindentation hardness (9.5–15.5?GPa), with up to 1–1.5 times greater strength than other oxide dispersion-strengthened ferritic steels (<1200?MPa). These extraordinary levels of mechanical properties can be attributed to the typical microstructure of uniform dispersion of 10–20-nm Y₂Ti₂O₇ or Y₂O₃ particles in a high-alloy ferritic matrix.