Density functional study on chirospectra of hydrogen-bonded systems X-（H2O）3（X=F,Cl,Br,I）

This paper calculates the molecular structures,infrared,Raman,circular dichroism spectra and optical rotatory powers of some hydrogen-bonded supramolecular systems as a cyclic water trimer,(H2O)3 and its pyramidal halide complexes,X-(H2O)3(X = F,Cl,Br,I) with the gradient-corrected density functional theory method at the B3LYP/6-311++G(2d,2p) and B3LYP/Aug-cc-pVTZ levels.It finds that the complexation of halide anions with the water trimer can efficiently modulate the chirally optical behaviors.The calculated vibrational circular dichroism spectrum illuminates that the vibrational rotational strength of S(+)-(H2O)3 mostly originates from the O-H rocking modes,whereas chirality of S(-)-X-(H2O)3(X = F,Cl,Br,I) has its important origin in the O-H stretching modes.The calculated optical rotatory power demonstrates that S(+)-(H2O)3 and S(+)-F(H2O)3 are positively chiral,whereas S(-)-X-(H2O)3(X = Cl,Br,I) are negatively chiral.With the polarizable continuum model,calculated bulk solvent effect in the solvents water and carbontetrachloride and argon shows that the positive chirality of S(+)-(H2O)3 is enhanced and the negative chirality of S(-)-X-(H2O)3(X = Cl,Br,I) and the positive chirality of S(+)-F(H2O)3 are reduced with an augmentation of the solvent dielectric constant.     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

Intense violet-blue photoluminescence in BaZrO3 powders: A theoretical and experimental investigation of structural order-disorder

Intense violet-blue photoluminescence (PL) emission at room temperature was verified in BaZrO₃ (BZO) powders with structural order-disorder. Ab-initio calculations, ultraviolet-visible absorption spectroscopy and PL were performed. Theoretical results showed that the local disorder in the network-formed Zr clusters present an important role in the formation of hole-electron pair. The experimental data and theoretical results are in agreement, indicating that the PL emission in BZO powders can be related to the structural order-disorder degree in the lattice.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

Lithium insertion into Co3O4: A preliminary investigation

Lithium has been inserted into the spinel Co₃O₄ both chemically and electrochemically. During lithiation the tetrahedral A-site Co²⁺ cations are displaced into neighbouring empty octahedral sites: the incoming Li⁺ ions occupy the remaining interstitial octahedra of the spinel structure to produce the partially ordered rocksalt compound LiCo₃O₄. The octahedral B-site cations of the A[B₂]X₄ spinel are unperturbed by this reaction: the oxide lattice expands by 8.6%. Lithium analyses and powder X-ray diffraction spectra indicate that further lithiation is possible. However, it is demonstrated that a fast lithiation in excess of LiCo₃O₄ is followed by a slow extrusion of B-site cobalt at 50°C until all the Li on the 8a sites has moved to the octahedral vacancies thus created.     

Thermoelectric transport coefficients of n-doped CaTiO3, SrTiO3 and BaTiO3: A theoretical study

Boltzmann transport equations and density functional theory calculations were employed to calculate the thermoelectric transport coefficients of CaTiO₃, SrTiO₃ and BaTiO₃. It was found that BaTiO₃ has the largest Seebeck coefficient and power factor. Then the transport coefficients were analyzed using the ‘Tight Binding Model’. The band narrowing, caused by the increasing lattice constants from CaTiO₃ to BaTiO₃, was the main reason for the increasing Seebeck coefficients and the decreasing electrical conductivity. The calculated electrical conductivity and electronic thermal conductivity were in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 2.45 for these oxides as degenerate semiconductors. Our theoretical results are helpful for seeking high performance thermoelectric oxides.     

A theoretical analysis of adsorption and dissociation of CH3OH on the stoichiometric SnO2(110) surface

A theoretical analysis based on the Hartree–Fock pseudopotential method and a density-functional theory calculation using a hybrid combination of general gradient approximation with pseudopotential procedure has been carried out to study the adsorption and dissociation of methanol on the stoichiometric SnO₂(110) surface. The dependence of the results upon model system and computing method is discussed. An optimization procedure of adsorbate and substrate atom positions on a six-layer slab model has been selected to characterize the corresponding geometric parameters, adsorption energy and charge-transfer processes related with the molecularly adsorbed CH₃OH and dissociative channels to yield methoxy or methyl fragments. In the high-coverage limit (θ=1), we find that dissociation of the methanol molecule via the heterolytic cleavage of the C---O bond is favoured. At lower coverage (θ=1/2), this channel and the molecularly adsorbed methanol present similar adsorption energies.     

Sol-gel synthesis of iron(III) oxyhydroxide nanostructured monoliths using Fe(NO3)3·9H2O/CH3CH2OH/NH4OH ternary system

Iron(III) oxyhydroxide xerogels were prepared through sol-gel technology, using iron(III) nitrate nonahydrate as precursor, ethanol as solvent and ammonium hydroxide as gelation agent. This base is used for propylene oxide substitution, which was the gelation agent in previous works. Synthesis of a gel using NH₄OH as a gelation agent is an innovative result with this type of precursor, since with metal salts the addition of a strong base commonly results in precipitation of the solid. The gel synthesis was achieved by controlling the base addition time. The dried material has a residual amount of organic impurities, in contrast with the significant amount detected in xerogels prepared using propylene oxide. The iron phase prevailing in the produced xerogels can be defined as γ-FeO(OH) (lepidocrocite), according to FTIR and Mössbauer analyses. The xerogels are formed by large clusters of well connected nanocrystallites of this phase. XRD revealed a crystalline phase retained inside the iron oxyhydroxide amorphous structure, which corresponds to NH₄NO₃ and results from the combination of NO₃⁻ and NH₄⁺ ions in solution. The produced xerogel has a promising composition to be an oxidizing composite for the energetic materials area.     

Spatial charge distribution and conductivities of the LaAlO3/SrTiO3 interfaces: A theoretical study

The electronic properties of the charge carriers at the LaAlO₃/SrTiO₃ interfaces are investigated by first principles studies. For the n-type interface, the carriers are located only on the SrTiO₃ side. For the p-type interface, the carriers are highly localized at the interface. A critical thickness of the LaAlO₃ overlayer exists, below which, the interface is insulating. Moreover, we show that the effective masses and mobilities of the carriers are spatially anisotropic and have a strong disparity for the two types of carriers. These results are consistent with experimental observations and are explained by the band structures and alignments of the consisting oxides and their interaction at the interfaces.     

Mössbauer investigation of hexagonal tungsten bronze type FeIII fluorides: (H2O)0.33 FeF3 and anhydrous FeF3

The new HTB (Hexagonal Tungsten Bronze) phases of FeF₃ and (H₂O)_0.33 FeF₃ have been characterized by ⁵⁷Fe Mössbauer spectroscopy; they have saturation hyperfine fields of (577 ±3) and (560±3) kOe and magnetic ordering temperatures (97±2) and (128.7 ± 0.5) K, respectively. The magnetic ordering temperature and the electric hyperfine interactions on iron are sensitive to the presence of zeolitic water in the system. Hydrolysed samples have also been investigated.     

A theoretical investigation of the special properties of SrFe1−xCoxO3

Using density-functional calculations, we investigate the special properties of SrFe_1−xCo_xO₃. The results show that the ground states have A-type antiferromagnetic order for x=0.1 and ferromagnetic order for x≥0.2 with Co ions distributed averagely. SrFe_1−xCo_xO₃ exhibits half-metallic nature for 0.2≤x≤0.7 and full-metallic nature for other values of x, and the half-metallic gap decreases with increasing x. The tunneling between the half-metallic ferromagnetic phases drives the large magnetoresistance. In addition, the Co cations are in the intermediate-spin state, while the Fe cations are in the intermediate-spin state for x≤0.5 and the high-spin state for x≥0.6.     

Correlation between P chemical shift tensor and local structure in lithium cyclohexaphosphates Li6P6O18·3H2O and Li6P6O18

To understand the surprising behavior between the variations of the P′–P–P″ angles and the correlated variations of the O′–P–O″ ones, two lithium cyclohexaphosphate compounds Li₆P₆O₁₈·3H₂O and Li₆P₆O₁₈ are studied by solid state nuclear magnetic resonance (NMR) spectroscopy. The two compounds exhibit the same [P₆O₁₈]⁶⁻ ring anions but with 3m or internal symmetry, respectively. Such symmetries induce local distortions that are exhibited by NMR spectroscopy. One-dimensional (1D) NMR gives information on structural sites of ⁷Li and ³¹P ions and the crystallographic non-equivalencies are observed. Nevertheless, in the anhydrous compound, X-ray diffraction and NMR results do not completely agree and some discrepancy exists between the number of sites observed with the first technique and the number of lines exhibited in the NMR spectra either for ⁷Li or ³¹P nuclei. This problem is elucidated by using 2D double quantum NMR spectroscopy coupled with theoretical considerations. We find that the ³¹P chemical shift tensor is dependent on the deviations of the O–P–O angles from those in the regular tetrahedron. Within the same empirical model, we suggest that the surprising behavior between the variations of the P′–P–P″ and the ones of the O′–P–O″ is related to the overall charge on the PO₄ group. We also find the positions of the isotropic lines for ⁷Li essentially depend on the site co-ordination of this nuclei.     

A theoretical investigation of the binding of TiCln to MgCl2

The structure of the (0 0 0 1) surface of the -MgCl₂ crystal has been investigated using DFT-GGA periodic calculations. The calculated surface relaxation is in agreement with LEED measurements. Motivated for the use of MgCl₂ as support for the Ziegler–Natta reaction, we have studied the adsorption of the catalyst (titanium chlorides as monomers or dimers) on the (1 0 0) and (1 1 0) MgCl₂ surfaces. The structures of adsorbed species are close to those previously found on cluster models: bridging chlorine atoms connect the Ti to the Mg atoms and the systems remain in high spin states. The (0 0 0 1) surface is the most stable face of the -MgCl₂ crystal; however it is Cl-terminated and henceforth poorly reactive; it had been suggested to deposit metallic Mg in order to improve its reactivity. Our modelling explains the failure of this tentative; the interaction between the deposited metal and the surface is repulsive and uncharged Mg atom does not bind.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.     

A better ferrimagnetic half-metal LuCu3Mn4O12: Predicted from first-principles investigation

Electronic structure calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) and GGA+U for manganite cuprate compound LuCu₃Mn₄O₁₂ have been performed, using the full-potential linearized augmented plane wave method. The calculated results indicate that LuCu₃Mn₄O₁₂ is ferrimagnetic and half-metallic in both GGA and GGA+U calculations. The minority-spin band gap is 0.7 eV within GGA, which is larger than that of LaCu₃Mn₄O₁₂ (0.3 eV), indicating its better half-metallicity. Further, the minority-spin gap enlarges from 0.7 to 2.8 eV with U taken into account, and simultaneously the Fermi level being shifted to the middle of the gap, making the half-metallic energy gap to be 1.21 eV. These results demonstrate that electronic correlation effect enhances the stability of half-metallic property. These facts make this system interesting candidates for applications in spintronic devices.     

Photoelectron spectroscopic investigation of transformation of trifluoroacetate precursors into superconducting YBa2Cu3O7−δ films

X-ray photoelectron spectroscopy (XPS) has been used to investigate the evolution of surface chemistry of YBa₂Cu₃O_7−δ (Y123) films prepared by the metalorganic deposition (MOD) process using trifluoroacetate (TFA) precursors. Detailed XPS core-level spectra obtained from the samples quenched from various points during the calcining and firing stages have been reported for the first time and are used to identify surface species. The XPS data show evidence of formation of intermediate phases such as Y-O-F, BaF₂, and CuO during the calcining process, which are the decomposition products of yttrium, barium, and copper trifluoroacetates, respectively. The TFA precursors are completely decomposed at the end of calcination. The change of binding energies for Y 3d_5/2, Ba 3d_5/2, and O 1s during the firing process indicates that Y123 starts to form at 800 °C after 0.5 h firing. Based on the experimental results, an alternative mechanism of the chemical evolution from precursor to final film in the TFA-MOD process is proposed.     

纳米晶ZrO2：Eu3＋-Bi3＋的制备及Bi3＋敏化Eu3＋特征发射的研究

采用化学共沉淀法制备了纳米晶ZrO2：Eu3＋-Bi3＋粉体,所制备的纳米晶粉体均能观测到Eu3＋离子的室温特征发射．研究了样品的晶体结构和发光性质．结果表明：经600℃煅烧后得到的纳米晶ZrO2：Eu3＋-Bi3＋粉体的晶相为四方相,经800,950及1100℃煅烧后的样品为四方相和单斜相的混合晶相．Eu3＋离子在四方相中的室温特征发射明显强于在混合相中的发射．Bi3＋离子的掺入对Eu3＋离子的室温特征发射有显著的敏化作用．     

Combined experimental and theoretical investigation of the CO2 adsorption on LaMnO3+y perovskite oxide

The surface interaction of CO₂ with the perovskite-type oxide LaMnO_3+y has been investigated by means of density functional theory calculations and experimental measurements of adsorption isotherms in the temperature range 298-473 K. A (1 0 0) oriented slab of the cubic structure was used for modeling CO₂ adsorption. The reference unit cell contains alternating LaO⁺ layers and layers; one slab is LaO⁺-terminated and the opposite surface is terminated. A Freundlich isotherm fitted the experimental data satisfactorily. Analysis of the isosteric heat revealed an energetically heterogeneous character for the lanthanum manganite oxide surface, mainly due to the degree of heterogeneity of the adsorption center and due to the adsorbate-adsorbate lateral interactions. Considering theoretical calculations and thermodynamical approaches, two types of active sites were found to be responsible for irreversible and reversible adsorption of CO₂ as a function of surface coverage and O₂ treatment. Strong adsorption takes place on the surface containing La cations. The strongest adsorption is associated with surface oxygen vacancies, center. The next strongest adsorption, a flat adaptation of CO₂ molecules with respect to the surface sites, with a strong binding to a surface oxygen, leads to chemisorbed carbonate species. These adsorption modes are chiefly indicative of a high basic character of the lanthanum manganite oxide surface. Several cationic sites formed by lanthanum and manganese cations are able to weakly adsorb CO₂ molecules in perpendicular or bridged forms. The latter adsorption modes suggest a weak acidic character of the manganite adsorbent.