Charge inversion at minute electrolyte concentrations

Anionic dimyristoylphosphatidic acid monolayers spread on LaCl3 solutions reveal strong cation adsorption and a sharp transition to surface overcharging at unexpectedly low bulk salt concentrations. We determine the surface accumulation of La3+ with anomalous x-ray reflectivity and find that La3+ compensates the lipid surface charge by forming a Stern layer with approximately 1 La3+ ion per 3 lipids below a critical bulk concentration, ct approximately 500 nM. Above ct, the surface concentration of La3+ increases to a saturation level with approximately 1 La3+ per lipid, thus implying that the total electric charge of the La3+ exceeds the surface charge. This overcharge is observed at approximately 4 orders of magnitude lower concentration than predicted in ion-ion correlation theories. We suggest that transverse electrostatic correlations between mobile ions and surface charges (interfacial Bjerrum pairing) may contribute to the charge inversion.     

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ～1 and ～10 nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)?Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.     

Soft X-ray resonant magnetic diffraction

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.     

两种药物与金属离子的作用

研究了磺胺间甲氧嘧啶、诺氟沙星与Ce3+,La3+,Tb3+,Co2+,Fe3+作用体系的荧光和紫外特性。结果表明,La3+和Tb3+对磺胺间甲氧嘧啶的荧光起增敏作用,而Fe3+,Co2+,Ce3+使磺胺间甲氧嘧啶的荧光猝灭;Ce3+,La3+,Tb3+,Co2+,Fe3+对诺氟沙星的荧光均起猝灭作用。紫外的测定结果与荧光基本相同。     

Charge order superstructure with integer iron valence in Fe(2)OBO(3)

Solution-grown single crystals of Fe(2)OBO(3) were characterized by specific heat, M?ssbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and M?ssbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe(2)OBO(3) is the clearest example of ionic charge order so far.     

La、Al在Fe晶界上协同作用的第一性原理研究

采用基于密度泛函理论的第一性原理,研究了稀土La和Al在bcc-Fe中Fe晶界处的协同作用影响.计算了La和Al在晶界、表面、晶内的形成能,建立La、Al共存于晶界模型,通过电荷密度、布居分布手段分析了La、Al原子对晶界的影响.结果表明:La、Al原子易偏聚在晶界处,且La-Al的原子间距与体系结合能成正比关系. La的掺杂改善了晶界处的电荷分布情况,促进了Al原子与周围Fe原子间的相互作用,态密度曲线的计算结果显示,La原子的加入能够使La、Fe及Al原子间的键的结合力更强,从而提高了界面结合强度.     

Electronic and magnetic reconstructions in La0.7Sr0.3MnO3/SrTiO3 heterostructures: a case of enhanced interlayer coupling controlled by the interface

We report on the magnetic coupling of La0.7Sr0.3MnO3 layers through SrTiO3 spacers in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures. Combined aberration-corrected microscopy and electron-energy-loss spectroscopy evidence charge transfer to the empty conduction band of the titanate. Ti d electrons interact via superexchange with Mn, giving rise to a Ti magnetic moment as demonstrated by x-ray magnetic circular dichroism. This induced magnetic moment in the SrTiO3 controls the bulk magnetic and transport properties of the superlattices when the titanate layer thickness is below 1 nm.     

Dielectric, magnetic and magnetoelectric properties of La and Nb codoped bismuth ferrite

Single-phase Bi(0.80)La(0.20)FeO(3) (BLFO) and Bi(0.80)La(0.20)Fe(1-x)Nb(x)O(3) (BLFNO) samples were prepared in order to study the dielectric, magnetic and magnetoelectric properties of La and Nb codoped BiFeO3. Rietveld refinement of x-ray diffraction patterns of La and Nb codoped samples has been performed using the R3c space group. Magnetic hysteresis loops revealed that codoping can effectively increase the spontaneous magnetization due to change in the bond angle of Fe-O-Fe as a result of distortion created by the Nb5+ doping. Magnetic field-induced relative change of the dielectric constant for BLFO and BLFNO samples is a signature of magnetoelectric coupling.     

(La,Ce,Tb)BO3的合成及光谱性质

采用高温固相法合成了(La,Ce,Tb)BO₃绿色发光粉,并对该发光粉进行了XRD和SEM分析。结果表明:(La,Ce,Tb)BO₃的晶体结构和LaBO₃相同,Ce³⁺、Tb³⁺的掺入并没有改变晶体的结构,发光粉颗粒大小均匀,形貌规则,粒度在5 μm左右。研究了(La,Ce,Tb)BO₃的光谱性质,在(La,Ce,Tb)BO₃的发射和激发光谱中除了有Tb³⁺的特征发射和激发峰外,还有Ce³⁺的特征发射和激发峰。比较了(La,Ce)BO₃发射光谱和(La,Tb)BO₃的激发光谱,两者存在重叠,这为Ce³⁺→Tb³⁺的能量传递提供了条件。将(La,Ce,Tb)BO₃的发射光谱与商品粉(La,Ce,Tb)PO₄进行比较,两者的发射主峰都在541 nm处, (La,Ce,Tb)BO₃在489 nm处的峰位稍有红移,通过计算表明,(La,Ce,Tb)BO₃的发光亮度达到商品粉(La,Ce,Tb)PO₄的94.7%。因此,(La,Ce,Tb)BO₃是一种很有应用前景的绿色发光粉。     

Charge-state distribution and Doppler effect in an expanding photoionized plasma

The charge state distributions of Fe, Na, and F are determined in a photoionized laboratory plasma using high resolution x-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter xi=20-25 erg cm s(-1) under near steady-state conditions. Line opacities are well fitted by a curve-of-growth analysis which includes the effects of velocity gradients in a one-dimensional expanding plasma. First comparisons of the measured charge state distributions with x-ray photoionization models show reasonable agreement.     

Comparison of the electronic structure of a thermoelectric skutterudite before and after adding rattlers: an electron energy loss study

Skutterudites, with rattler atoms introduced in voids in the crystal unit cell, are promising thermoelectric materials. We modify the binary skutterudite with atomic content Co(8)P(24) in the cubic crystal unit cell by adding La as rattlers in all available voids and replacing Co by Fe to maintain charge balance, resulting in La(2)Fe(8)P(24). The intention is to leave the electronic structure unaltered while decreasing the thermal conductivity due to the presence of the rattlers. We compare the electronic structure of these two compounds by studying the L-edges of P and of the transition elements Co and Fe using electron energy loss spectroscopy (EELS). Our studies of the transition metal white lines show that the 3d electron count is similar for Co and Fe in these compounds. As elemental Fe has one electron less than Co, this supports the notion that each La atom donates three electrons. The L-edges of P in these two skutterudites are quite similar, signalling only minor differences in electronic structure. This is in reasonable agreement with density functional theory (DFT) calculations, and with our multiple scattering FEFF calculations of the near edge structure. However, our experimental plasmon energies and dielectric functions deviate considerably from predictions based on DFT calculations.     

Structure of iron phosphate glasses modified by alkali and alkaline earth additions: neutron and x-ray diffraction studies

The structure of iron phosphate glasses modified by additions of K(2)O and BaO, with nominal molar compositions [(1 - x)(0.6P(2)O(5)-0.4Fe(2)O(3))]xMe(y)O, where x = 0-0.4 in increments of 0.1; Me=K or Ba; and y = 1 or 2, has been investigated using neutron diffraction and x-ray diffraction techniques. Fitted coordination numbers for P-O and Fe-O showed a notable change in the P-O(NBO) and P-O(BO) contributions at greater than 20 mol% modifier addition, with barium producing a markedly larger increase in P-O(NBO) contribution than potassium. Fitting of T(N)(r) and T(X)(r) provided average n(BaO) = 9 and n(KO) = 6. Iron occurs in a range of coordination sites in these glasses: ([6])Fe(2+), ([4])Fe(3+), ([5])Fe(3+) and ([6])Fe(3+). The partitioning between these sites gives average n(FeO) = 5.2-5.8, with barium-doped glasses exhibiting higher average n(FeO) than potassium-doped glasses. The stronger depolymerizing effect of Ba(2+) than K(+) on the phosphate network, coupled with its greater ionic charge and higher Me-O, Fe-O and O···O coordination numbers, explain previously observed divergences in physical properties between the barium-doped and the potassium-doped glasses.     

Direct observation of charge transfer in double-perovskite-like RbMn[Fe(CN)6

The charge density distribution has been determined for a transition metal cyanide, RbMn[Fe(CN)(6)], by means of the maximum entropy-Rietveld method combined with the highly angularly resolved synchrotron radiation x-ray powder diffraction at SPring-8 BL02B2. We directly observed a charge transfer from the Mn site to the Fe site in the low-temperature phase. On the basis of a local density approximation calculation, we discuss the origin for the anisotropic bonding electron distribution around the Mn3+ ion in the low-temperature phase.     

Stabilization of charge ordering in La1/3Sr2/3FeO3-delta by magnetic exchange

The magnetic exchange energies in charge ordered La1/3Sr2/3FeO3-delta (LSFO) and its parent compound LaFeO3 (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+-Fe5+ pairs (JF) and antiferromagnetic exchange between Fe3+-Fe3+ pairs (JAF) fulfills the criterion for charge ordering driven by magnetic interactions (|JF/JAF|>1). The 30% reduction of JAF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.     

Critical fluctuations and quenched disordered two-dimensional charge stripes in La(5/3)Sr(1/3)NiO4

Using high-resolution x-ray scattering, we have demonstrated the existence of quenched disordered charge stripes in a single crystal of La (5/3)Sr (1/3)NiO (4) at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (xi(a) approximately 185 A, xi(b) = 400 A, and xi(c) approximately 25 A) and by the critical exponents of the charge strip transition. The charge stripe ordering did not develop long-range order even at low temperatures, indicating that the charge stripes are disordered and that the length scale of the disorder is quenched.     

Modeling hyperfine parameters observed from the charge-ordered to valence-mixed state of NdBaFe(2)O(5)

Orbital populations of the minority-spin Fe(2+) electron in NdBaFe(2)O(5) are extracted from electric and magnetic hyperfine parameters of (57)Fe M?ssbauer spectra across the temperature interval where charge ordering of the valence state 2.5?+ of iron occurs. The previously used approach that assigns the minority-spin population to a single lowest-energy d orbital is expanded to three orbitals by setting up a system of equations in terms of the Fe(2+)/Fe(3+) balance from isomer shift, a point-charge model for the valence and ligand contributions to the electric-field gradient, a point-dipole model for the dipolar contribution and an iterative scheme for small orbital contributions to the internal magnetic field. This allows us to model the hyperfine fields (electric and magnetic) in the intermediate temperature range of partial charge ordering between T(V )?≈?210?K and T(p)?≈?285?K, for which the one-orbital model was insufficient.     

Collapse of spherical polyelectrolyte brushes in the presence of multivalent counterions

We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPB). The SPB result if linear polyelectrolyte (PE) chains (contour length: 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. Dispersed in water, the surface layer consisting of chains of the strong PE poly(styrene sulfonic acid) (PSS) will swell. We demonstrate that successive addition of trivalent ions (La3+) leads to a collapse in which the surface layer is shrinking drastically. All findings are discussed on the base of a theoretical mean-field approach using the Donnan equilibrium. The ion exchange and a strong binding of trivalent ions by PE chains is followed up by a drop in the osmotic pressure inside the brush. This reduction is the driving force for the collapse. The strong ion-chain correlation is discussed with results obtained from molecular dynamics simulations.     

稀土（La^3＋,Pr^3＋）—DBS—偶氮胂有色配合物组合和稳定常数的吸收光谱法测定

稀土（Ｌａ＾３＋,Ｐｒ＾３＋）在酸性条件下与ＤＢＳ－偶氮胂发生配位反应,首先对光谱数据进行因子分析法处理从而判断出只生成一类配合物,进而采用线性回归法计算配合物组成,摩尔吸光系数,表观稳定常数及配位体浓度,实验结果表明,在１ｍｏｌ．Ｌ＾－１ＨＣｌ介质中,Ｌａ＾３＋、Ｐｒ＾３＋与ＤＢＳ－偶氮胂形成Ｍ：Ｌ＝１：３的配合物,其摩尔吸光系数分别为１．３９９＊１０＾５和１．１５１７＊１０＾５Ｌ．ｍｏｌ＾－１．ｃｍ＾－１,表观稳定常数ｌｏｇβ３分别为１５．３６和１５．３４。     

Structural, spectroscopic, magnetic and electrical characterization of Ca-doped polycrystalline bismuth ferrite, Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1)

The crystal structure and physical properties of multiferroic polycrystalline Ca(2+)-doped BiFeO(3) samples have been investigated. The present experimental investigation suggests that Bi(1-x)Ca(x)FeO(3-x/2) (x ≤ 0.1) can be considered as a solid solution between BiFeO(3) and CaFeO(2.5). The oxidation state of Fe in these materials is + 3 and charge balance occurs through the creation of oxygen vacancies. For each composition, two structural phase transitions are revealed as anomalies in the variable-temperature in situ x-ray diffraction data which is consistent with the well-established high-temperature structural transformation in pure BiFeO(3). All compositions studied show antiferromagnetic behaviour along with a ferromagnetic component that increases with Ca(2+) doping. The resistivities of the Bi(1-x)Ca(x)FeO(3-x/2) samples at room temperature are of the order of 10(9) Ω cm and decrease with increasing Ca(2+) content. Arrhenius plots of the resistivity show two distinct linear regions with activation energies in the range of 0.4-0.7 and 0.03-0.16 eV. A correlation has been established between the critical temperatures associated with the structural phase transitions and the multiferroic properties. A composition of x = 0.085 is predicted to show maximum magneto-electric coupling.     

The interfacial structure of water/protonated α-Al2O3 (11 ?20) as a function of pH

Sum frequency vibrational spectroscopy (SFVS) was used to study the structure of the protonated α-Al(2)O(3) (11 ?20), and water/α-Al(2)O(3) (11 ?20) interfaces as a function of pH. By combining SFVS spectra with an oxygen-terminated model suggested by x-ray reflectivity, we are able to understand several details of the protonated α-Al(2)O(3) (11 ?20) interface structure. For example, the spectral changes observed for the water/α-Al(2)O(3) (11 ?20) interface with varying pH could be accounted for by the protonation/deprotonation of particular surface hydroxyls. Our spectra also indicate that the point of zero charge for this interface is at pH ~ 6.7.