Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below T(V)=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (approximately 80 meV) are shifted upwards in energy above T_{V} due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Delta(5) symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above T(V).     

Lithium dynamics in LiMn2O4 probed directly by two-dimensional (7)Li NMR

LiMn2O4 has been studied using magic-angle-spinning nuclear magnetic resonance (MAS NMR). 1D MAS NMR shows three Li resonances assigned to different crystallographic sites. At low temperatures an extra peak appears, indicating charge ordering of Mn3+ and Mn4+. Direct observation of the lithium dynamics was possible using rotor-synchronized 2D exchange NMR. A millisecond time scale exchange of lithium starts around 285 K between the 8a and the 16c site. At 380 K lithium even starts to hop between more than two sites. The activation energies and Li jump rates are derived and are in agreement with those determined macroscopically.     

X-Ray anomalous scattering study of a charge-ordered state in NaV2O5

Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an x-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below T(C) = 35 K. The charge ordering pattern is the fully charged zigzag-type ladder with the unit cell 2ax2bx4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as the average valence V(4.5+/-delta(c)/2) is observed below T(C), showing continuous variation of delta(c) as a function of temperature.     

基于香豆酰肼的高选择性的铜离子荧光探针

设计合成了含有香豆酰肼官能团的铜离子荧光探针,N,N-二乙基氨基香豆素-3-酰肼吡啶醛(XB),该探针在水溶液中[V(CH3CN):V(H2O)=1:1]对铜离子识别和检测具有较高的灵敏度和选择性。碱金属和碱土金属离子K+、Na+、Mg2+、Ca2+以及过渡金属离子Cr3+、Mn2+、Fe3+、Co2+、Ni2+、Zn2+、Cd2+、Hg2+和Ag+等对Cu2+离子的识别无显著影响。光谱滴定和ESI-MS谱表明Cu2+离子与XB以1:2的化学计量比形成配合物。     

Possible electron-phonon interaction-driven microscopic origin of the phase transitions in β-Na 0.33 V 2 O 5 and analogues

The microscopic origin of the various phase transitions which have been observed in g-Na 0.33 V 2 O 5 and its analogues can be explored experimentally by investigating the oxygen isotope effect on the phase transition temperatures. An absent oxygen isotope effect on the sodium-ordering transition would point to an antiferroelectrically driven instability, while a large isotope effect on the charge ordering and spin ordering transition temperatures would suggest the formation of a charge-density wave instability and phonon mediated long-range magnetic ordering. All instabilities can be attributed to strong electron-phonon interaction effects, which dominate the ground-state properties.     

Low to high spin-state transition induced by charge ordering in antiferromagnetic YBaCo2O5

The oxygen-deficient double perovskite YBaCo2O5, containing corner-linked CoO5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co(2+)/Co(3+) charge ordering occur as well as a change in the Co2+ spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.     

Charge-order-driven spin-peierls transition in alpha(')-NaxV2O5

We conclude from 23Na and 51V NMR measurements in alpha(')-NaxV2O5 (x = 0.996) (i) a charge-ordering transition starting at T相似文献   

Modeling the electronic behavior of gamma-LiV2O5: a microscopic study

We determine the electronic structure of the one-dimensional spin- 1 / 2 Heisenberg compound gamma-LiV2O5, which has two inequivalent vanadium ions, V(1) and V(2), via density-functional calculations. We find a relative V(1)-V(2) charge ordering of roughly 70:30. We discuss the influence of the charge ordering on the electronic structure and the magnetic behavior. We give estimates of the basic hopping matrix elements and compare with the most studied (alpha)'-NaV2O5.     

Ab initio evaluation of the charge ordering in alpha'NaV2O5

We report ab initio calculations of the charge ordering in alpha'NaV2O5 using large configurations interaction methods on embedded fragments. Our major result is that the 2p(y) electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.     

Ba2RV3O11(R=Y,Gd,La):Eu3+(或Dy3+,Bi3+)的合成及发光性能的研究

本文报导了Eu3+,Dy3+在Ba2RV3O11(R=Y,Gd,La)基质中的光谱性质、Bi3+对Dy3+发射强度的影响及温度对Dy3+发射强度的猝灭情况.研究了被取代离子R3+(R=Y,Gd,La)对基质电行迁移带、Eu3+的红橙比、Dy3+黄蓝比的影响,还给出了Dy3+的浓度猝灭值.     

Charge and orbital order in Fe3O4

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe3O4) is investigated using the local spin density approximation with Coulomb interaction correction method. While the difference between t(2g) minority occupancies of Fe(2+)(B) and Fe(3+)(B) cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.     

Enhancement of phosphor efficiency via composition modification

The luminescence efficiency of Mn4+-doped CaAl12O19 (Mn:CAO) is significantly improved by composition modification. The leading mechanism that quenches the Mn4+ photoluminescence is the formation of Mn4+ pairs in the lattice of CaAl12O19 (CAO) with interstitial O2- for charge compensation. Mixing Mg2+ ions into the CAO lattice may form Mn4+-Mg2+ pairs and reduce the number of Mn4+ pairs, thus enhancing the Mn4+ luminescence efficiency by more than three times. It is shown that the presence of Mg2+ leads to formation of additional phases that also affect the optical properties of Mn4+.     

Experimental evidence of coupling interaction between charge ordering and spin ordering

In order to experimentally probe into the complicated interaction between charge ordering and spin ordering in manganites,two sets of samples Nd0.5Sr0.5Mn1-xGaxO3(NSMGO) and Nd 0.5Sr0.5Mn1-y CryO3(NSMCO)(0.0 x,y 0.075),have been studied by means of electrical transport,magnetization,electron spin resonance and transmission electron microscopy analysis.By comparing the influence of Cr-doping and Ga-doping in the Nd0.5Sr0.5MnO3(NSMO) system,large difference between the evolution of charge ordering temperature T co in the Cr-doping and the Ga-doping cases is found.In the NSMCO system,the CE-type antiferromagnetic(AFM)/charge ordering(CO) phase disappears with only 2.5 percent Cr doping;but in the NSMGO system,the CE-type AFM/CO phase always exists.This phenomenon indicates that the charge ordering formation is dominated by the spin ordering.As a result,it is experimentally proved that there is strong coupling interaction between charge ordering and spin ordering in NSMO system.     

Magnetic-field switching of crystal structure in an orbital-spin-coupled system: MnV2O4

We studied the magnetic and structural properties of spinel MnV2O4, which has S=5/2 spin with no orbital degrees of freedom on the Mn2+ site and S=1 spin and three orbital degrees of freedom on the V3+ site. We found that the ferrimagnetic ordering at TN=56.5K and the structural phase transition at Ts=53.5K are closely correlated in this compound and found a switching of crystal structure between cubic and tetragonal phases by the magnetic field. This phenomenon can be explained by the coupling between orbital and spin degrees of freedom in the t2g states of the V site.     

Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2

Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt[3] strongly lessens correlation effects.     

NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na(0.5)CoO(2). Above T(N) = 86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co(3+) and Co(4+) sites. Below T(N), the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T(MIT) = 51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x = 0.5 Na ordering.     

特殊变色蓝宝石的紫外-可见光光谱研究

坦桑尼亚Umba出产颜色丰富的刚玉,该研究对象是一颗来自Umba的具有特殊变色效应的蓝宝石,D65光源（色温6 500 K）下呈现淡黄色,A光源（色温2 856 K）下呈现淡紫红色。为了研究这颗变色蓝宝石紫外-可见光光谱中的谱峰归属与变色成因,该研究使用电荷补偿理论来分析此样品紫外-可见光光谱中的谱峰归属。采用紫外-可见分光光度计（UV-Vis）和激光剥蚀电感耦合等离子体质谱仪（LA-ICP-MS）对这颗变色蓝宝石进行了测试。结果发现,变色蓝宝石紫外-可见光光谱中存在位于377,388和450 nm处的3个吸收峰和1个以560 nm为中心的宽缓吸收带。样品的颜色主要受450 nm处吸收峰和以560 nm为中心的吸收带影响,其中以560 nm为中心的吸收带造成了这颗蓝宝石的变色效应。根据激光剥蚀电感耦合等离子体质谱仪的测试结果,样品中主要杂质元素有Fe,Ti,Cr,V和Mg等。样品紫外-可见光光谱中377,388和450 nm处的吸收峰是由Fe3+导致。蓝宝石中的Cr3+,V3+,Fe2+-Ti4+对都可以在560 nm附近产生吸收,结合电荷补偿理论分析,刚玉中的Mg2+会优先和Ti4+进行电荷补偿,样品中Mg含量要稍微高于Ti,推测样品中几乎所有Ti4+会与Mg2+进行电荷补偿,因此样品中几乎不会存在Fe2+-Ti4+对。Fe2+-Ti4+对电荷转移产生的吸收特征具有很强的偏振性,尤其是在580 nm以后的吸收特征会随着偏振方向的改变而有很明显的变化。偏振紫外-可见光光谱测试发现以560 nm为中心的吸收带没有明显的偏振性,进一步验证了样品中几乎没有Fe2+-Ti4+对,因此以560 nm为中心的吸收带主要是由于Cr3+和V3+造成的。样品的颜色主要是由Fe3+,Cr3+和V3+引起的,而变色效应主要是由Cr3+和V3+导致。结合电荷补偿机制与偏振-紫外可见光光谱来解释这颗变色蓝宝石的紫外-可见光光谱中以560 nm为中心的吸收带的归属,为研究刚玉紫外-可见光光谱中较为常见的位于560 nm左右吸收带的归属提供了一种新的研究思路。     

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).     

BaMg2Si2O7:Eu3+和BaMg2Si2O7:Tb3+发光材料的合成及发光特性的研究

本文报道了首次合成的稀土离子激活的BaMg2Si2O7:Eu3+和BaMg2Si2O7:Tb3+光致发光材料,通过正交设计实验,确定了基质的最佳配比、烧结温度,探讨了影响材料发射强度的一些因素,测定了两种材料的发射光谱和激发光谱,对其发光性能也进行了分析和研究。     

Antiferromagnetic d-electron exchange via a spin-singlet pi-electron ground state in an organic conductor

Electron spin resonance reveals the spin behavior of conduction (pi) and localized (d) electrons in beta-(BDA-TTP)2MCl4 (M=Fe, Ga). Both the Ga3+(S=0) and Fe3+(S=5/2) compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the pi electron system of the donor molecules. The behavior is consistent with charge ordering in beta-(BDA-TTP)2MCl4 at the metal-insulator transition. At 5 K, the Fe3+ compound orders antiferromagnetically, even though the pi electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.