Search

We show that repulsive local Coulomb interaction alone can drive valence-skipping charge disproportionation in the degenerate d-band, resulting in effective negative-U. This effect is shown to originate from anisotropic orbitalmultipole scattering, and it occurs only for d1, d4, d6, and d9 fillings (and their immediate surroundings). Explicit boundaries for valence-skipping are derived, and the paramagnetic phase diagram for d4 and d6 is calculated. We also establish that the valence-skipping metal is very different, in terms of its local valence distribution, compared to the atomiclike Hund's metal. These findings explain why transition-metal compounds with the aforementioned d-band fillings are more prone to valence-skipping charge order and anomalous superconductivity. [ABSTRACT FROM AUTHOR]

59Co NMR experiments have been performed on single crystals of the layered cobaltate NaxCoO2 with x = 0.77, which is an antiferromagnet with Néel temperature TN = 22 K. In this metallic phase, six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies ʋQ, magnetic shifts KZZ and nuclear spin lattice relaxation rates 1/7). Contrary to the x = 1/2 or x = 2/3 phases, the 3D stacking of the Na planes is not perfect for x = 0.77 but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as KZZ and (1/T1)1/2 scale linearly with ʋQ . The data analysis allows us to separate the contribution ʋlattQ of the ionic charges to ʋQ from that ʋelQ due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with x of ʋlattQ is found to fit rather well numerical computations done in a point charge model. The second term ʋelQ allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations. [ABSTRACT FROM AUTHOR]

We have investigated the dynamics of spin-state crossover (SSC) for perovskite LaCoO3 through optical phonons by means of infrared and inelastic x-ray spectroscopy. Upon thermally induced SSC, anomalously dispersionless Co-0 bond stretching phonons coupled to the thermally excited spin state have been identified. The enhanced spin-state fluctuation irrespective of the presence of a clear charge gap suggests the emergence of complex spin-state disproportionation involving low-, intermediate-, and high-spin states due to the strong correlation among thermally activated spin states. [ABSTRACT FROM AUTHOR]

We performed 13C NMR measurements on the quasi-one-dimensional (Q1D) charge-ordered system (DIDCNQI)2 Ag under ambient and applied pressure to clarify the pressure-temperature phase diagram. For pressures up to 15 kbar, the NMR spectra exhibit complicated splitting at low temperatures, indicating a "generalized 3D Wigner crystal" state. In this pressure region, we find that increased pressure causes a decrease in the charge disproportionation ratio, along with a decrease in the transition temperature of the generalized 3D Wigner crystal. In the high-pressure region, near 20 kbar, where a 1D confined liquid crosses over to a 3D Fermi liquid at high temperatures, the ground state is replaced by a nonmagnetic insulating state that is qualitatively different from the generalized 3D Wigner crystal. [ABSTRACT FROM AUTHOR]

We report a density functional theory investigation of B-site doped CaFeO3, a prototypical charge ordered perovskite. At 290 K, CaFeO3 undergoes a metal-insulator transition and a charge disproportionation reaction 2Fe4+→Fe5++Fe3+. We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a two-dimensional Jahn-Teller-type distortion, where FeO6 cages on the xy plane elongate along x and y alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the Fe5+ and Fe3+ cations can be enhanced when the B-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the Fe5+O6 and Fe3+O6 octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural, and electrical relationships exploited in this paper can be extended to a variety of perovskites and nonperovskite oxides, providing guidance in the structural manipulation of electrical properties of functional materials. [ABSTRACT FROM AUTHOR]

We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaitlike lattice distortion of the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L denotes a ligand hole. For sufficiently large distortions (δdNi-O ∼ 0.05-0.10 Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge. [ABSTRACT FROM AUTHOR]

The article reports on the study of low-energy electronic structure of heterostructures formed by graphene on a three-dimensional (3D) topological insulator (TI). It states that the disproportion can be caused by a twist angle between the TI surface and the graphene. It mentions that the proximity of the TI induces a strong spin-orbit coupling enhancement in graphene.

A quartet of hydrogens can be dynamically bound to group-14 atoms, the resulting complexes having even valence. In macroscopic assemblies and at high pressures these can give way by dynamically assisted (quantum) disproportionation to complexes with odd valence and hence, in principle, to metallic tendencies. A new extended metallic composition of, for example, GeH3 is investigated by first-principles methods within density functional theory. Its stoichiometry and its very existence is a direct consequence of inclusion of nuclear quantum dynamical contributions to the free energy. From enthalpic comparisons GeH3 augmented with hydrogen appears preferred beyond 175 GPa, where three candidate structures are competitive, these being A15, P42/mmc, and Cccm. The pressure at which GeH3 makes its appearance is significantly influenced by zero-point energy, and quantum effects play an important role in a notable trend towards disproportionation. The ensuing systems are all metallic and the superconducting transition temperature of GeH3 has been estimated as about 100 K or higher (a common range for the three prospective structures). The general proposition of quantum disproportionation at elevated pressures appears extendable to other high hydrides. [ABSTRACT FROM AUTHOR]

Experimental X-ray absorption spectra at the fluorine K and transition metal L2,3 absorption edges of the MF2 (M = Cr-Ni) family are presented. Ligand field calculations in D4h symmetry show very good agreement with the transition metal L2,3 XAS spectra. To successfully explain nominal Cr2+ L2,3 XAS spectrum in CrF2, the inclusion of Cr+ and Cr3+ was needed implying the presence of a disproportionation reaction. The multiplet calculations were then modified to remove the structure of the 2p hole in the calculated M T2p → 3d absorption spectra. These results for the 3dn+1 states are in one to one correspondence with the leading edge structures found at the fluorine K edge. A direct comparison with the metal L2,3 edges also indicates that there is evidence of the metal multiplet at the fluorine K pre-edge structures. [ABSTRACT FROM AUTHOR]

An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO3 and YbNiO3 perovskites across the metal-insulator transition, with TMI = 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below TMI to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group P2\ /n, containing two inequivalent Ni1(3+σ)+ and Ni2(3-σ)+ cations; above TMI, the samples become orthorhombic, space group Pbnm, with a single site for Ni3+. The 57Fe Mossbauer spectra of iron-doped (at 1.5%) RNiOt, (R = Tm, Yb) samples recorded below Tm exhibit for Fel and Fe2 (replacing Ni1 and Ni2 sites) hyperfine parameters corresponding to large (NilO6) and small (Ni2O6) octahedra. The remarkable difference between the quadrupole splittings (Δ1 ≈0.3 mm/s and Δ2 ≈ 0.07 mm/s) of Fel and Fe2 sites in RNi0.0985Fe0.015is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at 57Fe ions, and estimate, by using the experimental ▵ and ▵2 values, the contributions of the 3d, 3p, and 2p electrons (overlap distortion and covalence effects). Above TMI, a unique state for iron atoms is observed, upon metallization of the sample. [ABSTRACT FROM AUTHOR]

Carbon-13 nuclear magnetic resonance (13C NMR) measurements have been carried out in the quasi-one-dimensional organic conductor (TMTTF)2SbF6, which exhibits charge ordering (CO) transition and antiferromagnetic (AF) transition at 157 K and 6 K at ambient pressure, respectively. We estimated the degree of charge disproportionation, Δρ=ρrich-ρpoor, in the CO state to be 0.25e from the analysis of the NMR shift and found the broadening of the NMR spectral peaks caused by the inhomogeneous distribution in association with the AF fluctuation at charge rich site below 20 K in the CO state. Moreover, we determined the magnetic structure with two kinds of staggered moments in the AF phase of (TMTTF)2SbF6, which suggests that the CO state remains even in the AF phase. Consequently, we found the difference between two AF phases appearing on both sides of the spin-Peierls phase in the universal P-T phase diagram. [ABSTRACT FROM AUTHOR]

While the formal valence and charge state concepts have been tremendously important in materials physics and chemistry, their very loose connection to actual charge leads to uncertainties in modeling behavior and interpreting data. We point out, taking several transition metal oxides (La2VCuO6, YNiO3, CaFeO3, AgNiO2, V4O7) as examples, that while dividing the crystal charge into atomic contributions is an ill-posed activity, the 3d occupation of a cation (and more particularly, differences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or disproportionation) systems, where different "charge states" of cations have identical 3d orbital occupation. Implications for theoretical modeling of such charge states and charge-ordering mechanisms are discussed. [ABSTRACT FROM AUTHOR]

We report the molecular dipole effect on conduction electrons in the title superconductor. The angular-dependent magnetoresistance has a peak for fields nearly parallel to the conducting layer, and the peak width scales as the field component perpendicular to the layer, indicating incoherent interlayer transport. However, two closed Fermi surfaces are observed in quantum oscillation. Accordingly, crystallograph-ically independent layers have different charge densities in a bulk single crystal. The electric dipole of tetrahydrofuran gives rise to interlayer charge disproportionation. [ABSTRACT FROM AUTHOR]

Previous x-ray diffraction measurements revealed the pressure-induced decomposition of an fee LaH2 3 into H-rich and H-poor fee phases around 11 GPa. The present neutron diffraction measurements on LaD2 confirm the formation of NaCl-type LaD as a counterpart of the D-rich LaD2+a by disproportionation. First-principles enthalpy and lattice dynamic calculations demonstrate that the NaCl-type LaH is stabilized at high pressures and can be recovered at ambient conditions. Finding the NaCl-type LaH will pave the way for investigations on the site-dependent nature of hydrogen-metal interactions. [ABSTRACT FROM AUTHOR]

51V NMR measurements have been conducted on a single crystal of the quasi-one-dimensional conductor β-Li0.33V2O5 which undergoes a metal-insulator (MI) transition at TMI~170 K. In the metallic phase, we obtain 51V Knight shift and electric field gradient tensors. From the analysis of the 51V Knight shifts, we find that the charge disproportionation appears even in the metallic state and the electronic structure is represented within a model of weakly coupled ladders containing two types of ladders with distinct carrier densities. Based on the 51V nuclear spin-lattice relaxation rate, we discuss the spin dynamics within the one-dimensional electron gas model. From the analysis of several nonmagnetic V5+ spectra observed in the insulating phase, we propose a possible charge-order pattern which has a superlattice modulation larger than those in other family members of β-A0.33V2O5 (A=Na and Ag). Finally, we discuss the A-ion dependence of the electronic structure, the charge disproportionation, and the charge order in β-A0.33V2O5. [ABSTRACT FROM AUTHOR]

Recently, an unusual metallic state with a substantially nonuniform distribution of the charge and magnetic density in CoO2 planes was found experimentally in the NaxCoO2 compound with x>0.6. We have investigated the origin of such an electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of the strength of the electron correlations in the d(Co) shells within the GGA+U approximation for a system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sodium cations and enhanced correlations. Two important magnetic states of cobalt lattice competing with each other at realistic values of the correlation parameter were found--low-spin hexagons lattice (LS) and higher-spin kagome lattice (HS-KSL). In the heterogeneous metallic HS-KSL phase, magnetic Co ions form a kagome structure. In LS phase, the kagome pattern is decomposed into hexagons and the Co ions possess the minimal values of their spin. Coexistence of these states could explain the emergence of the disproportionation with the peculiar kagome structure experimentally revealed in previous studies of the cobaltates. [ABSTRACT FROM AUTHOR]

We present a theory describing the local electronic properties of the perovskite rare-earth nickelates--materials which have negative charge transfer energies, strong O 2p - Ni 3d covalence, and breathing-mode lattice distortions at the origin of highly studied metal-insulator and antiferromagnetic ordering transitions. Utilizing a full-orbital, full-correlation double-cluster approach, we find strong charge fluctuations, in agreement with a bond disproportionation interpretation. The double-cluster formulation permits the inclusion of necessary orbital degeneracies and Coulomb interactions to calculate resonant x-ray spectral responses, with which we find excellent agreement with well-established experimental results. This previously absent, crucial link between theory and experiment provides validation of the recently proposed bond disproportionation theory, and provides an analysis methodology for spectroscopic studies of engineered phases of nickelates and other high-valence transition-metal compounds. [ABSTRACT FROM AUTHOR]

B-site-disordered perovskite CaFe0.5Mn0.5O3 with unusually high valence Fe4+ was synthesized using a high-pressure technique. Fe4+ randomly distributed at half of the B sites shows charge disproportionation to Fe3+ and Fe5+. The spins of Fe3+,Fe5+, and Mn4+ order below 90 K. Analysis of low-temperature neutron powder-diffraction data revealed a G-type antiferromagnetic structure--where all the nearest-neighboring spins of Fe3+,Fe5+, and Mn4+ couple antiparallel--and the small ordered moment of 0.58μB reveals local charge ordering that gives rise to predominant Fe3+-O-Fe5+ antiferromagnetic arrangements. Despite the identical chemical compositions of CaFe0.5Mn0.5O3 and Ca2FeMnO6, the magnetic structure of the present CaFe0.5Mn0.5O3 is very different from the noncollinear one of layered B-site-ordered Ca2FeMnO6. [ABSTRACT FROM AUTHOR]

The phase diagram of the Be-H system is measured up to 100 GPa. BeH2 is the only stoichiometry observed. Three crystalline phases are identified using synchrotron x-ray diffraction. The Ibam structure, already known at ambient pressure, is stable up to 27 GPa. The P41212 structure, isotypical to the crystoballite-low SiO2 structure, is stable above 600 K over the same pressure range. The well-known P3m1 layered structure of transition metal dichalcogenides is stable above 80 GPa, making BeH2 a quasi-two-dimensional hydride. Surprisingly, between 27 and 72 GPa, a miscibility gap opens up in the Be-H system and an equilibrium between pure Be and pure H is observed. That is at odds with the current prevailing view of an increase in solubility of H in metals under pressure and contradicts calculations of stable compounds in the Be-H system. [ABSTRACT FROM AUTHOR]

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites XBiO3 (X=Ba,Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated by using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecularlike orbitals of the A1g symmetry composed of O-2pσ and Bi-6s atomic orbitals of collapsed BiO6 octahedra. The primary importance of oxygen p states is thus revealed, in contrast to a popular picture of a purely ionic Bi3+/Bi5+ charge disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a nonuniform band narrowing. We argue that the formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice, and we expect similar hybridization effects in other perovskites involving formally high oxidation state cations. [ABSTRACT FROM AUTHOR]

A correction to the article "Disproportionation and Metallization at Low-Spin to High-Spin Transition in Multiorbital Mott Systems" in the January 2, 2013 issue is presented.

In this article, the authors reflect on the magnetite Verwey transitions for observing charge ordering. They discus the modification of the Verwey transitions with the help of x-ray diffraction and resonant x-ray scattering technique at the iron K edge. They also investigate the charge disproportion between octahedral iron sites and several inconsistencies and errors in the methodology.