Motivated by the colossal negative thermal expansion recently found in BiNiO3, the valence transition accompanied by the charge transfer between the Bi and Ni sites is theoretically studied. We introduce an effective model for Bi-6s and Ni-3d orbitals with taking into account the valence skipping of Bi cations, and investigate the ground-state and finite-temperature phase diagrams within the mean-field approximation. We find that the valence transition is caused by commensurate locking of the electron filling in each orbital associated with charge and magnetic orderings, and the critical temperature and the nature of the transitions are strongly affected by the relative energy between the Bi and Ni levels and the effective electron-electron interaction in the Bi sites. The obtained phase diagram well explains the temperature-and pressure-driven valence transitions in BiNiO3 and the systematic variation of valence states for a series of Bi and Pb perovskite oxides. PACS numbers: 71.10.Fd , 71.30.+h , 75.25.Dk, 75.30.Kz Perovskite transition metal (TM) oxides (general formula: ABO 3) have been providing central issues of phase transitions and strong electron correlations in condensed matter physics [1, 2]. They exhibit a wide range of novel magnetic , dielectric, and transport properties: for example, the large negative magnetoresistance in La 1−x Sr x MnO 3 [3-5], the spin-state transition in La 1−x Sr x CoO 3 [6, 7], the metal-to-insulator transition in RNiO 3 (R: rare earth element) [8], and the ferroelectric to quantum paraelectric transition in Ba 1−x Sr x TiO 3 [9, 10]. In these phenomena, the central players are the electrons in 3d orbitals of the B-site TMs hy-bridized with oxygen 2p orbitals. The A-site cations, on the other hand, are usually inert and have been regarded as "stagehands": they control the electron filling and bandwidth through their valence state and ionic radius, respectively. Peculiar exceptions to the above standards have recently been found in several perovskite TM oxides, in which the A-site cations play an active role as "valence skipper". In these compounds, not only the B-site 3d electrons but also the va-lence s electrons in the A-site cations significantly contribute to the electronic properties. In the valence skippers, the outer-most s orbital prefers closed-shell configurations s 0 or s 2 , and tends to skip the intermediate valence s 1. This is attributed to the effective attractive interaction between s electrons [11-13], and hence the A-site valence state can be actively controlled through electronic degrees of freedom. Owing to the multiple electronic instabilities in both A-and B-site cations, the TM oxides with the A-site valence skipper have a potential of new electronic phases and functions. The colossal negative thermal expansion (CNTE) material BiNiO 3 [14] is one of such candidates; both Bi-6s and Ni-3d electrons are expected to play a key role in the large volume change [15, 16]. At ambient pressure, BiNiO 3 has a unique valence state, where the average valence of ...

We report on an investigation of the lattice dynamical properties in a range of Fe1+yTe1−xSex compounds, with special emphasis on the c-axis polarized vibration of Fe with B1g symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se x and excess Fe y concentrations. In parent compound Fe1+yTe, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature TN . The situation smoothly evolves toward a regular anharmonic behavior as Te is substituted for Se and long range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B1g phonon at low temperatures. We performed Density Functional Theory ab initio calculations within the local density approximation to calculate the phonon frequencies including magnetic polarization and Fe non-stoichiometry in the virtual crystal approximation. We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low energy magnetic fluctuations that can not be treated accurately within these approaches. As recently revealed by neutron scattering and muon spin rotation studies, these phenomena occur in the temperature range where anomalous decay of the B1g phonon is observed and suggests a peculiar coupling of this mode with local moments and spin fluctuations in Fe1+yTe1−xSex.

International audienceWe studied the structural, magnetic, and electronic properties of the geometrically frustrated layered AuCrS2 system by means of x-ray and neutron powder diffraction, specific heat, dc magnetization, and dc electrical resistivity measurements. The room-temperature structural refinement is consistent with a hexagonal centrosymmetric R-3m symmetry and with formal valence states Au+ and Cr3+, where the Cr3+ ions form a regular triangular lattice within the hexagonal planes. On cooling, we observe a first-order structural phase transition to a monoclinic C2/m symmetry concomitant to an antiferromagnetic order at TN = 47 K. The atomic displacements associated with this transition stretch the triangular lattice, thus suppressing the geometric frustration. This accounts for the magnetic order observed and gives evidence of a large magnetoelastic coupling. The refined magnetic structure is commensurate and consists of double ferromagnetic chains along the stretching direction with μ = 2.54 μB/Cr3+; the residual frustration stabilizes an elegant pattern of alternate ferromagnetic and antiferromagnetic intra- and interplane couplings between adjacent chains. The electrical transport of our sintered powder samples is found to be semiconducting-like with ρ300K ∼ 157 cm and an activation energy of 0.38 eV

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PACS indexing codes Abstract:The antiferromagnetic order and structural distortion in the LaFe(As 1-x Sb x )O system have been investigated by powder neutron diffraction and physical properties measurements.Polycrystalline samples of LaFe(As 1-x Sb x )O (x<0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe magnetic moment is observed.

͑Co/ Au͒ and ͑Au/ Co/ Au͒ nanomagnet arrays grown on nanostructured self-organized SiGe templates have been characterized by means of x-ray photoemission electron microscopy, x-ray magnetic circular dichroism, and by extended x-ray absorption spectroscopy using synchrotron radiation. In-plane magnetization is observed at room temperature for practically all Co thicknesses, a stable macroscopic perpendicular magnetic order only at low temperature. The spin reorientation transition in these dot arrays takes place for smaller Co thicknesses over a broader thickness range than in two-dimensional systems. This finding appears to be related with structural relaxation modifications, occurring within the local Co atom environment, which are not necessarily connected with the orbital moment variations. These variations appear in the form of a systematic increase, correlated with the existence of out-of-plane magnetization.

(EDT-TTF-CONH2)6[Re6Se8(CN)6] is a molecular solid with R3 space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic P1 phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH2)6[Re6Se8(CN)6] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties.

The effect of selenium substitution by sulphur on the structural and physical properties of antiferromagnetic TlFe1.6+δSe2 has been investigated via neutron, x-ray and electron diffraction, and transport measurements. The √5a×√5a×c super-cell related to the iron vacancy ordering found in the pure TlFe1.6Se2 selenide is also present in the S-doped TlFe1.6+δ(Se1-xSx)2 compounds. Neutron scattering experiments show the occurrence of the same long range magnetic ordering in the whole series i.e. the 'block checkerboard' antiferromagnetic structure. In particular, this is the first detailed study where the crystal structure and the √5a×√5a antiferromagnetic structure is characterized by neutron powder diffraction for the pure TlFe1.6+δS2 sulphide over a large temperature range. We demonstrate the strong correlation between occupancies of the crystallographic iron sites, the level of iron vacancy ordering and the occurrence of block antiferromagnetism in the sulphur series. Introducing S into the Se sites also increases the Fe content in TlFe1.6+δ(Se1-xSx)2 which in turn leads to the disappearance of the Fe vacancy ordering at x = 0.5 ± 0.15. However, by reducing the nominal Fe content, the same √5a×√5a×c vacancy ordering and antiferromagnetic order can be recovered also in the pure TlFe1.6+δS2 sulphide with a simultaneous reduction in the Néel temperature from 435 K in the selenide TlFe1.75Se2 to 330 K in the sulphide TlFe1.5S2. The magnetic moment remains high at low temperature throughout the full substitution range, which contributes to the absence of superconductivity in these compounds.