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.