Pressure-Induced Intermetallic Valence Transition in BiNiO₃

Abstract

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 ...

Theory of Valence Transition inBiNiO3

“…This is attributed to the effective attractive interaction between s electrons [11][12][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 Ni3d electrons are expected to play a key role in the large volume change [15,16] [15,18,19]. Under pressure, this phase transition is also observed by raising temperature (T ), which is termed as the CNTE.…”

“…The colossal negative thermal expansion (CNTE) material BiNiO 3 [14] is one of such candidates; both Bi-6s and Ni3d electrons are expected to play a key role in the large volume change [15,16] [15,18,19]. Under pressure, this phase transition is also observed by raising temperature (T ), which is termed as the CNTE.…”

Orthorhombic Phase of La0.5Bi0.5NiO3 Studied by First Principles

“…1. According to the sketch, La ions may occupy the following Bi positions: (1,3,6,8), (2,4,6,8), (4,5,6,8), (1,2,3,4), (3,5,6,8) or (2,5,6,8). Moreover, four types of magnetic ordering at Ni cations sites are considered for the studied orthorombic phase: an ferromagnetic (FM) one and three antiferromagnetic (A-, C-, and G-AFM) ones.…”

“…multiferroicity (BiFeO 3 [1,2]), charge ordering (BiNiO 3 [3,4]), or as a lead-free ferro-and piezoelectric materials (BiAlO 3 [5]) to name the few. Variation of valence states can lead to notable phenomena such as superconductivity, magnetoresistance, negative thermal expansion in transition metal oxides.…”

“…Variation of valence states can lead to notable phenomena such as superconductivity, magnetoresistance, negative thermal expansion in transition metal oxides. Much efforts has been focused on BiNiO 3 proved to undergo a phase transition accompanied by the charge redistribution between Bi and Ni ions [3,4,6,7]. In ambient conditions, BiNiO 3 in triclinic structure (P-1) displays the antiferromagnetic insulating ground state (G-type AFM, T N =300 K) [8], as well as exhibits an unusual charge distribution of Bi .…”