Charge Transfer Induced Multifunctional Transitions with Sensitive Pressure Manipulation in a Metal–Organic Framework

The metal–organic framework {[Fe­(2,2′-bipyridine)­(CN)₄]₂Co­(4,4′-bipyridine)}·4H₂O (Fe₂Co-MOF) with single-chain magnetism undergoes an intermetallic charge transfer that converts the Fe₂Co charge/spin configurations from Fe³⁺_LS–Co²⁺_HS–Fe³⁺_LS to Fe²⁺_LS–Co³⁺_LS–Fe³⁺_LS (LS = low spin, HS = high spin) around 220 K under ambient pressure. A series of coherent phase transitions in structure, magnetism, permittivity and ferroelectricity are found to take place accompanying with the charge transfer, making Fe₂Co-MOF a unique ferroelectric single-chain magnet at low temperature. Moreover, our detailed measurements of magnetization, dielectric constant, and Raman scattering under high pressures illustrate that the charge transfer as well as the resulting multifunctional transitions can be readily induced to occur at room temperature by applying a tiny external pressure of about 0.5 kbar. The present study thus provides a pressure well-controllable multifunctional material with potential applications in a broad temperature region across room temperature

A‑Site and B‑Site Charge Orderings in an <i>s–d</i> Level Controlled Perovskite Oxide PbCoO₃

Perovskite PbCoO₃ synthesized at 12 GPa was found to have an unusual charge distribution of Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with charge orderings in both the A and B sites of perovskite ABO₃. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ quadruple perovskite structure. It is shown that the average valence distribution of Pb^3.5+Co^2.5+O₃ between Pb³⁺Cr³⁺O₃ and Pb⁴⁺Ni²⁺O₃ can be stabilized by tuning the energy levels of Pb 6<i>s</i> and transition metal 3<i>d</i> orbitals