3 research outputs found
Charge Transfer Induced Multifunctional Transitions with Sensitive Pressure Manipulation in a MetalāOrganic Framework
The
metalāorganic framework {[FeĀ(2,2ā²-bipyridine)Ā(CN)<sub>4</sub>]<sub>2</sub>CoĀ(4,4ā²-bipyridine)}Ā·4H<sub>2</sub>O (Fe<sub>2</sub>Co-MOF) with single-chain magnetism undergoes an
intermetallic charge transfer that converts the Fe<sub>2</sub>Co charge/spin
configurations from Fe<sup>3+</sup><sub>LS</sub>āCo<sup>2+</sup><sub>HS</sub>āFe<sup>3+</sup><sub>LS</sub> to Fe<sup>2+</sup><sub>LS</sub>āCo<sup>3+</sup><sub>LS</sub>āFe<sup>3+</sup><sub>LS</sub> (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 Fe<sub>2</sub>Co-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
Pressure Induced Amorphization of Pb<sup>2+</sup> and Pb<sup>4+</sup> in Perovskite PbFeO<sub>3</sub>
Perovskite-type
oxides have been the subject of intense
research
due to their various fascinating physical properties stemming from
their charge degree of freedom. PbFeO3 has an unusual Pb2+0.5Pb4+0.5Fe3+O3 charge distribution with a long-ranged ordering of
Pb2+ and Pb4+ and two inequivalent Fe3+ sites in a perovskite structure. Combined synchrotron X-ray diffraction
and MoĢssbauer spectroscopy revealed a change to an orthorhombic
GdFeO3 structure with a unique Fe3+ site and
randomly distributed Pb2+ and Pb4+ at 29.0 GPa,
namely, pressure-induced amorphization of Pb2+ and Pb4+. The absence of a charge transfer transition to the Pb2+Fe4+O3 phase, which was expected from
the comparison with PbCrO3 and PbCoO3, was verified
using ab initio density functional theory calculations in the range
of 0ā70 GPa
AāSite and BāSite Charge Orderings in an <i>sād</i> Level Controlled Perovskite Oxide PbCoO<sub>3</sub>
Perovskite PbCoO<sub>3</sub> synthesized
at 12 GPa was found to have an unusual charge distribution of Pb<sup>2+</sup>Pb<sup>4+</sup><sub>3</sub>Co<sup>2+</sup><sub>2</sub>Co<sup>3+</sup><sub>2</sub>O<sub>12</sub> with charge orderings in both
the A and B sites of perovskite ABO<sub>3</sub>. 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 Pb<sup>2+</sup>Pb<sup>4+</sup><sub>3</sub>Co<sup>2+</sup><sub>2</sub>Co<sup>3+</sup><sub>2</sub>O<sub>12</sub> quadruple perovskite structure. It is shown
that the average valence distribution of Pb<sup>3.5+</sup>Co<sup>2.5+</sup>O<sub>3</sub> between Pb<sup>3+</sup>Cr<sup>3+</sup>O<sub>3</sub> and Pb<sup>4+</sup>Ni<sup>2+</sup>O<sub>3</sub> can be stabilized
by tuning the energy levels of Pb 6<i>s</i> and transition
metal 3<i>d</i> orbitals