Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO 4 -tetrahedra (T) and FeO 6 -octahedra (O) results in a polar crystal structure. The layered structure consists of the stacking sequence of RE/Ca-RE/Ca-Ba-RE/Ca layers in combination with the TOOT sequence in a unit cell. A polar moment of 33.0 μC/cm 2 for the Gd-oxide (23.2 μC/cm 2 for the Tb one) is determined from the displacements of the cations, mainly Fe, and oxygen atoms along the b-axis. These oxides present antiferromagnetic ordering doubling the c-axis, and the magnetic structure in the Tb-compound remains up to 690 K, which is one of the highest transition temperatures reported in Fe perovskites.
All fluorochemicals—including elemental fluorine and nucleophilic, electrophilic, and radical fluorinating reagents—are prepared from hydrogen fluoride (HF). This highly toxic and corrosive gas is produced by the reaction of acid-grade fluorspar (>97% CaF
2
) with sulfuric acid under harsh conditions. The use of fluorspar to produce fluorochemicals via a process that bypasses HF is highly desirable but remains an unsolved problem because of the prohibitive insolubility of CaF
2
. Inspired by calcium phosphate biomineralization, we herein disclose a protocol of treating acid-grade fluorspar with dipotassium hydrogen phosphate (K
2
HPO
4
) under mechanochemical conditions. The process affords a solid composed of crystalline K
3
(HPO
4
)F and K
2−
x
Ca
y
(PO
3
F)
a
(PO
4
)
b
, which is found suitable for forging sulfur-fluorine and carbon-fluorine bonds.
Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr 2 YCu 2 O 7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions (magnetic ordering temperature T N = 110 K > superconducting critical temperature T c = 70 K). The somewhat unexpected A-type AFM structure, with a μ(Fe) ∼ 2 μ B magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.
Layered perovskites
of the Gd0.8–x
Ba0.8Ca0.4+x
Fe2O5+δ system
show oxygen reduction reaction (ORR)
activity. The layered crystal structure of these oxides is established
by the interplay of the Gd3+, Ba2+, and Ca2+ locations with the ordering of the coordination polyhedra
of the Fe3+ cations. Substitution of Gd3+ by
Ca2+ increases the oxygen deficiency that is accommodated
by the formation of layers of FeO5-squared pyramids intercalated
with A–O layers containing mainly Gd3+. The presence
of FeO5-squared pyramids in the crystal structure promotes
the oxygen diffusion and then the ORR activity. Therefore, GdBa2Ca2Fe5O13 is the oxide of
the system which presents lower area specific resistance (ASR) values
when it is applied as an electrode in symmetrical cells using Ce0.9Gd0.1O2−δ as an electrolyte.
Excellent performance as a cathode material for IT-SOFCs is found in (GdBa)0.8Ca0.4Co0.6Fe1.4O6−δ related to a complex crystal superstructure based on ordering of Gd, Ba and Ca in combination with different coordination polyhedral of Co and Fe and layered-location of anion vacancies.
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