A New Cation-Ordered Structure Type with Multiple Thermal Redistributions in Co₂InSbO₆

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO(3)) and LiNbO(3) type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO(3) derivative with R32 symmetry has never been observed. Here we show that Co(2)InSbO(6) recovered from high pressure has a new, ordered‐R32 A(2)BCO(6) variant of the corundum structure. Co(2)InSbO(6) is also remarkable for showing two cation redistributions, to (Co(0.5)In(0.5))(2)CoSbO(6) and then Co(2)InSbO(6) variants of the ordered‐LiNbO(3) A(2)BCO(6) structure on heating. The cation distributions change magnetic properties as the final ordered‐LiNbO(3) product has a sharp ferrimagnetic transition unlike the initial ordered‐R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation‐redistribution pathways, and may enable ABO(3) materials with the R32 structure to be discovered

Orthogonal antiferromagnetism to canted ferromagnetism in CaCo₃Ti₄O₁₂ quadruple perovskite driven by underlying kagome lattices

AA′₃B₄O₁₂ quadruple perovskites, with magnetic A′ and non-magnetic B cations, are characterized by a wide range of complex magnetic structures. These are due to a variety of competing spin-exchange interactions up to the fourth nearest neighbours. Here, we synthesize and characterize the magnetic behaviour of the CaCo₃Ti₄O₁₂ quadruple perovskite. We find that in the absence of an external magnetic field, the system undergoes antiferromagnetic ordering at 9.3 K. This magnetic structure consists of three interpenetrating mutually orthogonal magnetic sublattices. Under an applied magnetic field, this antiferromagnetic structure evolves into a canted ferromagnetic structure. In explaining these magnetic structures, as well as the seemingly unrelated magnetic structures found in other quadruple perovskites, we suggest a crucial role played by the underlying kagome lattices in these systems. All observed magnetic structures of these materials represent indeed one of the three possible ways to reduce spin frustration in the A′ site kagome layers. More specifically, our survey of the magnetic structures observed for quadruple perovskites AA′₃B₄O₁₂ reveals the following three ways to reduce spin frustration, namely to make each layer ferromagnetic, to adopt a compromise 120° spin arrangement in each layer, or to have a magnetic structure with a vanishing sum of all second nearest-neighbour spin exchanges

Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

Magnetite (Fe3O4) is of fundamental importance as the original magnetic material and also for the Verwey transition near T_V = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by strain or chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to an intriguing variation of the Verwey transition that elucidates the doping effects. Exposure to various fixed oxygen pressures at ambient temperature leads to an initial drop to TV minima as low as 70 K after 45-75 days, followed by recovery to a constant value of 95 K after 160 days that persists in all experiments for aging times up to 1070 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution and demonstrates that the persistent 95 K value corresponds to the lower limit for homogenously doped magnetite and hence for the first order regime. In comparison, further suppression down to 70 K results from inhomogeneous strains that characterize the second-order region. This work demonstrates that slow reactions of nanoparticles can give exquisite control and separation of homogenous and inhomogeneous doping or strain effects on an nm scale and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.Comment: 24 pages, 13 figures, 2 tables, the manuscript is accepted for publishing at Nature Communication

Multi- k spin ordering in CaFe₃Ti₄O₁₂ stabilized by spin-orbit coupling and further-neighbor exchange

Orthogonal spin ordering is rarely observed in magnetic oxides because nearest-neighbor symmetric Heisenberg superexchange interactions usually dominate. We have discovered that in the quadruple perovskite CaFe₃Ti₄O₁₂, where only the S = 2 Fe²⁺ ion is magnetic, long-range magnetic order consisting of an unusual arrangement of three interpenetrating orthogonal sublattices is stabilized. Each magnetic sublattice corresponds to a set of FeO₄ square planes sharing a common orientation. This multi-k magnetic spin ordering is the result of fourth-neighbor spin couplings with a strong easy-axis anisotropy. In an applied magnetic field, each sublattice tends towards ferromagnetic alignment, but remains polarized by internal magnetic fields generated by the others, thus stabilizing in a noncollinear canted ferromagnetic structure. CaFe₃Ti₄O₁₂ provides a rare example of how nontrivial long-range spin order can arise when near-neighbor Heisenberg superexchange is quenched

Multiferroism Induced by Spontaneous Structural Ordering in Antiferromagnetic Iron Perovskites

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE1.2Ba1.2Ca0.6Fe3O8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO4-tetrahedra (T) and FeO6-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/cm2 for the Gd-oxide (23.2 μC/cm2 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

3D to 2D Magnetic Ordering of Fe3+ Oxides Induced by Their Layered Perovskite Structure

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, and REBa2Ca2Fe5O13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Neel temperature ( ́ TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers

Coupled Electronic and Magnetic Phase Transition in the Infinite-Layer Phase LaSrNiRuO₄

Topochemical reduction of the ordered double perovskite LaSrNiRuO₆ with CaH₂ yields LaSrNiRuO₄, an extended oxide phase containing infinite sheets of apex-linked, square-planar Ni¹⁺O₄ and Ru²⁺O₄ units ordered in a checkerboard arrangement. At room temperature the localized Ni¹⁺ (d⁹, <i>S</i> = ¹/₂) and Ru²⁺ (d⁶, <i>S</i> = 1) centers behave paramagnetically. However, on cooling below 250 K the system undergoes a cooperative phase transition in which the nickel spins align ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. Features of the low-temperature crystal structure suggest a symmetry lowering Jahn–Teller distortion could be responsible for the observed diamagnetism of the ruthenium centers

Cation Exchange in a 3D PerovskiteSynthesis of Ni_0.5TaO₃

Reaction of NiCl₂ with NaTaO₃ leads the formation of the perovskite phase Ni_0.5TaO₃, via a topochemical nickel-for-sodium cation exchange in which the framework of apex-linked TaO₆ octahedra present in the parent phase is retained. Neutron powder diffraction data indicate Ni_0.5TaO₃ adopts a structure analogous to the paraelectric phase of LiTaO₃, with triclinic <i>P</i>1̅ crystallographic symmetry. Although Ni_0.5TaO₃ has features which make it a good candidate phase for magnetoelectric multiferroic behavior, the phase remains paramagnetic in the temperature range 15 < <i>T</i> (K) < 300, and detailed crystallographic characterization and analysis of SHG activity indicate it retains a centrosymmetric structure down to the lowest temperatures measured (5 K). Topochemical cation exchange reactions of 3D perovskite oxides offer the opportunity to prepare a wide range of novel metastable phases in a rational manner with a high degree of synthetic control