Mn2FeWO6 : A new Ni3TeO6-type polar and magnetic oxide

A New Cation‐Ordered Structure Type with Multiple Thermal Redistributions in Co 2 InSbO 6

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO₃) and LiNbO₃ type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO₃ derivative with R32 symmetry has never been observed. Here we show that Co₂InSbO₆ recovered from high pressure has a new, ordered‐R32 A₂BCO₆ variant of the corundum structure. Co₂InSbO₆ is also remarkable for showing two cation redistributions, to (Co_0.5In_0.5)₂CoSbO₆ and then Co₂InSbO₆ variants of the ordered‐LiNbO₃ A₂BCO₆ structure on heating. The cation distributions change magnetic properties as the final ordered‐LiNbO₃ 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 ABO₃ materials with the R32 structure to be discovered.

Iridate Li8IrO6: An Antiferromagnetic Insulator

A polycrystalline iridate Li₈IrO₆ material was prepared via heating Li₂O and IrO₂ starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li₈IrO₆ adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state ⁷Li NMR. This results in a crystal structure consisting of LiO₄ tetrahedral layers alternating with mixed IrO₆ and LiO₆ octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir⁴⁺ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li₈IrO₆ indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (T_N) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment μ_eff (Ir) = 1.73 μ_B and insulating character indicate that Li₈IrO₆ is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

New polymorphism for BaTi(IO3)6 with two polymorphs crystallizing in the same space group

Polymorphic α- and β-BaTi(IO₃)₆ have been synthesized. They crystallize in the same space group and exhibit highly similar structures. But the different powder X-ray patterns and crystal morphologies indicate that they belong to different phases, which represents the first phase transitions with two polymorphs possessing the same space group and similar cells.

Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling

Understanding the electric dipole switching in multiferroic materials requires deep insight of the atomic-scale local structure evolution to reveal the ferroelectric mechanism, which remains unclear and lacks a solid experimental indicator in high-pressure prepared LiNbO₃-type polar magnets. Here, we report the discovery of Zn-ion splitting in LiNbO₃-type Zn₂FeNbO₆ established by multiple diffraction techniques. The coexistence of a high-temperature paraelectric-like phase in the polar Zn₂FeNbO₆ lattice motivated us to revisit other high-pressure prepared LiNbO₃-type A₂BB'O₆ compounds. The A-site atomic splitting (∼1.0-1.2 Å between the split-atom pair) in B/B'-mixed Zn₂FeTaO₆ and O/N-mixed ZnTaO₂N is verified by both powder X-ray diffraction structural refinements and high angle annular dark field scanning transmission electron microscopy images, but is absent in single-B-site ZnSnO₃. Theoretical calculations are in good agreement with experimental results and suggest that this kind of A-site splitting also exists in the B-site mixed Mn-analogues, Mn₂FeMO₆ (M = Nb, Ta) and anion-mixed MnTaO₂N, where the smaller A-site splitting (∼0.2 Å atomic displacement) is attributed to magnetic interactions and bonding between A and B cations. These findings reveal universal A-site splitting in LiNbO₃-type structures with mixed multivalent B/B', or anionic sites, and the splitting-atomic displacement can be strongly suppressed by magnetic interactions and/or hybridization of valence bands between d electrons of the A- and B-site cations.

High-Pressure, High-Temperature Synthesis and Characterization of Polar and Magnetic LuCrWO6

A new polar and magnetic oxide, LuCrWO₆, was synthesized under high pressure (6 GPa) and high temperature (1673 K). LuCrWO₆ is isostructural with the previously reported polar YCrWO₆ (SG: Pna2₁, no. 33). The ordering of CrO₆ and WO₆ octahedra in the edge-shared dimers induce the polar structure. The effective size of rare earth, Ln cation does not seem to affect the symmetry of LnCrWO₆. Second harmonic generation measurements of LuCrWO₆ confirmed the noncentrosymmetric character and strong piezoelectric domains are observed from piezoresponse force microscopy at room temperature. LuCrWO₆ exhibits antiferromagnetic behavior, T_N, of ∼18 K with a Weiss temperature of -30.7 K.

Data-driven computational prediction and experimental realization of exotic perovskite-related polar magnets

Rational design of technologically important exotic perovskites is hampered by the insufficient geometrical descriptors and costly and extremely high-pressure synthesis, while the big-data driven compositional identification and precise prediction entangles full understanding of the possible polymorphs and complicated multidimensional calculations of the chemical and thermodynamic parameter space. Here we present a rapid systematic data-mining-driven approach to design exotic perovskites in a high-throughput and discovery speed of the A 2 BB'O6 family as exemplified in A 3TeO6. The magnetoelectric polar magnet Co3TeO6, which is theoretically recognized and experimentally realized at 5 GPa from the six possible polymorphs, undergoes two magnetic transitions at 24 and 58 K and exhibits helical spin structure accompanied by magnetoelastic and magnetoelectric coupling. We expect the applied approach will accelerate the systematic and rapid discovery of new exotic perovskites in a high-throughput manner and can be extended to arbitrary applications in other families.

High-Pressure Synthesis and Ferrimagnetism of Ni3TeO6-Type Mn2ScMO6 (M = Nb, Ta)

The corundum-related oxides Mn₂ScNbO₆ and Mn₂ScTaO₆ were synthesized at high pressure and high temperature (6 GPa and 1475 K). Analysis of the synchrotron powder X-ray diffraction shows that Mn₂ScNbO₆ and Mn₂ScTaO₆ crystallize in Ni₃TeO₆-type noncentrosymmetric crystal structures with space group R3. The asymmetric crystal structure was confirmed by second harmonic generation measurement. X-ray absorption near-edge spectroscopies indicate formal valence states of Mn²⁺₂Sc³⁺Nb⁵⁺O₆ and Mn²⁺₂Sc³⁺Ta⁵⁺O₆, also supported by the calculated bond valence sums. Both samples are electrically insulating. Magnetic measurements indicate that Mn₂ScNbO₆ and Mn₂ScTaO₆ order ferrimagnetically at 53 and 50 K, respectively, and Mn₂ScTaO₆ is found to have a field-induced magnetic transition.

Reversible Structural Transformation between Polar Polymorphs of Li2GeTeO6

Li₂GeTeO₆ prepared at ambient pressure adopts the corundum derivative ordered ilmenite structure (rhombohedral R3). When heated at 1073 K and 3-5 GPa, the as-made Li₂GeTeO₆ can convert into a LiSbO₃-derived Li₂TiTeO₆-type phase (orthorhombic Pnn2), which is the third LiSbO₃-derived double A₂BB'O₆ phase in addition to Li₂TiTeO₆ and Li₂SnTeO₆. This Pnn2 Li₂GeTeO₆ phase spontaneously reverts to the R3 phase if annealed up to 1023 K at ambient pressure. Although the crystal structural analyses and second harmonic generation measurements clearly demonstrate the polar nature of both the R3 and Pnn2 phases, P( E) and dielectric measurements do not show any convincing ferroelectric response. Given the large estimated spontaneous polarization (17 and 80 μC/cm²), the absence of ferroelectric behavior could be attributed to the random domain distribution and leakage due to Li-ion migration.

Magnetostriction-polarization coupling in multiferroic Mn2MnWO6

Double corundum-related polar magnets are promising materials for multiferroic and magnetoelectric applications in spintronics. However, their design and synthesis is a challenge, and magnetoelectric coupling has only been observed in Ni₃TeO₆ among the known double corundum compounds to date. Here we address the high-pressure synthesis of a new polar and antiferromagnetic corundum derivative Mn₂MnWO₆, which adopts the Ni₃TeO₆-type structure with low temperature first-order field-induced metamagnetic phase transitions (T _N = 58 K) and high spontaneous polarization (~ 63.3 μC·cm^-2). The magnetostriction-polarization coupling in Mn₂MnWO₆ is evidenced by second harmonic generation effect, and corroborated by magnetic-field-dependent pyroresponse behavior, which together with the magnetic-field-dependent polarization and dielectric measurements, qualitatively indicate magnetoelectric coupling. Piezoresponse force microscopy imaging and spectroscopy studies on Mn₂MnWO₆ show switchable polarization, which motivates further exploration on magnetoelectric effect in single crystal/thin film specimens.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Comparative studies on the room-temperature ferrielectric and ferrimagnetic Ni3TeO6-type A2FeMoO6 compounds (A = Sc, Lu)

First-principles calculations have been carried out to study the structural, electric, and magnetic properties of Ni₃TeO₆-type A₂FeMoO₆ compounds (A = Sc, Lu). Their electric and magnetic properties behave like room-temperature ferrielectric and ferrimagnetic insulators where polarization comes from the un-cancelled antiparallel dipoles of (A(1), Fe³⁺) and (A(2), Mo³⁺) ion groups, and magnetization from un-cancelled antiparallel moments of Fe³⁺ and Mo³⁺ ions. The net polarization increases with A’s ionic radius and is 7.1 and 8.7 μCcm⁻² for Sc₂FeMoO₆ and Lu₂FeMoO₆, respectively. The net magnetic moment is 2 μ_B per formula unit. The magnetic transition temperature is estimated well above room-temperature due to the strong antiferromagnetic superexchange coupling among Fe³⁺ and Mo³⁺ spins. The estimated paraelectric to ferrielectric transition temperature is also well above room-temperature. Moreover, strong magnetoelectric coupling is also anticipated because the magnetic ions are involved both in polarization and magnetization. The fully relaxed Ni₃TeO₆-type A₂FeMoO₆ structures are free from soft-phonon modes and correspond to stable structures. As a result, Ni₃TeO₆-type A₂FeMoO₆ compounds are possible candidates for room-temperature multiferroics with large magnetization and polarization.

High pressure synthesis of polar and non-polar cation-ordered polymorphs of Mn2ScSbO6

Two new Mn₂ScSbO₆ polymorphs are presented: an antiferromagnetic double perovskite (DPv) and a multiferroic Ni₃TeO₆-type (NTO) induced by site-selective disorder.