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.

Understanding complex multiple sublattice magnetism in double double perovskites

Understanding magnetism in multiple magnetic sublattice system, driven by the interplay of varied nature of magnetic exchanges, is on one hand challenging and on other hand intriguing. Motivated by the recent synthesis of AA[Formula: see text]BB[Formula: see text]O[Formula: see text] double double perovskites with multiple magnetic ions both at A- and B-sites, we investigate the mechanism of magnetic behavior in these interesting class of compounds. We find that the magnetism in such multiple sublattice compounds is governed by the interplay and delicate balance between two distinct mechanisms, (a) kinetic energy-driven multiple sublattice double exchange mechanism and (b) the conventional super-exchange mechanism. The derived spin Hamiltonian based on first-principles calculations is solved by classical Monte Carlo technique which reproduces the observed magnetic properties. Finally, the influence of off-stoichiometry, as in experimental samples, is discussed. Some of these double double perovskite compounds are found to possess large total magnetic moment and also are found to be half-metallic with reasonably high transition temperature, which raises the hope of future applications of these large magnetic moment half-metallic oxides in spintronics and memory devices.

Complex magnetism in Ni3TeO6-type Co3TeO6 and high-pressure polymorphs of Mn3-xCoxTeO6 solid solutions

New Ni₃TeO₆-type (NTO) and double perovskite (DPv) polymorphs of Co₃TeO₆ are synthesised at pressures of 15 GPa. A complex elliptic helical magnetic order is observed in the NTO polymorph (T_N1 = 58 K) that reorientates (42 K) and further splits (T_N2 = 23.5 K) creating a coexisting helix. Increasing Co content within the Mn_3-xCo_xTeO₆ system changes the dominant DPv phase to NTO structural type and drastically modifies the magnetic behaviour. DPv Co₃TeO₆ is the first A-site double cobaltite.

Unconventional magnetism in the high pressure ‘all transition metal’ double perovskite Mn2NiReO6

Mn₂NiReO₆ has an unusual rotation of Mn spins from 80 down to 42 K where a collapse in weak ferromagnetism evidences switching of the canted components.

High pressure: a feasible tool for the synthesis of unprecedented inorganic compounds

After a simple classification of inorganic materials synthesized at high-temperature and high-pressure, this tutorial reviews the important research results in the field of high-temperature and high-pressure inorganic synthesis in the past 5 years.

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.

Magnetic frustration in the high-pressure Mn2MnTeO6 (Mn3TeO6-II) double perovskite

Magnetic frustration and bandgap engineering in the high pressure Mn₂MnTeO₆ perovskite.

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.