Exotic Magnetism in Perovskite KOsO_{3}

A new perovskite KOsO_{3} has been stabilized under high-pressure and high-temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at 230 K as shown from refining synchrotron x-ray powder diffraction (SXRD) data. The larger orbital overlap integral and the extended wave function of 5d electrons in the perovskite KOsO_{3} allow to explore physics from the regime where Mott and Hund's rule couplings dominate to the state where the multiple interactions are on equal footing. We demonstrate an exotic magnetic ordering phase found by neutron powder diffraction along with physical properties via a suite of measurements including magnetic and transport properties, differential scanning calorimetry, and specific heat, which provide comprehensive information for a system at the crossover from localized to itinerant electronic behavior.

Emergent Three-Dimensional Electric Dipole Sinewave in Bulk Perovskite Oxides

The magnetic and electric dipoles of ferroics play a central role in their fascinating properties. In particular, topological configurations have shown promising potential for use in novel electromechanical and electronic devices. Magnetic configurations from simple collinear to complex topological are well-documented. In contrast, many complex topological features in the electric counterpart remain unexplored. Here, we report the first example of three-dimensional electric dipole sinewave topological structure in a PbZrO3-based bulk perovskite, which presents an interesting triple-hysteresis loop macroscopically. This polar configuration consists of two orthogonal sinewave electric dipole modulations decoded from a polar incommensurate phase by advanced diffraction and atomic-resolution imaging techniques. The resulting topology is unraveled to be the competition between the antiferroelectric and ferroelectric states, stabilized by the modulation of the Pb 6s2 lone pair and the antiferrodistortive effect. These findings further reinforce the similarity of the magnetic and electric topologies.

Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 was attributed to the dynamic Jahn-Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed.

Spontaneous periodic polarization wave in helielectric fluids

By analogy with spin waves in ferromagnetic systems, the polarization (or dipole) wave is the electric counterpart that remains elusive. Here, we discover that the helielectricity, i.e. a polarization field with helicoidal helices that corresponds to a quasi-layered chiral nematic environment, causes a spontaneous formation of large-scale polarization waves in the form of the sinusoidal function. Both experimental and theoretical analyses reveal that the polarization ordering over a threshold polarization strength violates the inherent periodicity of the polarization helices, thus penalizing the compression energy. It drives a second-order structural transition to a periodically modulated polarization wave state. The roles of chirality and confinement condition are discussed.

γ-BaFe2O4: a fresh playground for room temperature multiferroicity

Multiferroics, showing the coexistence of two or more ferroic orderings at room temperature, could harness a revolution in multifunctional devices. However, most of the multiferroic compounds known to date are not magnetically and electrically ordered at ambient conditions, so the discovery of new materials is pivotal to allow the development of the field. In this work, we show that BaFe₂O₄ is a previously unrecognized room temperature multiferroic. X-ray and neutron diffraction allowed to reveal the polar crystal structure of the compound as well as its antiferromagnetic behavior, confirmed by bulk magnetometry characterizations. Piezo force microscopy and electrical measurements show the polarization to be switchable by the application of an external field, while symmetry analysis and calculations based on density functional theory reveal the improper nature of the ferroelectric component. Considering the present findings, we propose BaFe₂O₄ as a Bi- and Pb-free model for the search of new advanced multiferroic materials.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12

Perovskite-type ABO₃ oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm₂CuMn(MnTi₃)O₁₂, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P4₂/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with T_SG = 7 K, while specific heat, in the form of C_p/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm₂CuMn(MnTi₃)O₁₂ was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu²⁺ doping drastically modified the magnetic and dielectric properties of Sm₂CuMn(MnTi₃)O₁₂ in comparison with the parent compound Sm₂MnMn(MnTi₃)O₁₂, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu²⁺ and Mn²⁺ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm₂CuMn(MnTi₃)O₁₂.

Switching Crystallographic Chirality in Ba(TiO)Cu4(PO4)4 by Laser Irradiation

Switching crystallographic chirality is nontrivial because there is no simple conjugate field to chirality. Here we demonstrate chirality switching in an inorganic crystalline material by manipulating the boundaries of chiral domains with laser irradiation. Our study material is Ba(TiO)Cu₄(PO₄)₄, exhibiting a chiral structure at room temperature and a chiral-achiral phase transition at 710 °C. By irradiation of a laser beam with a wavelength at which Ba(TiO)Cu₄(PO₄)₄ exhibits strong optical absorption, local heating is induced. This leads to reconstructions of chiral domain boundaries, revealed by optical rotation measurements. In the reconstruction process, energetically unstable domain boundaries tend to be minimized, affecting resultant domain patterns. On the basis of this feature, we successfully manipulate chiral domain patterns by scanning the laser beam on the sample surface. Our findings provide a unique approach to controlling chirality in inorganic crystalline materials.

Discovery of electric devil's staircase in perovskite antiferroelectric

The devil's staircase, describing step-like function for two competing frequencies, is well known over a wide range of dynamic systems including Huyghens' clocks, Josephson junction, and chemical reaction. In condensed matter physics, the devil's staircase has been observed in spatially modulated structures, such as magnetic ordering. It draws widespread attentions because it plays a crucial role in the fascinating phenomena including phase-locking behaviors, commensurate-incommensurate phase transition, and spin-valve effect. Here, we report the observation of polymorphic phase transitions consisting of several steps in PbZrO₃-based system-namely, electric devil's staircase-originated from competing ferroelectric and antiferroelectric interactions. We fully characterize a specific electric dipole configuration by decomposing this competitive interaction in terms of basic structure and modulation function. Of particular interest is that the occurrence of many degenerate electric dipole configurations in devil's staircase enables superior energy storage performance. These observations are of great significance for exploring more substantive magnetic-electric correspondence and engineering practical high-power antiferroelectric capacitors.

Interplay of domain structure and phase transitions: theory, experiment and functionality

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices.

The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12

Perovskite-structure AMnO₃ manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron-phonon coupling, and Jahn-Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn₇O₁₂ were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn₇O₁₂ compounds host rich physics beyond the canonical AMnO₃ materials.

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties

In contrast to the corundum-type A₂X₃ structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX₃ ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe²⁺Ti⁴⁺O₃ ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn₂O₃) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn²⁺Mn⁴⁺O₃ (ε-Mn₂O₃) is an n-type semiconductor with an indirect narrow band gap of E_g = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn₂O₃ and previously discovered quadruple perovskite ζ-Mn₂O₃ phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn₂O₃ below 210 K. The synthesis of ε-Mn₂O₃ indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A₂O₃, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO₃-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

We synthesized single crystals of marokite (CaMn₂O₄)-type orthorhombic manganese (II,III) oxide, γ-Mn₃O₄, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn₃O₄ is a semiconductor with an indirect band gap E_g of 0.96 eV and two antiferromagnetic transitions (T_N) at ∼200 and ∼55 K. The phase stability of the γ-Mn₃O₄ crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn₃O₄ was determined to be B₀ = 235.3(2) GPa with B' = 2.6(6). The γ-Mn₃O₄ phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO₆ polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn³⁺ ions in the octahedra and the Mn²⁺ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

A New Class of Chiral Nematic Phase with Helical Polar Order

A novel chiral nematic phase with a polar helical order is realized via the introduction of helical twisting power into a polar nematogen. The properties of the induced polar nematic (polar cholesteric: Np*) phase differ from those of the conventional cholesteric (N*) phases existing thus far. Np*, which is a new class of N* structures, is characterized not only by its helically twisted nematic director, but also by a continuously twisted polarization. Transmission spectroscopy and helical pitch measurements in a wedge cell revealed that the half-helical pitch in the Np* phase vanished because of the polar response in the Np* helix. The inner polar director in the Np* phase is confirmed in dielectric and second-harmonic-generation studies. Furthermore, this unique Np*LC, which corresponds to a half-/full-pitch helix, enables ultrafast electro-optic switching (τ < 20 µs), and proposes new potential applications for electrically interchangeable photonic bandgaps.

Charge order textures induced by non-linear couplings in a half-doped manganite

The self-organization of strongly interacting electrons into superlattice structures underlies the properties of many quantum materials. How these electrons arrange within the superlattice dictates what symmetries are broken and what ground states are stabilized. Here we show that cryogenic scanning transmission electron microscopy (cryo-STEM) enables direct mapping of local symmetries and order at the intra-unit-cell level in the model charge-ordered system Nd_1/2Sr_1/2MnO₃. In addition to imaging the prototypical site-centered charge order, we discover the nanoscale coexistence of an exotic intermediate state which mixes site and bond order and breaks inversion symmetry. We further show that nonlinear coupling of distinct lattice modes controls the selection between competing ground states. The results demonstrate the importance of lattice coupling for understanding and manipulating the character of electronic self-organization and that cryo-STEM can reveal local order in strongly correlated systems at the atomic scale.

In this paper, the authors demonstrate that cryogenic scanning transmission electron microscopy allows for the direct mapping of the local arrangements and symmetries of electronic order, providing a useful method for studying strongly correlated systems. They show this using the example of Nd1/2Sr1/2MnO3, a model charge ordered material.

Observation of Ferrochiral Transition Induced by an Antiferroaxial Ordering of Antipolar Structural Units in Ba(TiO)Cu4(PO4)4

Ferrochiral transition, i.e., a transition involving an emergence of chirality, provides an unique opportunity to achieve a nonvolatile reversible control of chirality with external fields. However, materials showing pure ferrochiral transitions, which are accompanied by no other types of ferroic transition, are exceedingly rare. In this study, we propose that a pure ferrochiral transition is achieved by a combination of antipolar and antiferroaxial orderings of structural units, and substantiate this proposal through a study of the chiral compound Ba(TiO)Cu₄(PO₄)₄. Single crystal X-ray diffraction measurements have revealed that this material undergoes a second order ferrochiral transition whose order parameter is described by an antiferroaxial (staggered) rotation of antipolar structural units, thus demonstrating our proposal. Furthermore, by measuring spatial distributions of optical rotation, we successfully visualized a temperature evolution of ferrochiral domains across the transition temperature and demonstrated the relationship between chirality and optical rotation. This work provides a guide to find a pure ferrochiral transition, thus providing an opportunity to achieve a ferroic control of chirality.

Controlling multiple orderings in metal thiocyanate molecular perovskites A x {Ni[Bi(SCN)6]}

We report four new A-site vacancy ordered thiocyanate double double perovskites, , A = K⁺, NH₄⁺, CH₃(NH₃)⁺ (MeNH₃⁺) and C(NH₂)₃⁺ (Gua⁺), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders suggest a new route to non-centrosymmetric perovskites, and mean this family of materials could contain excellent candidates for piezo- and ferroelectric applications.

Metal thiocyanate hybrid perovskites can have multiple simultaneous complex orderings and combining these orderings appropriately can produce non-centrosymmetric structures.

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.