Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4 O12.

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi4 O12 . This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below TN ≈12 K, Mn2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice.

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.

Pressure-induced Jahn-Teller switch in the homoleptic hybrid perovskite [(CH3)2NH2]Cu(HCOO)3: orbital reordering by unconventional degrees of freedom.

Through in situ, high-pressure X-ray diffraction experiments we have shown that the homoleptic perovskite-like coordination polymer [(CH3)2NH2]Cu(HCOO)3 undergoes a pressure-induced orbital reordering phase transition above 5.20 GPa. This transition is distinct from previously reported Jahn-Teller switching in coordination polymers, which required at least two different ligands that crystallize in a reverse spectrochemical series. We show that the orbital reordering phase transition in [(CH3)2NH2]Cu(HCOO)3 is instead primarily driven by unconventional octahedral tilts and shifts in the framework, and/or a reconfiguration of A-site cation ordering. These structural instabilities are unique to the coordination polymer perovskites, and may form the basis for undiscovered orbital reorientation phenomena in this broad family of materials.

Controlling Magnetic Anisotropy in a Zero-Dimensional S = 1 Magnet Using Isotropic Cation Substitution.

The [Zn1-xNix(HF2)(pyz)2]SbF6 (x = 0.2; pyz = pyrazine) solid solution exhibits a zero-field splitting (D) that is 22% larger [D = 16.2(2) K (11.3(2) cm-1)] than that observed in the x = 1 material [D = 13.3(1) K (9.2(1) cm-1)]. The substantial change in D is accomplished by an anisotropic lattice expansion in the MN4 (M = Zn or Ni) plane, wherein the increased concentration of isotropic Zn(II) ions induces a nonlinear variation in M-F and M-N bond lengths. In this, we exploit the relative donor atom hardness, where M-F and M-N form strong ionic and weak coordinate covalent bonds, respectively, the latter being more sensitive to substitution of Ni by the slightly larger Zn(II) ion. In this way, we are able to tune the single-ion anisotropy of a magnetic lattice site by Zn-substitution on nearby sites. This effect has possible applications in the field of single-ion magnets and the design of other molecule-based magnetic systems.

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.

High-Pressure Synthesis, Crystal Structures, and Properties of A-Site Columnar-Ordered Quadruple Perovskites NaRMn2Ti4O12 with R = Sm, Eu, Gd, Dy, Ho, Y.

The formation of NaRMn2Ti4O12 compounds (R = rare earth) under high pressure (about 6 GPa) and high temperature (about 1750 K) conditions was studied. Such compounds with R = Sm, Eu, Gd, Dy, Ho, Y adopt an A-site columnar-ordered quadruple-perovskite structure with the generic chemical formula A2A'A″B4O12. Their crystal structures were studied by powder synchrotron X-ray and neutron diffraction between 1.5 and 300 K. They maintain a paraelectric structure with centrosymmetric space group P42/nmc (No. 137) at all temperatures, in comparison with the related CaMnTi2O6 perovskite, in which a ferroelectric transition occurs at 630 K. The centrosymmetric structure was also confirmed by second-harmonic generation. It has a cation distribution of [Na+R3+]A[Mn2+]A'[Mn2+]A″[Ti4+4]BO12 (to match with the generic chemical formula) with statistical distributions of Na+ and R3+ at the large A site and a strongly split position of Mn2+ at the square-planar A' site. We found a C-type long-range antiferromagnetic structure of Mn2+ ions at the A' and A″ sites below TN = 12 K for R = Dy and found that the presence of Dy3+ disturbs the long-range ordering of Mn2+ below a second transition at lower temperatures. The first magnetic transition occurs below 8-13 K in all compounds, but the second magnetic transition occurs only for R = Dy, Sm, Eu. All compounds show large dielectric constants of a possible extrinsic origin similar to that of CaCu3Ti4O12. NaRMn2Ti4O12 with R = Er-Lu crystallized in the GdFeO3-type Pnma perovskite structure, and NaRMn2Ti4O12 with R = La, Nd contained two perovskite phases: an AA'3B4O12-type Im3̅ phase and a GdFeO3-type Pnma phase.

Spin-induced multiferroicity in the binary perovskite manganite Mn2O3.

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

Intrinsic Triple Order in A-site Columnar-Ordered Quadruple Perovskites: Proof of Concept.

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A2 A'A''B4 O12 quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy2 CuMnMn4 O12 (1) and Ho2 MnGaMn4 O12 (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho2 ]A [Mn]A' [Ga0.66 Mn0.34 ]A'' [Mn3.66 Ga0.34 ]B O12 and [Dy2 ]A [Cu0.73 Mn0.27 ]A' [Mn0.80 Dy0.20 ]A'' [Mn1.89 Cu0.11 ]B1 [Mn2 ]B2 O12 . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.

High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb).

A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1- xMn x)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.

Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound.

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2 K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.

Sc2NiMnO6: A Double-Perovskite with a Magnetodielectric Response Driven by Multiple Magnetic Orders.

Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.35281 Å, c = 7.34496 Å, and ß = 90.7915°), exhibiting sequential magnetic transitions at T1 = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector k1 = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k1 = (1/2, 0, 1/2) and k2 = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k1 is associated with the Mn sublattice and k2 with the Ni sublattice, suggesting that Mn-Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchange-striction between the two magnetic sublattices belonging to k1 and k2. The behavior of Sc2NiMnO6 demonstrates 3d double-perovskites with small A-site cations as a promising avenue in which to search for magnetoelectric materials.

Realization of a three-dimensional spin-anisotropic harmonic honeycomb iridate.

Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectly--via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) introduce strong spin-orbit coupling directly, such that these numbers become entwined together and the Mott physics attains a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spin-anisotropic magnetic interactions, coupling the spin to a given spatial direction of exchange and leading to strongly frustrated magnetism. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb and exhibits striking evidence for highly spin-anisotropic exchange. The basic structural units of this material suggest that a new family of three-dimensional structures could exist, the 'harmonic honeycomb' iridates, of which the present compound is the first example.

Collaboration of homologous recombination and nonhomologous end-joining factors for the survival and integrity of mice and cells.

Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are mechanistically distinct DNA repair pathways that contribute substantially to double-strand break (DSB) repair in mammalian cells. We have combined mutations in factors from both repair pathways, the HR protein Rad54 and the DNA-end-binding factor Ku80, which has a role in NHEJ. Rad54(-/-)Ku80(-/-) mice were severely compromised in their survival, such that fewer double mutants were born than expected, and only a small proportion of those born reached adulthood. However, double-mutant mice died at lower frequency from tumors than Ku80 single mutant mice, likely as a result of rapid demise at a young age from other causes. When challenged with an exogenous DNA damaging agent, ionizing radiation, double-mutant mice were exquisitely sensitive to low doses. Tissues and cells from double-mutant mice also showed indications of spontaneous DNA damage. Testes from some Rad54(-/-)Ku80(-/-) mice displayed enhanced apoptosis and reduced sperm production, and embryonic fibroblasts from Rad54(-/-)Ku80(-/-) animals accumulated foci of gamma-H2AX, a marker for DSBs. The substantially increased DNA damage response in the double mutants implies a cooperation of the two DSB repair pathways for survival and genomic integrity in the animal.