Room-temperature polar ferromagnet ScFeO3 transformed from a high-pressure orthorhombic perovskite phase.

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R3c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 µC/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe(3+) spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.

Electric-field-induced phase transitions in co-doped Pb(Zr1-xTix)O3 at the morphotropic phase boundary.

The strain- and polarization-electric field behavior was characterized at room temperature for Pb0.98Ba0.01(Zr1-x Ti x )0.98Nb0.02O3, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.

Large easy-plane anisotropy induced spin reorientation in magnetoelectric materials (Co4-x Mn x )Nb2O9.

Neutron powder diffraction experiments were carried out on the magnetoelectric compound series (Co4-x Mn x )Nb2O9 (x = 0, 1, 2, 3, 3.5, 3.9, 3.95 and 4) from base temperature to above their Néel temperatures. Their magnetic structures were analysed by using the irreducible representation analysis and Rietveld refinement method. Similar to Co4Nb2O9, the compounds with x ⩽ 3.9 have noncollinear in-plane magnetic structures (Γ6) with magnetic moments lying purely in the ab plane with certain canting angles. Mn4Nb2O9 has a collinear antiferromagnetic structure (Γ2) with magnetic moments aligning along the c axis. The compound of x = 3.95 shows two magnetic phases in the magnetization, which was confirmed to have the Γ2 magnetic structure above 60 K and develop a second Γ6 local phase in addition to the main Γ2 phase due to doping. This study indicates 2.5 at% Co2+ doping is sufficient to alter the collinear easy-axis magnetic structure of Mn4Nb2O9 into the noncollinear easy-plane magnetic structure, which is attributed to the large easy-plane anisotropy of Co2+ and relative small Ising-like anisotropy of Mn2+. The doping effects on the Néel temperature and occupancy are also discussed.

Extreme compressibility in LnFe(CN)6 coordination framework materials via molecular gears and torsion springs.

The mechanical flexibility of coordination frameworks can lead to a range of highly anomalous structural behaviours. Here, we demonstrate the extreme compressibility of the LnFe(CN)6 frameworks (Ln = Ho, Lu or Y), which reversibly compress by 20% in volume under the relatively low pressure of 1 GPa, one of the largest known pressure responses for any crystalline material. We delineate in detail the mechanism for this high compressibility, where the LnN6 units act like torsion springs synchronized by rigid Fe(CN)6 units performing the role of gears. The materials also show significant negative linear compressibility via a cam-like effect. The torsional mechanism is fundamentally distinct from the deformation mechanisms prevalent in other flexible solids and relies on competition between locally unstable metal coordination geometries and the constraints of the framework connectivity, a discovery that has implications for the strategic design of new materials with exceptional mechanical properties.

Electric-field-induced AFE-FE transitions and associated strain/preferred orientation in antiferroelectric PLZST.

Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field dependent dielectric anomalies as well as unrecovered strain change which appear in AFE materials after exposure to sufficiently high electric fields.

Susceptible Ferroelectric/Antiferroelectric Phase Transition near the Surface of Nb-Doped Lead Zirconate Stannate Titanate from Surface Processing.

This work systematically investigated the structure and property of the near-surface and bulk regions of Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 ceramics using a combination of X-ray and neutron diffraction, piezoresponse force microscopy, and conventional ferroelectric/piezoelectric characterization. It is found that mechanical force can induce an antiferroelectric/ferroelectric phase transition within micrometers of the surface. Such a phase transition is strongly dependent on the processing scenario, leading to differences from the bulk region. This work provides crucial insights into the sensitivity of this class of AFE materials. Clearly, surface processing conditions must be taken into account for both accurate structural determination and practical applications.