In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion-preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden-Popper HIF Sr 3 Zr 2 O 7 , which is the first ternary lead-free zirconate ferroelectric, is reported and room-temperature polarization switching is demonstrated. This compound undergoes a first-order ferroelectric-to-paraelectric transition, involving an unusual change in the "sense" of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first-principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper "antiferroelectricity" is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next-generation thermoelectrics.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches based on group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T C). Here, we establish a predictive and quantitative relationship between T C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A 3 B 2 O 7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr 1−x Ca x) 3 Sn 2 O 7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic-size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d 0 and 6s 2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T C = 410 K for Sr 3 Sn 2 O 7. We also find that the T C increases linearly up to 800 K with increasing the Ca 2+ content, i.e., with decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A 3 B 2 O 7 ferroelectrics, indicating that T C correlates with the simple crystal-chemistry descriptor, t, based on the ionic-size mismatch. This study provides a predictive guideline for estimating T C of a given material, which would complement the grouptheoretical and first-principles design approach. Additional ND and SXRD analyses, first-principles calculation results, and Mössbauer spectroscopy (PDF).

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.

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground state planar configuration with a saturated local moment of 1.5 µB, and a metastable non-planar configuration with a vanishing magnetic moment, at a modest energy expense of 50 meV. Though non-planarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (vσ, vπ). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of vπ yielding a fractional moment, which is aligned parallel to the unpaired vσ electron through Hund's coupling. As a byproduct, the Dirac states (dπ) of the two sublattices undergo a minor spin-polarization and couple antiferromagnetically. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between vσ and local dπ states, to form a hybridized vσ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between vσ and vπ lowers the energy. The gradual spin reversal of vπ with increasing non-planarity opens up the possibility of an intermediate structure with balanced vπ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to vσ based spin-1/2 paramagnetism. Carrier doping, electron or hole, does not alter the structural stability. However, the doping proportionately changes the occupancy of vπ state and hence the net magnetic moment. arXiv:1605.03921v1 [cond-mat.mtrl-sci]