Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO Thin Films.

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern.

The influence of the 6s configuration of Bi on the structures of A'BiNbO (A' = Rb, Na, Li) layered perovskite oxides.

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The 'hybrid improper' mechanism - in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure - offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNbO, LiBiNbO and NaBiNbO, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi cations which are often observed to stabilize acentric crystal structures due to their 6s electronic configurations.

Interplay between Phonons and Anisotropic Elasticity Drives Negative Thermal Expansion in PbTiO_{3}.

We use first-principles theory to show that the ingredients assumed to be essential to the occurrence of negative thermal expansion (NTE)-rigid unit phonon modes with negative Grüneisen parameters-are neither sufficient nor necessary for a material to undergo NTE. Instead, we find that NTE in PbTiO_{3} involves a delicate interplay between the phonon properties of a material (Grüneisen parameters) and its anisotropic elasticity. These unique insights open new avenues in our fundamental understanding of the thermal properties of materials and in the search for NTE in new materials classes.

La2SrCr2O7: Controlling the Tilting Distortions of n = 2 Ruddlesden-Popper Phases through A-Site Cation Order.

Structural characterization by neutron diffraction, supported by magnetic, SHG, and μ(+)SR data, reveals that the n = 2 Ruddlesden-Popper phase La2SrCr2O7 adopts a highly unusual structural configuration in which the cooperative rotations of the CrO6 octahedra are out of phase in all three Cartesian directions (ΦΦΦz/ΦΦΦz; a(-)a(-)c(-)/a(-)a(-)c(-)) as described in space group A2/a. First-principles DFT calculations indicate that this unusual structural arrangement can be attributed to coupling between the La/Sr A-site distribution and the rotations of the CrO6 units, which combine to relieve the local deformations of the chromium-oxygen octahedra. This coupling suggests new chemical "handles" by which the rotational distortions or A-site cation order of Ruddlesden-Popper phases can be directed to optimize physical behavior.

Interplay of Octahedral Rotations and Lone Pair Ferroelectricity in CsPbF3.

CsPbF3 is the only experimentally synthesized ABF3 fluoride perovskite with a polar ground state. We use CsPbF3 as a guide in our search for rules to rationally design new ABX3 polar fluorides and halides from first-principles and as a model compound to study the interactions of lone pairs, octahedral rotations, and A- and B-site driven ferroelectricity. We find that the lone pair cation on the B-site serves to stabilize a polar ground state, analogous to the role of lone pair cations on the A-site of oxide perovskites.

Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments.

ABO(3) perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity of properties found among perovskites, very few of these materials are ferroelectric, or even polar, in bulk. In this Perspective, we highlight recent theoretical and experimental studies that have shown how a combination of non-polar structural distortions, commonly tilts or rotations of the BO(6) octahedra, can give rise to polar structures or ferroelectricity in several families of layered perovskites.

Origin of ferroelectricity in a family of polar oxides: the Dion-Jacobson phases.

Recent work on layered perovskites has established the group theoretical guidelines under which a combination of octahedral distortions and cation ordering can break inversion symmetry, leading to polar structures. The microscopic mechanism of this form of ferroelectricity-so-called hybrid-improper ferroelectricity-has been elucidated in two families of layered perovskites: AA'B2O6 double perovskites and Ruddlesden-Popper phases. In this work, we use symmetry principles, crystal chemical models, and first-principles calculations to unravel the crystal chemical origin of ferroelectricity in the Dion-Jacobson phases, and show that the hybrid improper mechanism can provide a unifying explanation for the emergence of polar structures in this family of materials.

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics.

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects.

Mechanical control of electroresistive switching.

Hysteretic metal-insulator transitions (MIT) mediated by ionic dynamics or ferroic phase transitions underpin emergent applications for nonvolatile memories and logic devices. The vast majority of applications and studies have explored the MIT coupled to the electric field or temperarture. Here, we argue that MIT coupled to ionic dynamics should be controlled by mechanical stimuli, the behavior we refer to as the piezochemical effect.

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1).

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.

Hybrid improper ferroelectricity: a mechanism for controllable polarization-magnetization coupling.

First-principles calculations are presented for the layered perovskite Ca3Mn2O7. The results reveal a rich set of coupled structural, magnetic, and polar domains in which oxygen octahedron rotations induce ferroelectricity, magnetoelectricity, and weak ferromagnetism. The key point is that the rotation distortion is a combination of two nonpolar modes with different symmetries.

A genetic algorithm for predicting the structures of interfaces in multicomponent systems.

Recent years have seen great advances in our ability to predict crystal structures from first principles. However, previous algorithms have focused on the prediction of bulk crystal structures, where the global minimum is the target. Here, we present a general atomistic approach to simulate in multicomponent systems the structures and free energies of grain boundaries and heterophase interfaces with fixed stoichiometric and non-stoichiometric compositions.

Density functional theory study of hydrogen bonding in ionic molecular materials.

Crystal structures are usually described in geometric terms. However, it is the energetics of intermolecular interactions that determine the chemical and physical properties of molecular materials.(1) In this paper, we use density functional theory (DFT) in combination with numerical basis sets to analyze the hydrogen bonding interactions in a family of novel ionic molecular materials.