Competing structural instabilities in cubic perovskites

Ferrielectric twin walls in CaTiO3

Sizeable spontaneous polarization has been found in the (100) twin walls of CaTiO3, a definitely nonpolar material. Theoretical simulations of these walls show an extremely rich texture of the local polarization at and close to the walls, including a strong antiferroelectric component, and local nonzero contributions perpendicular to the wall plane, which do not contribute to the net dipole. Individual Ti displacements of 2 pm off the octahedron center give rise to a net polarization corresponding to a displacement of 0.6 pm in the direction of the bisector of the twin angle.

New phenomena at the interfaces of very thin ferroelectric oxides

We present a brief review of the role of interfacial physics in ferroelectric oxides, with an emphasis on the importance of boundary conditions that determine the properties of very thin ferroelectric films and superlattices. As well as discussing the screening problem, and the role of strain and electrostatics in ferroelectrics, we highlight some of the possibilities in fine period superlattices where the high density of interfaces can lead to new and potentially useful phenomena.

Polaron melting and ordering as key mechanisms for colossal resistance effects in manganites

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn-Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid-liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAM).

Frustration of tilts and A-site driven ferroelectricity in KNbO3-LiNbO3 alloys

Density functional calculations for K(0.5)Li(0.5)NbO(3) show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling.

Antiferrodistortive reconstruction of the PbTiO(3)(001) surface

We present in situ x-ray scattering measurements of the surface structures of PbTiO(3) (001) in equilibrium with PbO vapor. At 875 to 1025 K, a reconstruction having c(2x2) symmetry is present under most conditions, while a 1 x 6 reconstruction occurs under PbO-poor conditions. The atomic structure of the c(2x2) phase is found to consist of a single layer of an antiferrodistortive structure with oxygen cages counter-rotated by 10 degrees about the titanium ions.

Novel features of Sr1-xCaxTiO3 phase diagram: evidence for competing antiferroelectric and ferroelectric interactions

Structural and dielectric evidences are advanced to show that the Sr1-xCaxTiO3 (SCT) system undergoes an antiferroelectric (AFE) phase transition in the composition range 0.18</=x</=0.40. Stabilization of the AFE phase, instead of the ferroelectric (FE) phase known for 0<x<0.12, is shown to be intimately linked with the nonpolar antiferrodistortive phase transitions driven by the R and M point instabilities preceding the AFE transition. It is proposed that the disorder and frustration introduced by CaTiO3 substitution in the incipient FE matrix of SrTiO3 leads to the smearing of the varepsilon(')(T) response in SCT for 0.002<x<0.12 akin to dipole glasses and relaxor ferroelectrics.