Serge’s recent presentation about functional layered oxides by design at the 2015 APS March meeting is available here.

2010: First principles simulations predict a new functionality in a layered perovskite oxide with constrained polarization

With the help of first-principles-based computational techniques, we predicted that Goldstone-like states (i.e., collective, close to zero frequency excitations of the system requiring practically no consumption of energy) can be artificially induced in a layered-perovskite ferroelectric compound. Through precise control of the compound’s structure and composition its electric polarization can be constrained within the individual layer planes. This confinement then gives rise to easy, Goldstone-like rotations of polarization within these planes — in a similar fashion to other well-known perovskite materials with morphotropic phase boundaries. Such phason excitations had been shown to exist in ferroelectric liquid crystals (which actually have very weak polarization) and certain magnetic structures (as spin waves) but never before in a “hard,” strongly polar material. In the paper cited below, we predicted that in a layered Ruddlesden-Popper-type oxide PbSr₂Ti₂O₇ the presence of such excitations can lead to an emergence of a variety of useful physical properties that include large, tunable dielectric constants, close to anomalous light absorption and an ability to easily form vortex polar states in a nanodot geometry. Unusual functionalities of polar-phason compounds, like this one, can be exploited for improved energy generation, storage and conversion, opening up new avenues for disruptive advances in electroactive materials design.

After our first paper came out in 2010, we have also elucidated the electronic-structure underpinnings of the Goldstone-like states in the perovskite-oxide layers by examining the properties of individual atomic planes and tracking their evolution as these planes are stacked together. In contrast with ferroelectric PbTiO₃ that has no Goldstone-like excitations, their appearance in the layered structures can be attributed to the breakdown of the network of cooperative pseudo Jahn-Teller distortions on the Ti⁴⁺ sites, which leaves the Pb²⁺ site network in the PbO atomic planes as the primary source of the polar instability. We found that both the lone electron pair activity of the Pb²⁺ sites and the presence of the bracing SrO planes are paramount for the emergence of the Goldstone-like excitations, while the Ruddlesden-Popper structure, in principle, is not.

This project was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract No. DE-AC02-06CH11357.

Citation Information:

S. M. Nakhmanson and I. Naumov, Goldstone-like states in a layered perovskite with frustrated polarization: a first-principles investigation of PbSr₂Ti₂O₇, Phys. Rev. Lett. 104, 097601 (2010).

2015: A hint of Goldstone-like polar excitation  predicted in a layered oxide with no electron lone pairs.

A layered Ruddlesden-Popper-type oxide Ba₂ZrO₄, which has already been synthesized as a bulk ceramic, exhibits a yet poorly understood affinity for absorbing small molecules, like water and CO₂. An investigation of the properties of a (as of now fictitious) epitaxial thin-film variant of this material reveals that under compression it undergoes a transition into an incommensurate state, while under tension it shows hints of a Goldstone-like polar instability, which surprisingly occurs without the presence of a lone-pair active ions like Pb or Sn. In both cases, we observe anomalies in the planar static dielectric susceptibility of the system, with large dielectric response predicted for the phase displaying the Goldstone-like instability.

Citation Information:

L. Louis and S. M. Nakhmanson, Structural, vibrational, and dielectric properties of Ruddlesden-Popper Ba₂ZrO₄ from first principles, Phys. Rev. B 91, 134103 (2015).