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ion Modeling Ceramics – Warts and All

Rick Ubic, Boise State University, DMR 1052788

Outcome: Researchers at Boise State University and the University of Illinois at Urbana Champaign are investigating the effects of crystal defects on the structure and properties of functional perovskite ceramics.

Impact: Microwave resonators, superconducting filters, and ionic conductors commonly adopt the perovskite structure (idealized in the figure to the right) and are extensively used in multi-billion-dollar industries. Improving their properties could aid size reduction, speed, and quality of a variety of devices.

Explanation: Novel perovskite ceramics of generic composition ABO3 have two atomic positions, A site and a B site, which can be readily modified. Varying the chemistry of these sites, relationships between composition, structure, and properties can be derived. Towards that end, ceramics of precisely controlled chemistries are being synthesized via both conventional and chemical routes. The subsequent compounds are then characterized by advanced techniques, and the correlated results will aid in the development of a unique predictive model for the structure of realistic defect-containing perovskites. Computer modeling may allow for an understanding of the underlying physics of these effects.

The oxide perovskite structure contains A-site and B-site cations as well as oxygen. Each B-site is surrounded by six oxygen ions in octahedral coordination (blue above). Determining the exact atomic positions is complicated by the simultaneous possibilities of A- and B-cation displacements, cation ordering, and octahedral tilting.

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ion A-Site Vacancies both Push and Pull

Rick Ubic, Boise State University, DMR 1052788

Several models have been proposed to predict the structure/stability of perovskite phases; however, none are suited to realistic, complex or defective structures. This project is an attempt to develop such a model.

In the case of SrTiO3-based compounds, low A-site vacancy concentrations (≤2%) result in vacancies with a negative effective size as a result of geometrical relaxation of anions towards the vacant site. Vacancies essentially pull anions into them. For higher concentrations, vacancies have an effective positive size due to mutual Coulombic repulsion of the coordinating anions. In this case, vacancies push anions away. The equation which described this phenomenon is:

-1

-0.5

0

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2

0 0.05 0.1 0.15 0.2 0.25 0.3[V]

r V/r A

(ideal)

R2 = 0.99984

Sr1-3xLa2xTiO3

Sr1-3xCe2xTiO3

Sr1-3xNd2xTiO3

C2/c

Pm3m

R3c

rV = (– 0.016602/[V] + 0.922812 + 3.28839[V] - 4.2046[V]2)rA(ideal)

The effective size (normalized) of A-site vacancies, negative for [V] ≤ 2%, increases

with concentration, and symmetry decreases.

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ion Telecommunications - Getting the Word Out

Rick Ubic, Boise State University, DMR 1052788

The Idaho Academy of Science held its annual symposium 22-24 March 2012 in Idaho Falls. Dr. Ubic attended with one of the undergraduate project students, Billy Chan, who gave an oral presentation titled "Modeling the effect of point defects in perovskites." The talk won second place ($100) in the undergraduate oral-presentation category. A photo of the award winners is below, in which Billy is on the far right.

Senior undergraduate Billy Chan (far right) was runner-up in the undergraduate oral presentation category at the annual Idaho Academy of Science symposium.