Electrical Behavior of TiO 2 Grain Boundaries E.C. Dickey (PI), Pennsylvania State University, DMR Electroceramics are utilized in a wide variety of electrical, dielectric and sensing applications. Most of these materials are composed of an ensemble of crystals (grains) and it is often the properties of the interfaces between the grains (grain boundaries) that dictate the macroscopic electrical properties. Thorough judicious chemical doping the grain boundaries, and thus the macroscopic properties, can be controlled. Utilizing complementary transmission electron microscopy and impedance spectroscopy, this research has directly correlated the atomic-scale structure and chemistry of TiO 2 grain boundaries with their electrical behaviors. These findings provide the experimental foundation for developing predictive, quantitative models for grain boundary electrical behavior for a broad class of ceramics used in electrical applications. 5nm (Figure below) Z-contrast scanning transmission electron micrograph of a grain boundary in TiO 2. The bright intensity corresponds to Y segregation to the boundary. (Figure right) Model of a grain boundary in Y-doped TiO 2 that gives rise to a blocking effect for electronic conduction, increasing the resistivity of the material.

First-Principles Calculations of Intrinsic Defects in Bulk TiO 2 S.B. Sinnott (Co-PI), University of Florida, DMR Point defects play an important role in many applications of metal oxides, including rutile TiO 2. First- principles and thermodynamic calculations are used to determine defect formation enthalpies (DFEs) in TiO 2 in the reduced state (P O2 = ). The results indicate that at room temperature, undoped TiO 2 exhibits p-type or n-p behavior when E F =1.5 eV, while at 1400 K, the system clearly exhibits n-type behavior. This theoretical research helps explain why rutile TiO 2 exists experimentally as an oxygen deficient oxide at high temperature, and indicates how its electronic properties may be tailored by controlling the nature and density of point defects. This work is performed in collaboration with Mike Finnis (Queen’s University) and Elizabeth Dickey (Penn. State). V Ti -2 DFEs (eV) Ti i +4 Ti i +3 Ti i +2 Ti i +1 Ti i 0 V O +2 V O +1 VO0VO0 V Ti -1 Oi0Oi0 V Ti -4 V Ti -3 O i -2 T= 300 K Ti i +4 Ti i +3 Ti i +2 Ti i +1 Ti i 0 V O +1 VO0VO0 V Ti -1 Oi0Oi0 V Ti -4 V Ti -3 V Ti -2 O i -2 V O +2 T= 1400 K DFEs (eV)

Training of High School Students in Computational Materials Science and Engineering S.B. Sinnott (Co-PI), University of Florida, DMR Rutile TiO 2 Pyrolusite MnO 2 Cassiterite SnO 2 Cation radius (Å) Cation electron configuration [Ar]4S 2 3d 2 [Ar]4S 2 3d 5 [Kr]5S 2 4d 10 5p 2 Bond energy (kJ/mol, gaseous diatomic species) O vacancy formation energy (eV) Prof. Sinnott and graduate student, Mr. Jun He (left), mentored and worked with high school student, Mr. Leemen Weaver (right) on the described research. Mr. Weaver was a University of Florida Student Science Training Program participant during the summer of Mr. Weaver’s project was to understand oxygen vacancy formation in three rutile, metal oxides using first principles, density functional theory calculations. A sample unit cell is shown in the top, right-most figure. His preliminary results are shown in the table. They indicate that the cation electron configuration and bond energies are dominant factors in the formation of oxygen vacancies. Oxygen vacancy Cation (Ti, Mn, Sn)