Strong Electric Fields Tune the Stability of Ionic Defects in Oxides

Intellectual Merit
No ceramic crystal is perfect, and structural imperfections including point defects are responsible for many technologically desirable properties of ceramics. Applications such as modern computer memories rely on controlling defects inside a crystal by exposing them to large electric fields. High field effects on defective crystals, however, remain challenging to control and address.

MIT MRSEC researchers have demonstrated that strong electric fields can tune the stability of ionic defects in semiconducting or insulating oxides. Combining density functional theory and the modern theory of polarization, they were able to formulate and predict the effect of electric fields on charged defects in oxides. By applying this approach to simple binary oxides, they elucidated the rich thermodynamics underlying the free energy landscape of oxygen vacancies.

Intellectual Merit Visualizations of the charge density of two electrons trapped in an oxygen vacancy in four different ceramic materials. The top row of four images illustrates a zero-electric field condition and the second bottom row of four images, the same materials in a high-electric field condition. Four different ceramic materials are presented in each row from left to right: magnesium oxide followed by calcium oxide then strontium oxide and finally barium oxide. The boxes are labeled from A to H, beginning with the box on the top left, progressing from left to right, A, B, C, D; then listed on the bottom row E, F, G, H. The boxes, all with a black background, contain similar images showing symmetric grids comprised of four by four rows of spheres of varying color combinations. Where there would be a sixteenth sphere in the middle of the upper right quadrant of the grid, suspends a three-dimensional, cushioned yellow shape, comparatively larger than the fifteen spheres. These yellow forms suspended in each grid indicate spatial distribution of the two electrons in each given environment. The yellow form varies in shape in each box. Each form each shows a round indentation in the center that also varies in size in each box. Nearly all of the boxes show yellow detail on the edge of the spheres directly surrounding the yellow shape. The color variations of the spheres in each box represent the following: Oxygen is red; magnesium is dark blue; calcium is cyan; strontium is green; and barium is grey. The first vertical row displays magnesium oxide in boxes A and E, with red and dark blue spheres. Box A shows the yellow object face-forward, a symmetric octagonal face indicating and the central round shape is small. Box E displays a similar image, only skewed as if stretched on the right side of the yellow form. The second vertical row displays calcium oxide, boxes B and F, with red and cyan. The yellow form in box B is a symmetrical square with a large round indentation. Box F shows a similar image, but skewed as if being stretched to the right. The third vertical row displays strontium oxide, boxes C and G, show red and green spheres with a square face as shown in the second row, but with a medium round indentation. Box G also varies from the last in that it is angled more to the right, almost as though it has pivoted to the right at a forty-five degree angle. The final vertical row, barium oxide, boxes D and H, display red and grey spheres. The yellow image in Box D displays a shape like that of a square face with pinched edges. The round indentation is large with another indented ring surrounding it. Box H varies slightly from all other seven boxes in that the form does not contain a round indentation and forms more like a bean shape positioned to face the left side of the box.

Figure 1. Visualizations of the charge density of the two electrons trapped in an oxygen vacancy at zero field in (a) MgO, (b) CaO, (c) SrO, and (d) BaO. Similar visualizations at a field of 22 MV/cm in the +x direction are shown for (e) MgO, (f), CaO, (g) SrO, and (h) BaO. Red, blue, cyan, green, and grey spheres represent O, Mg, Ca, Sr, and Ba ions, respectively.

Broader Impact
The ability to predict electric field effects in semiconducting oxides will guide the design of optimal conditions to promote desirable forms of electronic and ionic defects in electronic or electrochemical applications. For example, accounting for electric field effects is necessary for a better understanding and modeling of the performance of ultra-fast, low-energy redox based memristive device performance, as well as of photo-electro-chemical energy conversion devices to produce clean fuels. In addition, advancements in electrocaloric refrigeration and field assisted ceramic sintering will benefit from accurate theories of how electric fields redistribute and move ionic defects in related materials.