Understanding and controlling ZnO microstructure plays a fundamental role for the production of varistor materials. The properties of ZnO varistors are determined by the microstructure, or more precisely, by the number and character of the grain boundaries between the electrodes. The character of the boundary defines the so called Double Schottky Barriers (DSB). Due to the polar space group of wurtzite ZnO, a single crystal can feature two different terminations with opposite charge when cut perpendicularly to the [0001] direction. This leads to a special class of grain boundaries in wurtzite based polycrystals where the crystal structure is inverted at the boundary, called inversion boundaries (IBs). As a number of experimental works on Bi-based ZnO varistors show, these IBs may be one key to microstructure design.[1] Here, we report a study of the head-to-head (-O-Zn-O-) inversion boundaries (IB) in zinc oxide. We investigate the effect of impurities (Fe, Bi, Pr, Sn, Ti, and Sb) on the IB formation using ab-initio calculations in the framework of density-functional theory (DFT) for different atomic coverages. In agreement with experiment, DFT predicts spontaneous formation of IBs in Sb-, Sn-, and Fe-doped ZnO, with coverages of 1/3, 1/2, and 1 monolayer, respectively. Half a monolayer of Ti also lowers the IB formation energy, but it is not enough to trigger IB formation. Bi and Pr do not sufficiently promote IB formation at conditions relevant for ZnO sintering. Inspired by the remarkable success of DFT for evaluation of IB energies, we discuss the implications for the IB design and control.

[1] Aleksander Recnik, Slavko Bernik, and Nina Daneu. Microstructural engineering of ZnO-based varistor ceramics. Journal of Materials Science, 47(4):1655-1668, 2012.