Controlling Crystal Surface Termination by Cleavage Direction

We have investigated the cleaving behavior of potassium bichromate $\left(\mathrm{K}{}_2\mathrm{Cr}{}_2\mathrm{O}{}_7\right)$ crystals using atomic force microscopy. This crystal has a double layered $\mathrm{AB}$ structure along $\{001\}$. We find that, upon cleavage along the $\{001\}$ plane in the $〈100〉$ directions, one side is completely $A$ terminated, while the other is $B$ terminated. Moreover, the cleavage plane (between an $A$ and a $B$ layer, or between $B$ and $A$) depends on the imposed direction of cleavage, i.e., [100] or $\left[\overline{1}00\right]$. This means that the molecular layer that terminates the crystal surface can be controlled by choosing the macroscopic direction of the cleavage force. One of the two terminations is metastable and partly reconstructs to the stable termination.