Cleavage and surface energies of LiNbO₃

“Cleavage” is one of the most well-known modes of mechanical failure of crystals. It describes their ability to break easily along specifically oriented atomically sharp planes. Although understanding cleavage properties of any crystalline functional material is mandatory, the current knowledge about cleavage is limited to simplest crystals like cubic silicon. With the goal of expanding the knowledge to the broader class of crystalline materials, the cleavage of trigonal LiNbO₃ single crystal is investigated in this work. The experimental method to measure the cleavage energy is introduced. The method is based on adhering a pre-cracked specimen in a rectangular hole in aluminum loading frame; forces are applied to the cracked specimen when heating up the assembly and due to the thermal expansion coefficient mismatch between the specimen and the loading frame. The cleavage energies of two known cleavage planes are evaluated by this method followed by extraction of the surface energies, and third previously unknown cleavage plane is discovered. The findings are explained qualitatively by counting the number of Nb–O bonds in LiNbO₃ structure, broken by these planes. The evaluated fracture properties can be used to prevent catastrophic failure or for optimal design of the LiNbO₃ crystal at service.