What is the difference between polycrystalline and monocrystalline?
Monocrystals are complete single crystals without grain boundaries, while polycrystals are composed of many grains with different orientations, and there are obvious grain boundaries between these grains. Monocrystalline MgF2 has better physical properties than polycrystalline MgF2, with higher mechanical strength and resistance to thermal shock, and is less prone to cracking along grain boundaries.