Why is silicon so hard but so brittle?

Silicon is an atomic crystal, whose atoms are connected to each other by covalent bonds, forming a spatial network structure. In this structure, the covalent bonds between atoms are very directional and have high bond energy, which makes silicon show high hardness when resisting external forces to change its shape. For example, it takes a large external force to destroy the strong covalent bond connection between atoms.

However, it is precisely because of the regular and relatively rigid structural characteristics of its atomic crystal that when it is subjected to a large impact force or uneven external force, the lattice inside silicon is difficult to buffer and disperse the external force through local deformation, but will cause the covalent bonds to break along some weak crystal planes or crystal directions, which will cause the entire crystal structure to break and show brittle characteristics. Unlike structures such as metal crystals, there are ionic bonds between metal atoms that can slide relatively, and they can rely on the sliding between atomic layers to adapt to external forces, showing good ductility and not easy to break brittle.

Silicon atoms are connected by covalent bonds. The essence of covalent bonds is the strong interaction formed by the shared electron pairs between atoms. Although this bond can ensure the stability and hardness of the silicon crystal structure, it is difficult for the covalent bond to recover once it is broken. When the force applied by the outside world exceeds the limit that the covalent bond can withstand, the bond will break, and because there are no factors such as freely moving electrons like in metals to help repair the break, re-establish the connection, or rely on the delocalization of electrons to disperse the stress, it is easy to crack and cannot maintain the overall integrity through its own internal adjustments, causing silicon to be very brittle.

In practical applications, silicon materials are often difficult to be absolutely pure, and will contain certain impurities and lattice defects. The incorporation of impurity atoms may disrupt the originally regular silicon lattice structure, causing changes in the local chemical bond strength and the bonding mode between atoms, resulting in weak areas in the structure. Lattice defects (such as vacancies and dislocations) will also become places where stress is concentrated.

When external forces act, these weak spots and stress concentration points are more likely to cause the breaking of covalent bonds, causing the silicon material to begin to break from these places, exacerbating its brittleness. Even if it originally relied on the covalent bonds between atoms to build a structure with a higher hardness, it is difficult to avoid brittle fracture under the impact of external forces.