ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale.[5] Its elastic constants are smaller than those of relevant III-V semiconductors, such as GaN. The high heat capacity and heat conductivity, low thermal expansion and high melting temperature of ZnO are beneficial for ceramics.[21] ZnO exhibits a very long lived optical phonon E2(low) with a lifetime as high as 133 ps at 10 K