Compressibility of Lattice Parameters of Several Layered Compounds

The lattice parameters of AlB₂, MgB₂ and TiB₂ under pressures are determined with a high-energy synchrotron source in a diamond anvil cell. The experimental results indicate that these three compounds have different mechanical behaviour under pressures, TiB₂ is the hardest and MgB₂ is the softest among the three materials. The phenomena are explained in terms of bonding strength in the crystal. Our results may be helpful for understating the decrease of the superconducting transition temperature of MgB₂ under pressures.