Effect of temperature on the crystal structure of BaSi2

The effect of temperature on the crystal structure of BaSi2 was investigated by single crystal X-ray diffraction studies at temperatures ranging from 120 to 300 K. Upon reducing the temperature from 300 to 120 K, the unit-cell volume decreased by ∼1%, while the volume of the Si4 tetrahedra remained unchanged. First principles calculations revealed that the Si atoms form covalent bonds with three adjacent Si atoms in the Si4 tetrahedra. These covalent bonds are speculated to be responsible for the constant volume of Si4 upon cooling.