Phase Transition and Thermodynamics of Ruthenium Diboride via First-Principles Calculations

The pressure induced phase transitions of RuB₂ from the OsB₂-type structure to the ReB₂-type structure are investigated by first-principles calculations based on the plane-wave basis set with the generalized gradient approximation for exchange and correlation. It is found that the phase transition occurs at 18.6GPa. We predict the phase transition from the OsB₂-type RuB₂ to the ReB₂-type RuB₂ at high temperatures for the first time. The dependences of the heat capacity, thermal expansion coefficient, and the Grüneisen parameter on pressure and temperature for OsB₂-type RuB₂ and ReB₂-type RuB₂ are also investigated.