  Melting behaviors of defective crystals under extreme conditions are
theoretically investigated using the statistical moment method. In our
theoretical model, heating processes cause missing atoms or vacancies in
crystal structures via dislocating them from their equilibrium positions. The
coordination number of some atoms is assumed to be removed by one unit and the
defect depends on temperature and external pressure. We formulate analytical
expressions to directly connect the equilibrium vacancy concentration, the
elastic modulus, and the melting temperature. Numerical calculations are
carried out for six transition metals including Cu, Ag, Au, Mo, Ta, and W up to
400 GPa. The obtained results show that vacancies strongly drive the melting
transition. Ignoring the vacancy formation leads to incorrect predictions of
the melting point. The good agreement between our numerical data and prior
experimental works validates the accuracy of our approach.
