  Recent contrasting observations for halogen (X)-bridged binuclear platinum
complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O, that is, pressure-induced Peierls and
reverse Peierls instabilities, are explained by finite-temperature Hartree-Fock
calculations. It is demonstrated that increasing pressure transforms the
initial charge-polarization state into a charge-density-wave state at high
temperatures, whereas the charge-density-wave state oppositely declines with
increasing pressure at low temperatures. We further predict that
higher-pressure experiments should reveal successive phase transitions around
room temperature.
