Thermodynamic stability and pressure dependence of FePO4 polymorphs

Olivine LiFePO4 is a promising cathode material for lithium ion batteries. Its delithiated form FePO4 has attracted interest as potential cathode for rechargeable lithium ion batteries due to high charge/discharge capacity and low cost. This paper report pressure dependence DFT + U calculations and stability of FePO4 polymorphs. The heats of formation predict that berlinite polymorph is thermodynamically stable up to 4 GPa, while HP phase reaches a transition above 4 GPa at 0 K. The energy band gap is relatively wide, suggesting that FePO4 polymorphs are low in electronic conductivity. However, conductivity is expected to improve at high pressure (4 to 7.5 GPa). All independent elastic constants are positive corresponding to stability of FePO4 polymorphs, except for berlinite. Its phonon dispersion curves display negative vibrations along high symmetry direction (condition of instability) at 0 GPa. However, at pressure above 7 GPa, the berlinite becomes stable due to absence of soft modes.