A first-principles scheme to phonons of high temperature phase: No imaginary modes for cubic SrTiO₃

The issue of imaginary phonon modes predicted by first-principles calculations for high-temperature structures of most materials has been a longstanding problem for decades. We propose that the observed high-temperature structures are actually dynamic averages of related low-temperature structures. This theory is used to predict the phonon dispersions of cubic SrTiO ₃. The calculated phonon dispersions for the cubic phase, using the force constants calculated from the tetragonal phases, are found to be in remarkably good agreement with existing neutron data, without exhibiting any imaginary phonon modes.