Abstract
Using the local-density approximation, and calculating the Hellmann–Feynman forces, the stability of the perovskite-like structures of CaTiO3 have been studied by applying the direct method and deriving the phonon dispersion relations. The cubic Pm3̄m phase shows dispersionless soft phonon branch spreading from R to M points of the cubic Brillouin zone. Low-symmetry phase I4/mcm can be considered as a result of the soft mode condensation at R point. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups, being the result of soft mode condensation at R and M points, respectively. Thus, the Pmnb can be regarded as simultaneous condensations of two soft modes. The phonon dispersion relations of Pmnb suggest that this phase is stable.
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