First - principles theory of structural phase transitions for perovskites : competing instabilities
نویسندگان
چکیده
We extend our previous first-principles theory for perovskite ferroelectric phase transitions to treat also antiferrodistortive phase transitions. Our approach involves construction of a model Hamiltonian from a Taylor expansion, first-principles calculations to determine expansion parameters, and Monte Carlo simulations to study the resulting system. We apply this approach to three cubic perovskite compounds, SrTiO 3 , CaTiO 3 , and NaNbO 3 , that are known to undergo antiferrodistortive phase transitions. We calculate their transition sequences and transition temperatures at the experimental lattice constants. For SrTiO 3 , we find our results agree well with experiment. For more complicated compounds like CaTiO 3 and NaNbO 3 , which can have many different structures with very similar energy, the agreement is somewhat less satisfactory.
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