Jahn-Teller hexagonal perovskites

Low-temperature structural phases of Jahn-Teller hexagonal perovskites 656... 

In this chapter an effective Hamiltonian for a static cooperative Jahn-Teller effect is proposed. This Hamiltonian acts in the space of local active distortions only and possesses extrema points of the potential energy equivalent to those of the full microscopic Hamiltonian, defined in the space of all phonon and uniform strain coordinates. First we present the derivation of this effective Hamiltonian for a general case and then apply the theory to the investigation of the structure of Jahn-Teller hexagonal perovskites. 

We can conclude that orbital-dependent exchange interaction in hexagonal perovskites does not contribute essentially to the ordering of orbitals and Jahn-Teller distortions on sites but only results in a slight renormalization of the force constant Kt and, therefore, of A2 and, 42 parameters of the basic model in equation (37). 

Maaskant WJA (1995) On Helices Resulting from a Cooperative Jahn-Teller Effect in Hexagonal Perovskites. 83 55-88... 

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