Structural phase transformation and limited miscibility in Jahn-Teller systems

Immiscibility of crystal systems with Jahn-Teller (JT) ions is studied at l ow temperatures. Phase transformations under which concentrated JT systems are separated into phases with a high and low concentration of JT ions with structural JT transformations are analyzed. A dependence of the phase diag ram topology on the ratio between the cooperative interaction energy in the JT subsystem and the intensity of random crystal fields acting on the JT i ons is established. The possibility of low-temperature three-phase equilibr ium of two cubic and a tetragonal phase with different concentrations of JT centers is demonstrated for a high degree of substitutional-type disorder. (C) 1999 American Institute of Physics. [S1063-777X(99)01206-2].