Anharmonic effects in a Jahn-Teller phase transition

A mechanism is proposed to explain the double phase transitions found in K₂PbCu(NO₂)₆ and Tl₂PbCu(NO₂)₆. The higher temperature transition is caused by the cooperative Jahn-Teller effect. The lower temperature transition occurs as the fourth order anharmonic interaction restabilizes the lattice against instabilities caused by static distortions in the third order anharmonic interaction. Calculated curves of the order parameter and heat capacity reproduce all the features in the experimental curves.