In conventional spin crossover systems, the vibrational degrees of freedom enhances the entropic effect in excited high-spin terms resulting from the softening of vibrations [1]. Here, we show an opposite effect of vibration on the spin-crossover taking C₆₀^3- as an example [2]. C₆₀^3- anion takes either high (S = 3/2) or low (S = 1/2) spin state, and in the latter the dynamical Jahn-Teller effect arises. It is found that the large dynamical Jahn-Teller stabilization energy lowers the low-spin levels, resulting in the violation of Hund’s rule. The Jahn-Teller dynamics influences the thermodynamic properties via strong variation of the density of vibronic states with energy. Thus, the large vibronic entropy in the low-spin states enhances the effective spin gap of C₆₀^3- quenching the spin crossover. This finding is used for the rationalization of the experimental data on the spin gaps in various fullerides. The vibronic mechanism is not limited to fullerene: It can play a crucial role when Hund and Jahn-Teller couplings are comparable to each other.
[1] P. Gütlich, A. Hauser, and H. Spiering, Angew. Chem. Int. Ed. 33, 2024 (1994).
[2] D. Liu, N. Iwahara, and L. F. Chibotaru, Phys. Rev. B 97, 115412 (2018).