On the pressure-induced spin transition in CoCl_2. interplay between Jahn-Teller effect and spin sate in Co²⁺

This work investigates on the interplay between the Jahn-Teller effect and spin state in Co²⁺(3d⁷ configuration). High-pressure optical absorption experiments were carried out in the 0 - 50 GPa range in CoCl_2. This system is attractive since pressure increases the crystal-field splitting between e_g and t_2g levels from 0.9 eV to 1.6 eV, thus approaching CoCl₂ towards the high-spin-to-low-spin transition. The free energy associated with the two states is discussed in terms of crystal-field and stabilization energies due to lattice relaxation including the Jahn-Teller effect, which is expected to be particularly strong in the low-spin configuration (²E;t_2g⁶e_g¹) with respect to the weakly Jahn-Teller high-spin configuration (⁴T₁; t_2g⁵e_g²). The metallization process observed above 40 GPa is discussed in terms of spin transition and charge-transfer states.