Unveiling barocaloric potential in organometallic-sandwich compounds [Cp2M][PF6] (M: Fe3+, Co3+)

Abstract

Organometallic-sandwich salts are well-known materials that undergo order–disorder phase transitions, leading to a high-temperature phase characterized by the total or partial disorder of ionic species. Their potential for barocaloric applications has not previously been explored. Here, we focus on two salts derived from metallocenes with the formula [Cp₂M][PF₆] where Cp: cyclopentadienyl anion (C₅H₅)⁻ and M: Fe³⁺ or Co³⁺. These molecular salts exhibit two solid–solid phase transitions, shifting from a well-ordered crystalline phase at low temperatures to an orientationally disordered phase in the case of the Fe-compound, and a partially disordered phase in the case of the Co-compound above room temperature. We find a significant entropy change (∼40 J K⁻¹ kg⁻¹) and a moderate volume change (∼2%) associated with these phase transitions. Additionally, we observe that these transitions are highly sensitive to external applied pressure, leading to substantial barocaloric effects (exceeding 20 K kbar⁻¹). Very interestingly, we obtain substantial values for reversible adiabatic temperature change (ΔT_rev > 10 K) under an applied pressure as low as 1 kbar, comparable to those observed in the most promising barocaloric materials. We prove that the unique structure and chemical bonding of these sandwich organometallic cations are responsible for their interesting and unusual barocaloric response. These findings position the metallocenium family as promising candidates for eco-friendly solid-state refrigeration technologies.