First principles calculation of a large variation in dielectric tensor through the spin crossover in the CsFe[Cr(CN)₆] Prussian blue analogue.

Abstract

The dielectric response of spin-crossover (SCO) materials is a key property facilitating their use in next-generation information processing technologies. Solid state hybrid density functional theory calculations show that the temperature-induced and strongly hysteretic SCO transition in the Cs(+)Fe(2+)[Cr(3+)(CN(-))6] Prussian blue analogue (PBA) is associated with a large change (Δ) in both the static, Δɛ(0)(HS - LS), and high frequency, Δɛ(∞)(HS - LS) dielectric constants. The SCO-induced variation in CsFe[Cr(CN)6] is significantly greater than the experimental Δɛ values observed previously in other SCO materials. The phonon contribution, Δɛ(phon)(HS - LS), determined within a lattice dynamics approach, dominates over the clamped nuclei term, Δɛ(∞)(HS - LS), and is in turn dominated by the low-frequency translational motions of Cs(+) cations within the cubic voids of the Fe[Cr(CN)6](-) framework. The Cs(+) translational modes couple strongly to the large unit cell volume change occurring through the SCO transition. PBAs and associated metal-organic frameworks emerge as a potentially fruitful class of materials in which to search for SCO transitions associated with large changes in dielectric response and other macroscopic properties.