Nuclear inelastic scattering shows lattice softening induced by Zn(II) dilution of a Fe(II) spin crossover material

Abstract The complex Fe[HBpz₃]₂ with HBpz₃ being tris-pyrazolyl borate exhibits a temperature dependent spin crossover (SCO) between the low-spin (S = 0, LS) and the high-spin (S = 2, HS) state. During SCO, the increasing number of HS states influences the switching behavior of the residual LS states which is a general feature of such phase transitions in solid matrices. Since SCO is governed by phonons, we have investigated the effect of an HS Fe(II) matrix on the iron partial density of phonon states (pDOS) in Fe[HBpz₃]₂ by studying the structural analogue Zn₀.₉Fe₀.₁[HBpz₃]₂ with nuclear inelastic scattering (NIS). In this material, 90% of the iron(II) sites are replaced with zinc(II) which allows us to investigate the vibrational effect of a majority zinc(II) matrix. This represents a suitable structural model for a matrix of high-spin iron(II) sites. Our results show that the observed vibrational modes in the pDOS shift by ca. 10 cm⁻¹ towards lower frequencies for the Zn(II)-diluted complex. Analysis of the Debye region of the pDOS yields for the mean velocity of sound $$\:v_m$$ = 3.9 km·s − 1 for 57 Fe[HBpz₃]₂ and a reduction of this value by ~ 10% in the Zn(II) diluted compound Zn₀.₉Fe₀.₁[HBpz₃]₂. This observation is consistent with the action of a soft majority HS Fe(II) environment of residual LS Fe(II) centres, a situation which is modelled in our study by employing Zn(II) as a structural analogue of high‑spin Fe(II).