The debye-waller factor in spincrossover molecular crystals: a mössbauer study on [Fe_xZn_1−x(ptz)₆](BF₄)₂

Abstract

In a first approximation the Debye-Waller factor (DWF) of molecular crystals is factorized into a molecular and a lattice part:f=f _m f _l. In the case of spincrossover compounds there is the unique possibility to measure differences of both parts in the two spin states by switching the spin states with the LIESST effect. Measurements of the DWF depending on temperature, γ-ray direction and spin state were performed with⁵⁷Fe-Mössbauer spectroscopy on three single crystals (R¯3) of the spincrossover system [Fe_xZn_1−x(ptz)₆](BF₄)₂ (ptz=1-propyltetrazole), a concentrated sample withx=1 and diluted ones withx=0.30 andx=0.005. The DWF decreases from 10 to 300 K by a factor of ∼ 100 and its anisotropyf _{⊥/f ∥} increases from ∼ 1 to 5.4. The differences of the DWF in the low spin and the high spin state (after LIESST) for the diluted and the concentrated samples were measured at low temperatures (≤ 50 K). From these data both the Grüneisen constant of the lattice γ_G=2.9 and the lowest IR active intramolecular frequency (45 cm⁻¹ (HS), 58 cm⁻¹ (LS)) have been determined. The contribution of the DWF from the acoustic modes of the lattice is calculated from the elastic constants of rhombohedral [Fe _x Zn_1−x (ptz)₆](BF₄)₂. The contribution of the optical lattice vibrations is estimated. The influence of different DWF in the two spin states on the evaluation of the fraction of molecules in either spin state from Mössbauer area data is discussed.