The Debye-Waller factor in spincrossover molecular crystals: a Moessbauer study on [FexZn1-x(ptz)6](BF4)2

In a first approximation the Debye-Waller factor (DWF) of molecular crystals is factorized into a molecular and a lattice part: f = fmfl. 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 with 57Fe-Moessbauer spectroscopy on three single crystals (R anti 3) of the spincroosover system [FexZn1-x(ptz)6] (BF4)2 (ptz = 1-propyltetrazole), a concentrated sample with x = 1 and diluted ones with x 0.30 and x = 0.005. The DWF decreases from 10 to 300 K by a factor of ∼ 100 and its anisotropy f perpendicularto /f parallel 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 Grueneisen constant of the lattice γG= 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 FexZn1-x(ptz)6z(BF4)2. The contribution of the optical lattice vibration 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 Moessbauer area data is discussed. (orig.)