THE DEBYE-WALLER FACTOR IN SPINCROSSOVER MOLECULAR-CRYSTALS - A MOSSBAUER 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 = f(m)f(1). 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, gamma-ray direction and spin state were performed with Fe-57-Mossbauer spectroscopy on three single crystals (R (3) over bar) of the spincrossover 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 similar to 100 and its anisotropy f(perpendicular to) f(parallel to) increases from similar to 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 (less than or equal to 50 K). From these data both the Gruneisen constant of the lattice gamma(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 [FexZn1-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 Mossbauer area data is discussed.