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 cr ystals is factorized into a molecular and a lattice part: f = f(m)f(1) . In the case of spincrossover compounds there is the unique possibili ty to measure differences of both parts in the two spin states by swit ching the spin states with the LIESST effect. Measurements of the DWF depending on temperature, gamma-ray direction and spin state were perf ormed with Fe-57-Mossbauer spectroscopy on three single crystals (R (3 ) over bar) of the spincrossover system [FexZn1-x(ptz)(6)](BF4)(2) (pt z = 1-propyltetrazole), a concentrated sample with x = 1 and diluted o nes 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(p arallel 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 di luted and the concentrated samples were measured at low temperatures ( less than or equal to 50 K). From these data both the Gruneisen consta nt of the lattice gamma(G) = 2.9 and the lowest IR active intramolecul ar frequency (45 cm(-1) (HS), 58 cm(-1) (LS)) have been determined. Th e contribution of the DWF from the acoustic modes of the lattice is ca lculated from the elastic constants of rhombohedral [FexZn1-x(ptz)(6)] (BF4)(2). The contribution of the optical lattice vibrations is estima ted. The influence of different DWF in the two spin states on the eval uation of the fraction of molecules in either spin state from Mossbaue r area data is discussed.