Further studies on the spin cross-over phenomenon in di-isothiocyanatobis(1,10-phenanthroline)iron(II)

Abstract

Some inconsistencies in the results of earlier Mössbauer spectroscopic and magnetic susceptibility measurements on [Fe(phen)₂(NCS)₂](phen = 1,10-phenanthroline) demanded a systematic reinvestigation of this system. We have prepared the complex by two different methods: precipitation in methanol (A) and extraction from [Fe(phen)₃][NCS]₂ with acetone (B). The two samples (A and B) differ in crystal size and quality, which markedly influences the spin-transition behaviour. Mössbauer and magnetic susceptibility measurements down to ca. 4 K show that spin conversion is gradual in sample A but rather abrupt in sample B. This is supported by differential thermal analysis measurements. There is ‘residual paramagnetism’ of ca. 12% in sample A, but none in sample B. A sharp fall in the Mössbauer resonance line width and a distinct irregularity in the quadrupole splitting of the high-spin state near T_c suggests that a structural change occurs as the system undergoes spin cross-over. Temperature-dependent changes in the X-ray diffraction patterns support this suggestion.