The Phase Transformation in Ferreous Fluosilicate Hexahydrate

The structural change of FeSiF 6 , . 6 H 2 O at T , = 225 K (P2 1 /C to P3ml above T e ) is well known from X‐ray measurements. The temperature dependence of the Mössbauer parameters in the neighbourhood of the phase transformation (200 K ≦ T ≦ 250 K) is studied. At T e , the quadru‐pole splitting Δ E q changes with increasing temperature by (+0.009 ± 0.002) mm/s, the Lamb factor f slightly decreases by δ f = −0.011 ±0.004, while the anisotropy of f changes remarkably: δ( f ∥ ‐ f⊥) = −0.18. From the relaxation broadening of the absorption lines below and above T e , it is concluded that the non‐axial terms of the ligand field at the Fe 2+ ion do not vanish above the transition temperature.