Mössbauer effect and x-ray studies of the phase transition in iron hexammine salts

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Abstract
The Mössbauer effect of 57Fe has been studied between 1.6 and 300 K in iron hexammine salts Fe(NH3)6X2, where X = Cl, Br, I, ClO4, BF4. A phase transition has been inferred from the sudden splitting of the single line (observed at room temperature) into a quadrupole doublet below a critical temperature, Tc. A static distortion of the lattice is confirmed by x−ray determination of a monoclinic structure of Fe(NH3)6Cl2 in the low temperature phase. The strength of the Mössbauer resonant absorption was determined as a function of temperature. The measurements show an abrupt decrease at Tc in passing from the low to the high temperature phase. The small value of the latter is attributed to the scattering of acoustic phonons by the NH3 liberons. The temperature dependences of the quadrupole interactions below Tc cannot be described by conventional theories and suggest temperature dependent crystal fields. These may arise through tilting oscillations of the NH3 groups in the low temperature phase. This behavior in the low symmetry phase, together with the general features of the transition itself (hysteresis, coexistence of the two forms without line broadening) leads to the conclusion that the transition is of second order but turns to first order before reaching the second order critical point.