Nuclear γ-resonance in iron sulphates of the jarosite group

Mössbauer spectroscopy was used to study the temperature dependence (4.2 to 77 K) of the hyperfine structure (hfs) in jarosites H3OFe3[(OH)6(SO4)2], Pb{Fe3[(OH)6(SO4)2]}2, and K(Fe2.5Al0.5)[(OH)6(SO4)2]. For the two first compounds the magnetic hfs collapsed above the Néel temperatures, 17.5 and 50 K, respectively, to a paramagnetic quadrupole doublet. In the third compound, at all temperatures in the range from TN=54 K down to 4.2 K the coexistence of the magnetic hfs and paramagnetic quadrupole doublet appeared. In this paper, a qualitative explanation of the newly observed phenomenon is given.