The two crystallographic modifications of ferric fluoride trihydrate (agr;‐ and bgr;‐FeF3⋅3H2O) were synthesized and studied by magnetic and Mo¨ssbauer techniques. The two forms could be distinguished by their different Mo¨ssbauer parameters. The magnetic susceptibility versus temperature curves also showed distinct differences. The susceptibility decreased monotonically with increasing temperature for the agr; form. For the bgr; form, however, the susceptibility decreased initially and then went through a broad maximum around 130 K. The agr; form was found to convert to the bgr; form after a year. The broad maximum in the susceptibility versus temperature curve observed for bgr;‐FeF3⋅3H2O is explained in terms of cooperative interaction involving relatively small number of Fe3+ions. Postulates concerning the nature of the short‐range antiferromagnetic ordering within ‘‘clusters’’ of Fe3+ions are presented.
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