Evidence for differentiation in the iron-helicoidal chain in GdFe3(BO3)4

A single-crystal X-ray structure study of gadolinium triiron tetraborate, GdFe3(BO3)(4), at room temperature and at 90 K is reported. At room temperature GdFe3(BO3)(4) crystallizes in a trigonal space group, R32 ( No. 155), the same as found for other members of the iron borate family RFe3(BO3)(4). At 90 K the structure of GdFe3(BO3)(4) transforms to the space group P3(1)21 (No. 152). The low-temperature structure determination gives new insight into the weakly first-order structural phase transition at 156 K and into the related Raman phonon anomalies. The presence of two inequivalent iron chains in the low-temperature structure provides a new perspective on the interpretation of the low-temperature magnetic properties.