Evidence for differentiation in the iron-helicoidal-chain in GdFe$_{3}$(BO$_{3}$)$_{4}$

We report on a single-crystal X-ray structure study of $GdFe_{3}(BO_{3})_{4}$ at room temperature and at T=90 K. At room temperature $GdFe_{3}(BO_{3})_{4}$ crystallizes in a trigonal space group R32 (No. 155), the same as found for other members of iron-borate family $RFe_{3}(BO_{3})_{4}$. At 90 K the structure of $GdFe_{3}(BO_{3})_{4}$ has transformed to the space group $P3_{1}2_{1}$ (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 discovery of two inequivalent iron chains in the low temperature structure provide new point of view on the low-temperature magnetic properties.