X-ray study on phase transitions of ferroelectric iron iodine boracite Fe3B7O13I at low temperatures

The accurate lattice parameters of iron iodine boracite, Fe3B7O13I, in the low-temperature range between −40 and −120 °C have been successfully determined using a special X-ray diffraction technique. In order to investigate the symmetry change occurring in this temperature range Weissenberg diagrams were also studied. It has been found that ferroelectric Fe3B7O13I exhibits two kinds of monoclinic C and rhombohedral C phases, in addition to the well-known orthorhombic C phase. The two kinds of monoclinic modifications, which are stable in the narrow temperature regions between the orthorhombic and rhombohedral phases, appear separately depending on history of temperature change of the crystal: one phase arises between −70 and −82 °C with decreasing temperature, the other between −55 and −68 °C with increasing temperature. They have almost similar crystal structures but possess slightly different lattice parameters, thermal expansion coefficients, and optical birefringences. The deviation of the axial angle β from 90° in the monoclinic phases is less than half a minute. The deviation of the rhombohedral axial angle γ is also extremely small, the difference from 60° is 48″ at −120 °C. A simple model is proposed to explain the reason why the two monoclinic phases must bridge the orthorhombic and rhombohedral phases.