Crystal structure and structural phase transition in bis­muth-containing HoFe 3 (BO 3) 4 in the temperature range 11-500 K

An accurate single-crystal X-ray diffraction study of bismuth-containing HoFe₃(BO₃)₄between 11 and 500 K has revealed structural phase transition atT_str= 365 K. The Bi atoms enter the composition from Bi₂Mo₃O₁₂-based flux during crystal growth and significantly affectT_str. The content of Bi was estimated by two independent methods, establishing the composition as (Ho_0.96Bi_0.04)Fe₃(BO₃)₄. In the low-temperature (LT) phase belowT_strthe (Ho_0.96Bi_0.04)Fe₃(BO₃)₄crystal symmetry is trigonal, of space groupP3₁21, whereas at high temperature (HT) above 365 K the symmetry increases to space groupR32. There is a sharp jump of oxygen O1 (LT) and O2 (LT) atomic displacement parameters (ADP) atT_str. O1 and O2 ADP ellipsoids are the most elongated over 90–500 K. In space groupR32 specific distances decrease steadily or do not change with decreasing temperature. In space groupP3₁21 the distortion of the polyhedra Ho(Bi)O₆, Fe1O₆and Fe2O₆, B2O₃and B3O₃increases with decreasing temperature, whereas the triangles B1O₃remain almost equilateral. All BO₃triangles deviate from theabplane with decreasing temperature. Fe–Fe distances in Fe1 chains decrease, while distances in Fe2 chains increase with decreasing temperature. The Mössbauer study confirms that the FeO₆octahedra undergo complex dynamic distortions. However, all observed distortions are rather small, and the general change in symmetry during the structural phase transition has very little influence on the local environment of iron in oxygen octahedra. The Mössbauer spectra do not distinguish two structurally different Fe1 and Fe2 positions in the LT phase. The characteristic temperatures of cation thermal vibrations were calculated using X-ray diffraction and Mössbauer data.