Crystal structure and structural phase transition in bismuth-containing HoFe₃(BO₃)₄ in the temperature range 11-500 K

An accurate single-crystal X-ray diffraction study of bismuth-containing HoFe₃(BO₃)₄ between 11 and 500 K has revealed structural phase transition at T_str = 365 K. The Bi atoms enter the composition from Bi₂Mo₃O₁₂-based flux during crystal growth and significantly affect T_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 below T_str the (Ho_0.96Bi_0.04)Fe₃(BO₃)₄ crystal symmetry is trigonal, of space group P3₁21, whereas at high temperature (HT) above 365 K the symmetry increases to space group R32. There is a sharp jump of oxygen O1 (LT) and O2 (LT) atomic displacement parameters (ADP) at T_str. O1 and O2 ADP ellipsoids are the most elongated over 90-500 K. In space group R32 specific distances decrease steadily or do not change with decreasing temperature. In space group P3₁21 the distortion of the polyhedra Ho(Bi)O₆, Fe1O₆ and Fe2O₆, B2O₃ and B3O₃ increases with decreasing temperature, whereas the triangles B1O₃ remain almost equilateral. All BO₃ triangles deviate from the ab plane 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 FeO₆ octahedra 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.