Crystal structure, absolute configuration and characteristic temperatures of SmFe₃(BO₃)₄ in the temperature range...

The crystal structure of samarium iron borate was analyzed with regard to growth conditions and temperature. The inclusion of about 7% Bi atoms in the crystals grown using the Bi₂Mo₃O₁₂-based flux was discovered and there were no impurities in the crystals grown using the Li₂WO₄-based flux. No pronounced structural features associated with Bi inclusion were observed. The different absolute configurations of the samples grown using both fluxes were demonstrated. Below 80 K, a negative thermal expansion of the c unit-cell parameter was found. The structure of (Sm_0.93Bi_0.07)Fe₃(BO₃)₄ belongs to the trigonal space group R32 in the temperature range 90-400 K. A decrease in the (Sm,Bi)-O, Sm-B, Sm-Fe, Fe-O, Fe-B and Fe-Fe distances is observed with a lowering of the temperature, B1-O does not change, B2-O increases slightly and the B2O₃ triangles deviate from the ab plane. The strongest decrease in the equivalent isotropic atomic displacement parameters (U_eq) with decreasing temperature is observed for atoms Sm and O2, and the weakest is observed for B1. The O2 atoms have the highest U_eq values, the most elongated atomic displacement ellipsoids of all the atoms and the smallest number of allowed vibrational modes of all the O atoms. The largest number of allowed vibrational modes and the strongest interactions with neighbouring atoms is seen for the B atoms, and the opposite is seen for the Sm atoms. The quadrupole splitting Δ(T) of the paramagnetic Mössbauer spectra increases linearly with cooling. The Néel temperature [T_N = 31.93 (5) K] was determined from the temperature dependence of the hyperfine magnetic field B_hf(T), which has a non-Brillouin character. The easy-plane long-range magnetic ordering below T_N was confirmed.