Structural and magnetic phase transitions in Eu _{1– <i>x</i>} La _<i>x</i> Fe ₃ (BO ₃ ) ₄ ( <i>x</i>  = 0, 0.18)

The crystal structures and hyperfine magnetic parameters of EuFe₃(BO₃)₄ and mixed Eu_0.82La_0.18Fe₃(BO₃)₄ were studied over a wide temperature range in order to analyze correlations of the structural and magnetic features and the phase transitions in multiferroic compounds of the rare-earth iron borate family. The chemical compositions of the crystals are reported from X-ray fluorescence analysis. The crystal structures of EuFe₃(BO₃)₄ and Eu_0.82La_0.18Fe₃(BO₃)₄ were determined using single-crystal X-ray diffraction in the temperature range 25-500 K. A structural phase transition is observed in EuFe₃(BO₃)₄ below 89 K which is related to distortions in the interatomic distances and angles. The most significant of which are for R-O, R-B, R-Fe, Fe-O and Fe-Fe distances, and the angles between the BO₃ triangles and the ab plane. There is no structural phase transition in lanthanum-doped EuFe₃(BO₃)₄ based on specific heat measurements (2.2-101.3 K) and structure analysis (25-500 K), and the temperature dependences of the interatomic distances and angles are smooth. The lengths of the superexchange paths needed for the appearance of a structural phase transition in RFe₃(BO₃)₄ have been proposed. Negative thermal expansion is observed for both compounds below 90 K, resulting from a growth of the interatomic Fe-Fe distances in the iron chains during cooling. The largest atomic displacement parameters are observed for O atoms (O2), indicating that they have the highest mobility. The magnetic properties of EuFe₃(BO₃)₄ and Eu_0.82La_0.18Fe₃(BO₃)₄ were analyzed using Mössbauer spectroscopy in the temperature range 4.5-298 K. Néel temperatures (T_N) of 34.57 (1) and 32.22 (1) K are obtained based on Mössbauer spectroscopy for the pure and doped crystals, respectively. The maximum specific heat capacity temperature dependence related to the magnetic phase transition for the doped crystal is observed at 31.2 K. A violation of the strict arrangement of antiferromagnetic ordering in the ab plane in the La-doped crystals at low temperatures is suggested. The magnetic contributions of the two structural positions of the iron ions to the Mössbauer spectra could not be distinguished in either pure and doped compounds, regardless of whether they are in the paramagnetic and antiferromagnetic regions.