Crystal structure of bismuth-containing NdFe3(BO3)(4) in the temperature range 20-500 K

E. S. Smirnova, O. A. Alekseeva, A. P. Dudka, I. A. Verin, V. V. Artemov, M. V. Lyubutina, I. A. Gudim, K. V. Frolov and I. S. Lyubutin// Acta Crystallographica Section B//

Neodymium iron bor­ate NdFe3(BO3)4 is an intensively studied multiferroic with high electric polarization values controlled by a magnetic field. It is char­ac­ter­ized by a large quadratic magnetoelectric effect, rigidity in the base plane and a rather strong piezoelectric effect. In this work, the atomic structure of (Nd0.91Bi0.09)Fe3(BO3)4 was studied by single-crystal X-ray diffraction in the tem­per­ature range 20–500 K (space group R32, Z = 3). The Bi atoms found in the com­position partially substitute the Nd atoms in the 3a position; they entered the structure due to the growth conditions in the presence of Bi2Mo3O12. It was shown that in the tem­per­ature range 20–500 K there is no structural phase transition R32→P3121, which occurs in rare-earth iron bor­ates (RE = Eu–Er, Y) with an effective rare-earth cation radius smaller than that of Nd. The tem­per­ature dependence of the unit-cell c parameter reveals a slight increase on cooling below 90 K, which is similar to the results obtained previously for iron bor­ates of Gd, Y and Ho. The atomic distances (Nd,Bi)—O, (Nd,Bi)—B, (Nd,Bi)—Fe, Fe—O, Fe—B and Fe—Fe in the iron chains and between chains decrease steadily with decreasing tem­per­ature from 500 to 90 K, whereas the B1(3b)—O distance does not change and the average B2(9e)—O distance increases slightly. There is a uniform decrease in the atomic displacement parameters with decreasing tem­per­ature, with a more pronounced decrease for the Nd(3a) and O2(9e) atoms. The O2(9e) atoms are characterized by the maximum atomic displacement parameters and the most elongated atomic displacement ellipsoids. The characteristic Debye and Einstein tem­per­atures, and the static com­ponent in the atomic displacements were determined for cations using multi-tem­per­ature diffraction data. It was shown that the Nd cations have the weakest bonds with the surrounding atoms and the B cations have the strongest.