The bismuth ferrite BiFeO3 is promising as magnetoelectric material because both the ferroelectric and antiferromagnetic orders coexist in this material at room temperature. Doping of BiFeO3 or the substitution of bismuth by other chemical elements can modify its physical properties. The goal of this paper is to determine the influence of the bismuth substitution by gadolinium and neodymium on crystal lattice dynamics at room temperature.
The substituted solid solutions of BiFeO3 of the types Bi1-xGdxFeO3 and Bi1-xNdxFeO3 with the atomic part of the substitutive element x up to 0.20 were synthesized by means of the solid-state reaction method using powders of oxides Bi2O3, Nd2O3, Gd2O3, and Fe2O3 of pure grade quality. The X-ray diffraction method was applied using the diffractometer Dron-3 on Cu Kα radiation. The experimental data were collected during scanning repeated ten times in the 2Θ range from 20° to 90° at the scanning speed of 10°/6 min. The infrared reflection spectra of the samples in tablet powder mixtures were recorded with VERTEX 80v FT-IR spectrometer (Bruker). As standards for the recording of the absolute values of the reflection coefficient R, an aluminum mirror with R = 97% and a single crystal silicon wafer were used.
The X-ray patterns show the displacement of the positions of reflexes of BiFeO3 with the substitution of bismuth by gadolinium and neodymium. This confirms the formation of the solid solutions Bi1-xGdxFeO3 and Bi1-xNdxFeO3. The lattice constants of the formed solid solutions decrease with the increase of the content of the substitutive element Gd. This may be explained by the lower ionic radii of Gd3+ with respect to Bi3+. The small inclusion of the impurity phases of Bi25FeO39 and Bi2Fe4O9 in addition to the main phase of BiFeO3 is also indicated. Nevertheless the small presence of the impurity phases allows to make conclusions regarding physical properties of these materials, including their crystal lattice dynamics.
Two extremums at 18.2 μm (strong extremum) and 22.5 μm (rather weak extremum) on the infrared reflection spectra of the solid solutions Bi1-xGdxFeO3 and Bi1-xNdxFeO3 were discovered. The extremum at 18.2 μm is caused by the bending of the mechanical vibrations of the Fe – O bonds and the extremum at 22.5 μm is caused by the stretching of the mechanical vibrations of the Fe – O bonds. The increase of the atomic part of the substitutive element x up to 0.20 leads to the growth of the reflectivity and the absorption coefficients of the solid solutions Bi1-xGdxFeO3 and Bi1-xNdxFeO3 in comparison with BiFeO3. The displacement of the absorption band maximum into the side of the spectrum with longer wavelengths with the increase of the content of the substitutive elements was also found. Such behavior of the spectra can be explained by decreasing elastic constants of the solid solutions Bi1-xGdxFeO3 and Bi1-xNdxFeO3 occurring when the new substitution-induced bonds Gd – O and Nd – O are formed.