Structure and Lattice Dynamics of Bi_1−xNd_xFeO₃ and Bi_1−xGd_xFeO₃ Ceramics near the Morphotropic Phase Boundary

Abstract

The crystal structures of Bi1−xNdxFeO3 and Bi1−xGdxFeO3 solid solutions (0 ≤ x ≤ 0.2) with chemical compositions across structural transformations from the polar rhombohedral phase to the orthorhombic phase with an antipolar distortion and then to the nonpolar orthorhombic phase have been investigated using X-ray diffraction and infrared reflective spectrometry. The obtained results clarify details of the structural transitions assuming the changes that occurred in the crystal lattice dynamics of the compounds. Increase in the dopant content causes a notable change in the intensity and position of the reflectance lines at 18.2 μm and 22.6 μm (550 cm−1 and 440 cm−1) ascribed to the transverse optical phonon modes associated with Bi (Nd, Gd)–O and Fe–O bonds. In the concentration region attributed to the dominant rhombohedral phase, the chemical substitution leads to an increase in intensity of the modes A1 for solid solutions of both systems. Meanwhile, in the case of Gd doping, the mode A1 shifts towards the red side of the spectrum, but there is an opposite tendency in the case of Nd doping; the intensity of the modes E decrease regardless of both the dopant-ion type and concentration. This behavior is discussed assuming the change in mass for the chain of chemical bonds caused by different dopant ions and the structural transformations occurring in the compounds upon chemical doping.

Keywords

• multiferroics
• crystal structure
• structural transitions
• X-ray neutron diffraction
• Raman spectroscopy