China
Africa
Explore chapters and articles related to this topic
Multiferroism and Magnetoelectric Applications in Bismuth Ferrite
Published in Jiabao Yi, Sean Li, Functional Materials and Electronics, 2018
It is widely accepted that bulk BFO has a typical perovskite structure (ABO3) with rhombohedral symmetry (space group R3c) [21–24], as shown in Figure 3.1, where Bi atoms are located at the eight corners (A-site), Fe atom at the center of the pseudocubic (pc) (B-site), and oxygen atoms at the face centers, forming an octahedron cage. However, some recent reports suggest the real symmetry of bulk BFO is in fact lower, making its crystal structure somewhat controversy [25,26]. The basic unit cell of BFO comprises two vertex-connected simple perovskite units, which can be described in either rhombohedral (arh = 5.63 A, a = 59.35°) or hexagonal (ahex = 5.58 A, chex = 13.87 A) notations [23,27]. Each perovskite unit is rhombohedrally distorted along [1 1 1]pc direction, with a lattice constant of apc = 3.965 A and a rhom- bohedral angle of ~89.3-89.5° at room temperature [23,28]. Along the threefold [1 1 1]pc direction, the Fe3+ cation is displaced from the center of the oxygen octahedron by about ~0.26 A and Bi3+ cation is shifted by ~0.67 A away from its centrosymmetric position between two octahedron centers [24,28]. Both of these off-center displacements contribute to the large spontaneous polarization along [1 1 1]pc direction. In addition to the atomic displacement, the two neighbor octahedra along the polar axis rotate by ~11-14° clockwise and counterclockwise around the same axis, respectively [23,28,29]. Oxygen octahedral rotation, sometimes referred to as the antifer- rodistortive order, is commonly observed in perovskite oxides due to the imperfect atomic packing. The oxygen octahedral rotation pattern in BFO, under Glazer’s tilt system [30], is c-c-c-, consistent with the R3c symmetry. Generally, the Goldschmidt tolerance factor [31] is used to describe how well the ionic radii match, and thus the stability of the perovskite structure. The Goldschmidt tolerance factor is given by, t=rA+rB2rB+rA $$ t = \frac{{r_{A} + r_{B} }}{{\sqrt 2 \left( {r_{B} + r_{A} } \right)}} $$
Structural, electronic and magnetic properties of the double perovskite Ba2GdNbO6 with octahedral tilting effect: first-principles calculations
Published in Philosophical Magazine, 2023
Abdelkader Khouidmi, Fatima Zohra Dahou, Hadj Baltach, Amel Laref, Mohammed El Amine Monir
This prediction is focused on the study of the physical properties of the Ba2GdNbO6 double perovskite compound. The presence of lanthanide Gd atom at b-octahedral position gives rise to various applications due to their macroscopic magnetisation and electric polarisation beyond room temperature [20]. Some of double perovskite materials have been applied in different fields, such as giant magnetoresistance (GMR), substrates for high TC superconductors [21], electrode materials in spintronic devices [22], and dielectric resonators [23]. The electronic structure analysis shows that lanthanide element Gd (5s25p64f75d16s2) has half-empty 4f sub-shell, where its oxidation is maintained at 3+ in the stable state. Both Gd and Nb cations are represented in the second class of compound [24]. The structure of Ba2GdNbO6 compound was re-examined at room temperature by X-ray diffraction [25]. According to the available experimental data, the stable structure for the equilibrium Ba2GdNbO6 compound is tetragonal with the space group I4/m (no. 87) [25–27]. This structure is derived from the primitive cubic aristotype by ordering the octahedral GdO6 and NbO6 sites, and tilting the octahedra around the principal fourfold [001]P-axis [28,29]. The tilting angle (rotation of octahedral site) in this symmetry is given by the following relation: [30]. The Goldschmidt tolerance factor for the double perovskite structure is defined as follows:, where is the averaged ionic radius of the Gd3+ and Nb5+ cations [31], widely used to assess geometric stability and distortion of crystal structures. The tolerance factor for the cubic double perovskite under the space group Fm (no. 225) is about t ≈ 1; it means that this structure has no structural distortions [1,32]. If t factor deviates from the ideal value (t ≠ 1), the crystal distortions arise and the compound adopts structures with lower symmetry. Moreover, the tolerance factor of Ba2GdNbO6 compound is reported in the range from 0.968–0.978 [33], where the tilting of octahedra around the principal fourfold [001]P-axis produces seven (7) bond lengths: two of Gd-O, two of Nb-O, and three of Ba-O.