Research Article |
Corresponding author: Dmitry V. Karpinsky ( dmitry.karpinsky@gmail.com ) © 2023 Maxim V. Silibin, Polina A. Sklyar, Vadim D. Zhivulko, Sergey I. Latushko, Dmitry V. Zheludkevich, Dmitry V. Karpinsky.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Silibin MV, Sklyar PA, Zhivulko VD, Latushko SI, Zheludkevich DV, Karpinsky DV (2023) Crystalline structure of 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions in the vicinity of the morphotropic phase boundary. Modern Electronic Materials 9(4): 193-200. https://doi.org/10.3897/j.moem.9.4.116620
|
Complex transition metal oxides are distinguished for a close interrelation between their type of crystal structure and electrical and magnetic properties, thus determining their practical importance. Bismuth ferrite based solid solutions contain simultaneously both dipole electric and magnetic ordering thus expanding their potential applications as external impact sensors. The sensitivity of these compositions to external fields is largely dependent on their structural state. 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions (0 ≤ x ≤ 1) the compositions of which are close to the rhombohedral/cubic morphotropic phase boundary have metastable structures and are therefore promising functional materials. The crystal structure and morphology of 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions has been studied using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and energy dispersive X-ray spectroscopy. The chemical substitution of barium ions for strontium ones has been found to reduce the magnitude of rhombohedral distortions and decrease the unit cell parameters for all the substituted compounds. Solid solutions with x ≥ 0,25 have single-phase structure and cubic unit cells, their grain size decreasing with an increase in the concentration of the substituting ions. The results of structural studies obtained using Raman spectroscopy suggest the presence of rhombohedral distortions in the structures of all the compositions studied. The results of structural studies have allowed identifying the sequence of changes in the phase state and lattice parameter of the compounds in the vicinity of the rhombohedral/cubic morphotropic phase boundary. The concentration ranges in which the compounds have single-phase and two-phase structures have been found. The concentration stability range of the polar rhombohedral phase has been corrected on the basis of the structural data obtained using local and microscopic methods.
bismuth ferrite, multiferroics, X-ray diffraction, electron microscopy, structural phase transitions, morphotropic phase boundary
In the last decade multiferroics raise growing interest of researchers in the field of new functional materials. Multiferroics are well-known due to the possibility of controlling their properties originating from a close interrelation between their electric and magnetic subsystems, allowing to develop new materials offering new opportunities of practical applications [
Bismuth ferrite (BiFeO3) based solid solutions are the most renowned and promising single-phase multiferroics because their transition to magnetically ordered and ferroelectric states occurs at high temperatures: the Neel temperature TN ~ 650 K and the Curie temperature TC ~ 1100 K [
The magnitude of the magnetoelectric effect in bismuth ferrite based multiferroics is but moderate (~10–100 mV/cm/Oe) as compared with that for composites [
The available literary works dealing with the structural phase transitions in the BiFeO3–Ba(Sr)TiO3 systems often provide contradictory data on the concentration stability ranges of the coexisting structural phases. Furthermore, there are no convincing experimental data on the ratio between the magnetically active and ferroelectric subsystems in those solid solutions and on the formation conditions of metastable structural states in compounds close to the morphotropic phase boundaries. Presented below are data on the rhombohedral-to-cubic structural phase transition in the 0.65BiFeO3–0.3Ba1-xSrxTiO3 system of solid solutions (0 ≤ x ≤ 1). The data broaden the understanding of the structure of the bismuth ferrite based solid solutions and will favor the development and synthesis of new functional materials exhibiting magnetoelectric interaction.
Ceramic specimens of the 0.65BiFeO3–0.3Ba1-xSrxTiO3 system (x = 0, 0.15, 0.25, 0.5, 0.75 and 1) were synthesized using the method of solid phase reactions. The raw materials were high-purity oxides and carbonates BaСO3, SrСO3, Bi2O3, Fe2O3 and La2O3. The mixture of oxides taken in the stoichiometric ratio was crushed for 60 min in an ethyl alcohol media in a RETSCH 200 PM planetary mill. The resultant powders were uniaxially pressed at 0.1 GPa to 10 mm diam. 1–2 mm thick tablets. After intermediate crushing and compressing the specimens were synthesized for 10 h at 1000–1050 °C with a gradual increase in the synthesis temperature and an increase in the concentration of the dopant ions. The specimens were then quenched from the synthesis temperature to room temperature at a 100–200 K/s cooling rate.
X-ray phase and structural studies were conducted using an Adani PowDiX 600 X-ray diffractometer (CuKα radiation, wavelength λ = 0.15406 nm) at room temperature. The Kβ component was cut off with a graphite monochromator. The spectra were recorded in the Bragg–Brentano setup. The X-ray patterns were taken in the 20 to 60 deg. 2θ range with a 0.02 deg. scanning step. The X-ray data were analyzed using the Rietveld method (full-profile analysis). The spectra were refined with the FullProf software.
The grain morphology was studied under a Zeiss Evo 10 scanning electron microscope and the elemental composition of the specimens was analyzed using energy dispersive X-ray spectroscopy on an Oxford Instruments EDS attachment. The Raman spectra were recorded on a Confotec MR350 spectrometer (SOL Instruments, Belarus) with 532 nm excitation radiation.
Analysis of the X-ray diffraction patterns (Fig.
Room-temperature X-ray diffraction pattern of 0.65BiFeO3–0.35BaTiO3 solid solution composition (x = 0) refined in the two-phase model (space groups R3c and Pmm). Inset: X-ray diffraction patterns showing reflections typical of (C) cubic and (R) rhombohedral phases with compositions 0.65BiFeO3–0.35Ba1-xSrxTiO3 where x = 0, 0.25, 0.5, 0.75 and 1.0
The change of the lattice parameters (Fig.
The grain structure of the solid solutions was studied using scanning electron microscopy (SEM). Figure
Elemental analysis of the 0.65BiFeO3–0.35Ba1-xSrxTiO3 system of solid solutions with x = 0÷1 using energy dispersion X-ray spectroscopy suggests the absence of impurity phases, with the chemical composition of the specimens changing in accordance with the substitution model used. The presence of small quantities of the C and Al elements is caused by the experimental setup chosen (the specimens were attached to aluminum holders with carbon-containing tape). The concentration ratio of the main chemical elements confirms the rated chemical formulas for the 0.65BiFeO3–0.35Ba1-xSrxTiO3 system (data on the quantities of elements in the solid solutions are shown in insets of Fig.
SEM images of 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions: (a) x = 0.25; (b) 0.5; (c) 0.75; (d) 1.0
Energy dispersion X-ray spectra of 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions: (a) x = 0.25; (b) 0.5; (c) 0.75; (d) 1.0. Insets show elemental compositions of specimens
x 0.65BiFeO3–0.35Ba1- x SrxTiO3
0.17 Bi0.579Ba0.230Sr0.062Fe0.579Ti0.335O3
0.39 Bi0.584Ba0.175Sr0.136Fe0.614Ti0.355O3
0.67 Bi0.546Ba0.071Sr0.235Fe0.518Ti0.339O3
0.86 Bi0.617Sr0.301Fe0.671Ti0.350O3
These formulas suggest a over-stoichiometric concentration of oxygen anions due to a deficiency of cations in the A and B perovskite sublattices, in agreement with literary data on the chemical composition of the BiFeO3–BaTiO3 solid solutions [
The X-ray diffraction data on the crystalline structure for solid solutions of different compositions were added with Raman spectroscopy data. The Raman spectroscopy data delivered additional information on the structure of the compounds on a local level (nanometers) and thus allowed assessing the degree and type of structural distortions in the oxygen octahedra, as well as the lengths and angles of the Bi(Ba,Sr)–O and Fe(Ti)–O chemical bonds.
It is well-known that the Raman spectra of solid solutions having a cubic perovskite structure do not contain first-order Raman modes. The data obtained in this work suggest the presence of active Raman modes and diffuse bands in the spectra, possibly due to second and higher orders Raman scattering (Fig.
Raman spectra of 0.65BiFeO3–0.35Ba1-xSrxTiO3 solid solutions: (1) x = 0; (2) 0.25; (3) 0.5; (4) 0.75; (5) 1.0
Note that the initial BiFeO3 composition with rhombohedral unit cell distortions exhibits typical active E(1TO), E(2TO), A1(1TO) and A1(2TO) Raman modes at 77, 136, 142 and 170 cm-1, respectively, caused by bismuth ion oscillations in the oxygen cubic octahedra [
The presence of the diffuse bands at 275, 510, 650 and 1320 cm-1 can be caused either by two-phonon scattering processes or by oxygen anion tension-compression oscillation modes.
An increase in the concentration of strontium ions does not cause any significant changes to the main bands in the Raman spectra, this suggesting that the main structural state of the test solid solutions persists. With an increase in the concentration of strontium ions, the A(1TO) band shifts towards higher frequencies, indicating a change in the type of the Bi(Ba,Sr)–O bonds due to barium ion substitution for lighter strontium ions. Noteworthy, the uneven broadening of diffraction peaks observed in the X-ray diffraction patterns agrees with the structural model assuming that a matrix having a paraelectric cubic structure can contain nanosized clusters with rhombohedral unit cell distortions.
0.65BiFeO3–0.35Ba1-xSrxTiO3 system solid solutions with x = 0, 0.15, 0.25, 0.5, 0.75 and 1 corresponding to the region in the vicinity of the rhombohedral/cubic morphotropic phase boundary were synthesized using the method of solid phase reactions. The crystal structure and morphology of the solid solutions were studied using microscopic and local methods. The substitution of barium ions for strontium ones reduces the degree of rhombohedral distortions and leads to a decrease in the lattice parameters. It was shown that the solid solutions with x = 0 and 0.15 are in a two-phase structural state in which a polar rhombohedral structure and a cubic structure coexist, and the solid solutions with x = 0.25 are single-phase with a perovskite cubic structure. An increase in the concentration of strontium ions to x = 0.5 reduces the average grain size but slightly, while for the compounds with higher strontium ion concentrations the average grain size decreases much faster due to a lower chemical activity of strontium ions compared with those of bismuth and barium ions and an increase in the number of oxygen vacancies in the solid solutions with x > 0.5 which reduces grain growth during synthesis.
For all the test compounds Raman spectroscopy showed the presence of local distortions typical of compounds with a rhombohedral structure. The X-ray diffraction and Raman scattering structural data indicate the formation of inhomogeneous structural states in the solid solutions, i.e., the main matrix having a paraelectric cubic structure contains polar-active nanosized clusters with rhombohedral unit cell distortions.
The work was carried out with support of the Russian Science Foundation (grant No. 23-19-00347); the electron microscopic experiments were conducted with support from the Belarus Republican Foundation for Fundamental Research.