Elsevier

Journal of Alloys and Compounds

Volume 688, Part A, 15 December 2016, Pages 1151-1156
Journal of Alloys and Compounds

Chemical pressure induced red shift in band gap and d-d transition energies in Sr doped BiFeO3

https://doi.org/10.1016/j.jallcom.2016.07.158Get rights and content

Highlights

  • Chemical pressure effect on the optical properties of Bi1−xSrxFeO3 (0 ≤ x ≤ 0.45) samples has been studied.

  • Comparison in d-d transitions energies on divalent doped samples have been made with decrease and increase in chemical pressure.

  • A red shift in d-d transition energies have been observed with increase in Sr content.

  • Direct and indirect band gap decreases with increase in Sr content.

  • Decrease in band gap is attributed to the increase in chemical pressure.

Abstract

The optical response of multiferroic Bi1xSrxFeO3 (0 ≤ x ≤ 0.45) samples is studied in the spectral range from 1 eV to 4 eV. Optical response in the studied spectral range were dominated by two charge transfer transitions and two doubly degenerate d-d transitions (6A1g  4T2g and 6A1g  4T2g) for all samples. The d-d transitions were found to weaken as the Sr content was increased which may be due to the transformation of the crystal structure from rhombohedral to cubic. Moreover, a red shift in the d-d transition energies were observed as the Sr ion substitutes the BFO host lattice, which indicates the variation in the crystal field strength. The direct band gap was found to decrease from 1.99 eV to 1.89 eV, whereas the indirect band gap decreased from 1.40 to 0.49 eV as the Sr content is increased to x = 0.45. The decrease in the energy band gap values and red shift in d-d transition energies is ascribed to the chemical pressure induced reduction in unit cell volume. The results indicate a promising ability for tuning the BFO band gap to enable functionality as a multiferroic photovoltaic material.

Introduction

Ferroelectric (FE) materials have received attention in recent years as potential photovoltaic (PV) materials. FE materials based solar cells provide an alternative driving force –the polarization induced internal electric field-to separate photon excited charge carriers (e–h pairs) [1], [2]. Among such FE materials with potential for photovoltaic applications, multiferroic BiFeO3 (BFO) has shown promise with a comparatively small band gap compared to other oxides and exhibits a large open circuit voltage [3]. Its ferroelectric transition temperature is ∼830 °C while the antiferromagnetic Néel temperature is ∼370 °C [4], [5]. The multiferroic and optoelectronic properties of BFO make it a potential candidate for next generation ferroelectric random access memories, magnetic sensors, and photovoltaics [6], [7]. However, despite of the interest in BFO, several fundamental properties for this system remain controversial; most significantly there is still debate on the nature and size of the band gap. A number of recent theoretical studies have focused on this particular issue. Neaton et al. [8] studied the spontaneous polarization in BFO using local spin density approximation (LSDA+U) and they extracted a band gap of ∼1.9 eV. Clark et al. [9] employed the screened exchange density functional theory approximation to calculate the band gap and their estimated value was ∼2.8 eV. Both approximations yielded a slightly indirect band gap. Experimental studies have reported a band gap variously in the range between 1.9 eV and 2.8 eV at room temperature [9], [10], [11], [12]. According to some authors this band gap is direct [10], [13], although other reports suggest also the presence of an indirect band gap roughly 0.4–1.0 eV smaller than the direct one [11], [14].

The correct determination and tailoring of the band gap of BFO thus acquires special significance in photovoltaic applications. It is generally accepted that a band gap of 1.4 eV is most suitable for photovoltaic applications [15], [16], since about 93% of sunlight energy is concentrated in the visible and infrared range with wavelengths in the range of 350–4000 nm (3.18–0.31eV) [17]. Thus, if the band gap of BFO is 2.67 eV, then photons with λ >464 nm (which account for the 85% of the total energy) cannot be absorbed. Hence to improve the optical absorption of BFO, its band gap needs to be narrowed. A suitably narrower band gap will allow more energy to be absorbed in turn enabling more e-h pairs to be produced enhancing the photovoltaic performance. It has been suggested that the band gap can be controlled in an effective way by introducing strains. Strain can be produced by external stresses, such as uniaxial and hydrostatic pressure or by internal stresses, such as chemical pressure or lattice mismatch [18], [19], [20], [21].

Typically chemical pressure is developed in these systems with the substitution of parent elements of BFO by suitable elements of smaller size that produce shrinkage of unit cell volume and thereby affects the band gap as well as the multiferroic properties. Ramachandran et al. [22] studied the effect of chemical pressure on the optical properties, Raman modes and magnetic properties of multiferroic BFO. They found a red shift in the in the values of band gap and d-d transition bands of doped BFO samples and ascribed it to the dopant induced reduction of the unit cell volume. Arora et al. [23] also studied the optical properties of Ce doped BFO nanoparticles. They observed a small shift in d-d and charge transfer (CT) transition bands. Moreover they found a prominent red shift in the band gap which is ascribed to a significant change in the band structure of the doped nanoparticles. They attributed the reduction in band gap to the doping induced changes in internal chemical pressure as a result of change in FeO6 octahedral unit cell. Mocherla et al. [24] demonstrated that oxygen vacancies produced by aliovalent doping in BFO and associated structural changes due to oxygen vacancy ordering result in systematic alteration of the band gap. They also observed systematic variations in the relative intensities and peak positions of Fe d-d transition in Ca doped BFO. However producing band gap changes by introducing dopants generally also leads to inter-band electronic states that affect the conductivity mechanisms and intrinsic leakage current in multiferroics. Hence a clearer understanding of in-gap states and their changes with varying chemical pressure is required. In this view a better understanding of mechanisms governing the optical response in BFO can enable researchers to engineer band gap and conductivity to enhance the photo-ferroelectric properties [15]. With this view we aimed to reduce the band gap of BFO by chemical pressure induced reduction in unit cell volume by partial substitution of the parent Bi atom with the di-valent alkaline earth Sr, which is well known to substitute Bi in this system [25], [26]. However due to the different valence states between Sr and Bi ions, the oxygen vacancy and/or the hole doping into Fe ions should affect the electronic structure of compounds in addition to the chemical pressure effect, which is not considered in the current manuscript.

Section snippets

Experimental

Bi1-xSrxFeO3 (0 ≤ x ≤ 0.45) samples were synthesized by a rapid two stage solid state reaction method using the high purity starting oxides of Bi2O3, Fe2O3, and SrCO3. The atomic fraction of Sr, x, in the doped compositions was 0.15, 0.25, 0.35, and 0.45, respectively. Pure BFO sample was calcined at 1063 K for 30 min and sintered at 1073 K for 15 min. All the divalent doped samples were calcined at 1123 K for 30 min and sintered at 1263 K for 15 min. The detail of the synthesis procedure can

Results and discussion

From the XRD analysis of Bi1-xSrxFeO3 (0 ≤ x ≤ 0.45) samples (ref 25), we found that the pure BFO crystalized in rhombohedral structure which transformed to the cubic structure at x = 0.25 and onwards. For x = 0.15 both rhombohedral and cubic phases co-exist [25]. The X-ray diffraction pattern of pure BFO can be refined primarily using non-centrosymmetric R3c symmetry with lattice parameters a = b ∼5.5812(32) Å and c ∼13.8646(13) Å that emerge to be close to pseudocubic, approximately following

Conclusion

We have investigated the optical properties of Sr doped BFO. The observed effects are consistent with Sr substitution resulting in an increase in the internal pressure due to the accompanying volume changes. Both the structural changes (rhombohedral to cubic) and the volume changes generate changes in the FeO6 local environment. These factors affect the band gaps as well as the crystal field splitting which in turn changes in the energies of various transitions. The d-d transitions energies are

Acknowledgment

The authors would like to thank Higher Education Commission (HEC), Government of Pakistan for their financial support under the project “Development and study of Magnetic Nanostructures”.

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