Assignment of optical phonons at the zone center of distorted orthorhombic RCrO3 (R = La, Pr, Nd, Sm, Eu) perovskites using force-field lattice dynamics model
Graphical abstract
A short-range force-field model is applied to investigate the optical phonon distributions and complex behavior of atomic motions in distorted orthorhombic perovskites RCrO3 (R = La, Pr, Nd, Sm, Eu).
Introduction
The rare-earth (R) orthochromites RCrO3 are perovskite-type oxides, having general formula unit ABO3, where A3+ is rare-earth ions or Y, and B3+ is a transition or post-transition metal, which have recently drawn immense interest due to their multiferroic properties, such as ferromagnetism, ferroelectricity, and/or ferroelasticity, which are exploited in terms of multifunctional designed novel devices, such as high-density data-storage devices, multistate memories, spintronics, and magnetoelectric sensors [[1], [2], [3], [4], [5]]. Further these materials are well-studied for their extremely rich magnetic properties, like temperature-induced magnetization reversal, spin-flipping, spin-reorientation (SR) and exchange-bias, due to the presence of various magnetic interactions, namely, the Cr3+–Cr3+, Cr3+–R3+ and R3+–R3+ interactions [[6], [7], [8], [9]]. Orthochromites exhibit antiferromagnetic structure and ferroelectric order appearing below the magnetic ordering temperature TNéel (120–291 K) [[10], [11], [12], [13], [14]]. Some of these materials were also studied from the perspective of the magnetocaloric effect and were found to be promising candidates for low-temperature magnetic refrigeration [8,[15], [16], [17]], which properties are associated with the suppression of the spin entropy associated with the suppression of SR phase transition [8]. These SR phenomena are considered to be a typical displacive-type second-order phase transition. It was found that orthochromites undergo temperature-dependent structural changes from centro-symmetric Pbnm to non-centrosymmetric Pna21 space group of distorted orthorhombic structure, [7]. Such structural distortions in perovskites can be described by three main features with respect to their ideal cubic structure [18]: (i) a rotation (tilt) of essentially rigid CrO6 octahedra, (ii) R3+ cation displacements, which often lead to ferroelectricity, and (iii) distortions of the octahedra, such as the Jahn-Teller distortion. Octahedral tilt angle modify Cr
OCr bond angles and CrO bond lengths as well as distortion of RO12 polyhedra, which in turn tunes to intriguing magnetic phase diagram, metal-insulator phase transition and spectroscopic properties over the orthochromite series.
An investigation of Raman and infrared (IR) wave numbers is very useful for describing the details of structural transitions and allows in some cases to investigate the local structural distortions and symmetry lowering through optical phonon activity study, and spin-phonon coupling [18]. In addition, Raman spectra can be employed in studying various solid-state properties of chromites. Weber et al. [18] reported a systematic investigation of orthorhombic (Pbnm) perovskite-type RCrO3 powder samples by Raman scattering for nine different R3+ cations (R = Y, La, Pr, Sm, Gd, Dy, Ho, Yb, and Lu) at room temperature and assigned the observed phonon modes taking advantage of reported literature pre-assigned modes for several orthochromites, e.g., LaCrO3, GdCrO3, YCrO3, and HoCrO3. Their results showed that the high wave number Raman modes (ω > 300 cm−1) can easily be monitored and thus assigned as a function of the R3+ ion. However for low wave number modes (ω < 300 cm−1), mode monitoring becomes difficult because of the complex crossing of several of them due to changes in the orthorhombic distortion notably for large lanthanides, e.g., LaCrO3, PrCrO3, NdCrO3 and SmCrO3. In order to overcome this problem, recently Camara et al. [19] reinvestigated polarized Raman scattering of some of these compounds, R = La, Pr, Nd, Sm, in single crystal form, and computed different Raman modes for these compounds based on the lattice dynamics calculation (LDC) within the density-functional perturbation theory (DFPT) framework, but only for LaCrO3. As complementary study to the Raman spectra, IR spectra experiments were also carried out by several authors for LaCrO3 [[20], [21], [22], [23]], PrCrO3 [20], NdCrO3 [24], SmCrO3 [25], EuCrO3 [26]. For these samples except SmCrO3, only two strong absorption IR bands at ∼600 cm−1 and ∼400 cm−1 were observed instead of theoretically expected 25 bands [[20], [21], [22], [23], [24]]. For SmCrO3, at least 18 IR modes were identified at 300 K [25]. However, we noted that in these works, neither IR mode was assigned to the specific modes which are group-theoretically expected for orthorhombic Pbnm structure, nor was coordinated with the observed Raman modes.
Since the optical phonons associated with the atom motions are very sensitive to the force constants (FCs) and hence to the bonds, an attempt was, therefore, undertaken to study the zone center phonons in RCrO3 (R = La, Pr, Nd, Sm, Eu) for the first time in the context of a short-range force-constant model (SRFCM) taking account of all observed Raman and IR bands simultaneously. Thus based on the atomic vibrations model, the specific modes observed in the experimental Raman and IR spectra were assigned. It was observed that a total of 16 interatomic FCs are required to obtain a good agreement between the theory and experiment for the Raman and IR phonon wave numbers.
Section snippets
Theoretical outline
The Pbnm crystal structure of RCrO3 contains four formula units (Z = 4, ZB = 4) per Bravais unit cell, sketched in Fig. 1. The irreducible representation for vibration modes of R, Cr and O atoms at four different Wyckoff sites can be obtained using group theory and standard correlation method [27], given in Table S1 (Supplementary material). The total irreducible representation of the crystal at the Γ-point of Brillouin zone consists of 60 zone-center normal modes, Γ = 7Ag + 7B1g + 5B2g + 5B3g
Results and discussion
The n.n. atomic bond lengths and bond angles required for SRFCM were calculated in Table 1 using the structural parameters and atomic coordinates reported in Ref. [32] for RCrO3 (R = La, Pr, Nd, Sm, Eu). The inter-atomic FCs are also quoted in Table 1 and plotted in Fig. S1 (Supplementary material) for stretching FCs against ionic radius of R-cations in order to understand the variation of bond strengths over this series. We noted that the FCs vary systematically for RCrO3 containing smaller
Conclusions
In summary, we have successfully utilized the short-range force-field model to assign the Raman and infrared modes at Γ-point of orthorhombic (space group: Pbnm) perovskite compounds, e.g., LaCrO3, PrCrO3, NdCrO3, SmCrO3 and EuCrO3. The SRFCM is based on lattice dynamics calculations within Wilson’s GF matrix method and represents a rapid and very efficient approach to investigate and assign the vibrational spectra of an arbitrary crystalline structure by generating the complete set of
Authors’ contributions
J. Saha and S. Nandi have done theoretical calculations in this work. Y.M. Jana wrote the manuscript and supervised the overall project. All authors equally contributed and participated in discussion and analyses.
Research data statement
The calcualted data in this work may be available from authors upon request.
Declaration of Competing Interest
The authors declare that they have no financial interest and personal relationships with other people or organizations that could inappropriately influence their work or state in this paper.
Acknowledgements
The authors gratefully acknowledge the helpful discussions with Prof. H.C. Gupta, IIT-Delhi, India.
References (36)
- et al.
J. Matter. Chem. C
(2016) - et al.
Mater. Today: Proc.
(2016) - et al.
Magn. Mater.
(2014) - et al.
Mater. Chem. Phys.
(2020) - et al.
Nat. Mater.
(2007) - et al.
Nat. Mater.
(2007) - et al.
Adv. Phys.
(2011) - et al.
Crit. Rev. Solid State Mater. Sci.
(2014) - et al.
Appl. Phys. Lett.
(2014) - et al.
EPL (Europhys. Lett.
(2014)