Raman spectra of fine-grained materials from first principles

Abstract

Raman spectroscopy is an advantageous method for studying the local structure of materials, but the interpretation of measured spectra is complicated by the presence of oblique phonons in polycrystals of polar materials. Whilst group theory considerations and standard ab initio calculations are helpful, they are often valid only for single crystals. In this paper, we introduce a method for computing Raman spectra of polycrystalline materials from first principles. We start from the standard approach based on the (Placzek) rotation invariants of the Raman tensors and extend it to include the effect of the coupling between the lattice vibrations and the induced electric field, and the electro-optic contribution, relevant for polar materials like ferroelectrics. As exemplified by applying the method to rhombohedral BaTiO 3 , AlN, and LiNbO 3 , such an extension brings the simulated Raman spectrum to a much better correspondence with the experimental one. Additional advantages of the method are that it is general, permits automation, and thus can be used in high-throughput fashion.

Origin of Relaxor Behavior in Barium‐Titanate‐Based Lead‐Free Perovskites

“…We explain here the role of defects and defect-induced disorder in the onset of relaxor behavior in a heterovalent (Nb 5+ -substituted) BTO system (BNbT), and compare it against the well-studied Zr 4+ -substituted BTO-based homovalent relaxor system (BZT). The discussion we provide is supported by local structural methods, such as Raman and nuclear magnetic resonance (NMR) spectroscopy, evidencing the evolution of lattice disorder upon substitution in both systems, and the presence of defect-related phonon modes in the Raman spectra-calculated by the recently developed spherical averaging method [20] -to reveal the effect of substitution type in the local structure. In addition, density functional theory (DFT) simulations of unit cell volume, strain and electrostatic potential landscape showing the effect of defect dipoles in supercells of the substituted BTO lattice are presented.…”

“…in pure BTO is described in detail in previous work. [20] From the weakening of the ferroelectric-related mode around 300 cm -1 (ref. [29]), it is evident that in BZT ferroelectric long-range order is lost between 20% and 40% of Zr 4+ content, whereas in BNbT this occurs at a lower concentration of heterovalent B-site cation, namely between 5% and 7% of Nb 5+ content, in agreement with dielectric measurements.…”

“…To clarify the origin of extra modes 2 and 5, we calculated the full Raman spectral signature of both BZT40 and BNbT15 within 3 × 3 × 3 supercells, following the spherical averaging method we developed recently based on ab initio Raman-active phonon calculations. [20] In BZT, each supercell was constructed with either Ti 4+ or Zr 4+ at the B-site in the right proportion to realize 40% Zr 4+ amount. Whereas, for BNbT the presence of either 1 V Ti ′′′′ and 4 Nb 5+ at the B-site or 1 V Ba ′′ at the A-site and 2 Nb 5+ at the B-site was considered for the 3 × 3 × 3 supercell.…”

Strategies to Improve the Energy Storage Properties of Perovskite Lead-Free Relaxor Ferroelectrics: A Review

“…Both are made increasingly available nowadays. Examples of this approach are the determination of the local structure in relaxor materials by Reverse Monte Carlo modeling of diffuse neutron scattering [ 242 ], the detection of defects in BaTiO 3 solid solutions aided by ab-initio phonon calculations [ 41 , 243 ], the use of machine learning to quantify phases and atomic structures in ferroelectrics by correlative spectroscopy/microscopy [ 244 , 245 , 246 ], the model-aided calculation of local, atomic-scale polarization in relaxors from drift-free STEM analyses [ 247 ], and the quantification of porosity and tortuosity in ceramics from tomography scans aided by image correlation based on convolutional neural networks [ 248 ].…”

In situ combined stress‐ and temperature‐dependent Raman spectroscopy of Li‐doped (Na,K)NbO3

“…Among laboratory-scale structural investigation techniques, Raman spectroscopy is particularly attractive. [31][32][33] Despite being a surface-sensitive technique, the higher spatial resolution, relatively easy access, low cost, and fast data acquisition make in situ Raman spectroscopy an important method for studying the structure of functional materials. Moreover, the Raman spectroscopy probes the chemical bonds, which is advantageous for the investigation of the local structure of disordered systems.…”