Jonathan E Spanier

Calculation of Raman scattering from molecular dynamics (MD) simulations
requires accurate modeling of the evolution of the electronic polarizability of
the system along its MD trajectory. For large systems, this necessitates the
use of atomistic models to represent the dependence of electronic
polarizability on atomic coordinates. The bond polarizability model (BPM) is
the simplest such model and has been used for modeling the Raman spectra of
molecular systems but has not been applied to solid-state systems. Here, we
systematically investigate the accuracy and limitations of the BPM
parameterized from density functional theory (DFT) results for a series of
simple molecules such as CO2, SO2, H2S, H2O, NH3, and CH4, the more complex
CH2O, CH3OH and CH3CH2OH and thiophene molecules and the BaTiO3 and CsPbBr3
perovskite solids. We find that BPM can reliably reproduce the overall features
of the Raman spectra such as shifts of peak positions. However, with the
exception of highly symmetric systems, the assumption of non-interacting bonds
limits the quantitative accuracy of the BPM; this assumption also leads to
qualitatively inaccurate polarizability evolution and Raman spectra for systems
where large deviations from the ground state structure are present.

Antiferroelectric perovskite oxides exhibit exceptional dielectric properties and high structural/chemical tunability, making them promising for a wide range of applications from high energy-density capacitors to solid-state cooling. However, tailoring the antiferroelectric phase stability through alloying is hampered by the complex interplay between chemistry and the alignment of dipole moments. In this study, correlations between chemical order and the stability of the antiferroelectric phase are established at anti-phase boundaries in \ce{Pb2MgWO6}. Using multislice ptychography, we reveal the three-dimensional nature of chemical order at the boundaries and show that they exhibit a finite width of chemical intermixing. Furthermore, regions at and adjacent to the anti-phase boundary exhibit antiferroelectric displacements in contrast to the overall paraelectric film. Combining spatial statistics and density functional theory simulations, local antiferroelectric distortions are shown to be confined to and stabilized by chemical disorder. Enabled by the three-dimensional information of multislice ptychography, these results provide insights into the interplay between chemical order and electronic properties to engineer antiferroelectric material response.

The application of molecular dynamics (MD) simulations to the interpretation of Raman scattering spectra is hindered by inability of atomistic simulations to account for the dynamic evolution of electronic polarizability, requiring the use of either ab initio method or parameterization of machine learning models. More broadly, the dynamic evolution of electronic-structure-derived properties cannot be treated by the current atomistic models. Here, we report a simple, physically-based atomistic model with few (maximum 10 parameters for the systems considered here) adjustable parameters that can accurately represent the changes in the electronic polarizability tensor for molecules and solid-state systems. Due to its compactness, the model can be applied for simulations of Raman spectra of large (~ 1,000,000-atom) systems with modest computational cost. To demonstrate its accuracy, the model is applied to the CO2 molecule, water clusters, and BaTiO3 and CsPbBr3 perovskites and shows good agreement with ab-initio-derived and experimental polarizability tensor and Raman data. The atomistic nature of the model enables local analysis of the contributions to Raman spectra, paving the way for the application of MD simulations for the interpretation of Raman spectroscopy results. Furthermore, our successful atomistic representation of the evolution of electronic polarizability suggests that the evolution of electronic structure and its derivative properties can be represented by atomistic models, opening up the possibility of studies of electronic-structure-dependent properties using large-scale atomistic simulations.

Prediction of properties from composition is a fundamental goal of materials science and can greatly accelerate development of functional materials. It is particularly relevant for ferroelectric perovskite solid solutions where compositional variation is a primary tool for materials design. To advance beyond the commonly used Landau-Ginzburg-Devonshire and density functional theory methods that despite their power are not predictive, we elucidate the key interactions that govern ferroelectrics using 5-atom bulk unit cells and non-ground-state defect-like ferroelectric domain walls as a simple as possible but not simpler model systems. We also develop a theory relating properties at several different length scales that provides a unified framework for the prediction of ferroelectric, antiferroelectric and ferroelectric phase stabilities and the key transition temperature, coercive field and polarization properties from composition. The elucidated physically meaningful relationships enable rapid identification of promising piezoelectric and dielectric materials.