Extensive reference set and refined computational protocol for calculations of ⁵⁷Fe Mössbauer parameters

Area of Science:

• Computational chemistry
• Materials science
• Spectroscopy

Background:

• Mössbauer spectroscopy is crucial for analyzing iron compound electronic structures.
• Accurate prediction of isomer shift and quadrupole splitting using computational methods is a long-standing goal.
• Existing density functional theory (DFT) protocols have limitations in scope and accuracy.

Purpose of the Study:

• To develop and validate an optimized computational protocol for predicting Mössbauer parameters (isomer shift and quadrupole splitting) in iron compounds.
• To create a comprehensive, curated reference dataset of iron compounds for protocol optimization and validation.
• To assess the performance of various DFT functionals and computational approaches.

Main Methods:

• Compilation of an extensive and diverse reference set of iron compounds, categorized into low-temperature and high-temperature experimental subgroups.
• Optimization of a computational protocol using the scalar exact 2-component (X2C) Hamiltonian with finite nucleus approximation and an accurate all-electron basis set for iron.
• Systematic evaluation of multiple DFT functionals, focusing on hybrid functionals with varying percentages of exact exchange.

Main Results:

• Hybrid DFT functionals with approximately 25-30% exact exchange demonstrate superior accuracy for predicting isomer shifts across a wide range of iron compounds.
• The refined computational protocol achieves good predictive performance for isomer shifts, surpassing previous literature benchmarks in scope and accuracy.
• Significant limitations of DFT in accurately predicting quadrupole splittings were identified.

Conclusions:

• A refined, broadly applicable computational protocol has been established for accurate isomer shift prediction in diverse iron compounds.
• The study highlights the strengths of hybrid DFT functionals for isomer shift calculations but also underscores their limitations for quadrupole splitting.
• An empirical correction factor is necessary when comparing calculated values with high-temperature experimental data to account for the second-order Doppler shift.