First-Principles ELNES Simulation of P-O Based Materials

Phosphate materials have been attracted attention for as energy materials and used for cathode materials in rechargeable batteries. In particular, the phosphate materials are expected to be used as storage electrodes storing sodium because they have many voids [1]. The physical properties of phosphate materials are greatly affected by the <i>Qⁿ</i> value (the number of bridging oxygens coordinated to phosphorus). We found that the difference in P-<i>Qⁿ</i> values greatly affects the diffusion of Na⁺ ions and protons in phosphate glasses using first-principles molecular dynamic simulations [2]. This suggests that ionic conductivity can be designed by controlling the P-<i>Qⁿ</i> distribution of phosphate materials. In the experiment, <i>Qⁿ</i> values are usually measured using magic angle spinning nuclear magnetic resonance (MAS-NMR) techniques. But they are average information, and spatial information cannot be obtained. In recent years, electron energy-loss spectroscopy (EELS) in transmission electron microscopy (TEM) has become increasingly important for the structural characterization of materials in high spatial resolution. In particular, the near-edge structure (ELNES) can provide detail and local information on chemical bonding, valence states, and coordination. In this study, the spatial distribution of P-<i>Qⁿ</i> value in phosphate materials will be observed in the combination of TEM-EELS observation and first-principles calculations.<br/>We performed a series of first-principles PAW calculations of P <i>L</i>-edge ELNES spectra with QMAS code [3] for phosphate crystal models with different P-<i>Qⁿ</i> values. These crystal models were taken from Materials project [4]. In the calculated spectra, the first peak shifted to the lower energy side as the P-<i>Qⁿ</i> value decreased. This trend agrees with experimental results . This suggests that the P-<i>Qⁿ</i> value can be determined using first peaks of ELNES spectra.To investigate the origin of each peak, we compared the partial density of states (PDOS) to the ELNES spectra. It was found that peak positions of ELNES spectra clearly coincided with the PDOS peak. This suggests that the first peak p1 originates from excitation from the core 2p state to the 4s state, while the second peak originates from excitation from the core 2p state to the 3d state.<br/>In conclusion, TEM-EELS will play an important role in the practical application of high-performance phosphate-based materials.<br/><br/>1)A. Sanaa et al., Mater. Sci. Technol. 6, 493-501(2023)<br/>2) K. Takada et al., Phys. Chem. Chem. Phys. 23, 14580-14586 (2021).<br/>3) T. Tamura et al., Phys. Rev B 85, 205210 (2012).<br/>4) A. Jain et al., APL Materials, 1, 011002 (2013).