Thermodynamic Routes to Novel Metastable Nitrogen-Rich Nitrides

Compared to oxides, the nitrides are relatively unexplored, making them a promising chemical space for novel materials discovery. Of particular interest are nitrogen-rich nitrides, which often possess useful semiconducting properties for electronic and optoelectronic applications. However, such nitrogen-rich compounds are generally metastable, and the lack of a guiding theory for their synthesis has limited their exploration. Here, we review the remarkable metastability of observed nitrides, and examine the thermodynamics of how reactive nitrogen precursors can stabilize metastable nitrogen-rich compositions during materials synthesis. We map these thermodynamic strategies onto a predictive computational search, training a data-mined ionic substitution algorithm specifically for nitride discovery, which we combine with grand-canonical DFT-SCAN phase stability calculations to compute stabilizing nitrogen chemical potentials. We identify several new nitrogen-rich binary nitrides for experimental investigation, notably the transition metal nitrides Mn₃N₄, Cr₃N₄, V₃N₄, and Nb₃N₅, the main group nitride SbN, and the pernitrides FeN₂, CrN₂, and Cu₂N₂. By formulating rational thermodynamic routes to metastable compounds, we expand the search space for functional technological materials beyond equilibrium phases and compositions

Effects of Antisite Defects on Li Diffusion in LiFePO₄ Revealed by Li Isotope Exchange

Li–Fe antisite defects are commonly found in LiFePO₄ particles and can impede or block Li diffusion in the single-file Li diffusion channels. However, due to their low concentration (∼1%), the effect of antisite defects on Li diffusion has only been systematically investigated by theoretical approaches. In this work, the exchange between Li in solid LiFePO₄ (92.5% enriched with ⁶Li) and Li in the liquid Li electrolyte solution (containing natural abundance Li, 7.6% ⁶Li and 92.4% ⁷Li) was measured as a function of time by both ex situ and in situ solid-state nuclear magnetic resonance experiments. The experimental data reveal that the time dependence of the isotope exchange cannot be modeled by a simple single-file diffusion process and that defects must play a role in the mobility of ions in the LiFePO₄ particles. By performing kinetic Monte Carlo simulations that explicitly consider antisite defects, which allow Li to cross over between adjacent channels, we show that the observed tracer exchange behavior can be explained by the presence of channels with paired Li–Fe antisite defects. The simulations suggest that Li diffusion across the antisite is slow (10^–16 cm² s^–1) and that the presence of antisite defects is widespread in the LiFePO₄ particles we examined, where ∼80% channels are affected by such defects