Generally, the orthorhombic (Fmmm) phase of the La₃Ni₂O₇ compound is considered as the one exhibiting superconductivity at high pressure among several of its polymorphs [1]. However, a recent high-pressure and low-temperature X-ray diffraction study of the compound revealed that the I4/mmm phase of the compound may be the actual one responsible for the superconductivity [2].  Thus, an understanding of the phase stability of the compound as a function of pressure and temperature is needed. In this work, we used both the conventional density-functional theory (DFT) structural optimizations and the temperature-dependent DFT structural optimizations to study the stabilities of the Amam, Fmmm, and I4/mmm phases of La₃Ni₂O₇ from 0 – 50 GPa, and from 0 – 300 K at 20 GPa. Results show that at 0 K, the I4/mmm phase is the most stable phase at a pressure above ~ 2 GPa, and at 20 GPa, it is the most stable phase in 0 – 300 K. This is consistent with the experimental results of ref. [2]. Future work will include entropy calculations to improve the phase stability analysis.


References
[1] Sun, et al., Nature, https://doi.org/ 10.1038/s41586-023-06408-7 (2023).
[2] Wang, et al., arXiv:2311.09186, https://doi.org/10.48550/arXiv.2311.09186.
 

phase stability,superconductivity,structural optimizations,high pressure,density functional theory