Comparative analysis of structural, elastic, electronic, phonon, thermal and optical properties of two $\text{Na}_6\text{Ge}_2\text{Se}_6$ phases from first principles calculations

TL;DR

This study uses first-principles calculations to compare two phases of Na6Ge2Se6, revealing differences in stability, electronic, optical, and thermal properties, and providing insights into their potential applications.

Contribution

It introduces a computationally predicted phase of Na6Ge2Se6 with lower energy and compares its properties to the known phase, expanding understanding of this material family.

Findings

Both phases are mechanically and dynamically stable.

The predicted phase has a 2.97 eV indirect band gap.

The predicted phase is more stable below 907 K.

Abstract

The demand for new alkali metal chalcogenide materials is continuously increasing due to their potential applications across various technological fields. Recently, a new compound, $\text{Na}_6\text{Ge}_2\text{Se}_6$, was computationally predicted, representing a new phase distinct from the experimentally observed $\text{Na}_6\text{Ge}_2\text{Se}_6$ reported in 1985. Notably, this newly predicted phase displays a lower total energy compared to the previously known experimental phase, as determined by first-principles calculations. In this study, we undertake a thorough comparative analysis of the structural, elastic, electronic, phonon, thermal, and optical properties of these two $\text{Na}_6\text{Ge}_2\text{Se}_6$ phases. Our results show that both phases meet mechanical and dynamical stability criteria. The electronic band structure analysis confirms the semiconducting nature of both materials, with a 2.97 eV indirect band gap for the predicted phase and a 2.93 eV direct band gap for the observed phase. Optically, both phases exhibit strong absorption in the ultraviolet region. Thermal properties analysis reveals that the predicted phase is more thermodynamically stable below 907 K, while the observed phase shows greater thermodynamic stability above this temperature.