  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.
