Figure 1.

a) Primitive unit cell for Cs₂NaYbCl₆ obtained from full structural optimization. The equilibrium lattice parameters are a = b = c = 7.709~Å and α = β = γ = π/3. b) 40 atoms conventional cubic cell obtained by remapping the ten atoms primitive cell. The conventional cubic cell was used for harmonic and anharmonic vibrational‐related properties. The bulk modulus B ₀ is obtained by fitting the DFT energies obtained at different lattice parameters using the Birch–Murnaghan equation of state, resulting in B ₀ = 16.28~GPa.