Photoluminescence measurements confirmed a direct band gap of ∼1.41 eV for the cubic ${\mathrm{Na}}_2\mathrm{KSb}$ phase. At the highest excitation powers, a secondary emission peak emerged alongside the original band-edge peak, indicating the formation of a Fermi-degenerate electron-hole plasma. Polarization- and temperature-resolved Raman spectroscopy, supported by density functional theory (DFT), revealed the vibrational and structural properties of multialkali antimonide high-vacuum photocathode thin films. Polycrystalline ${\mathrm{Na}}_2\mathrm{KSb}$ cubic phase of the as-grown film was explicitly identified by its characteristic transverse optical (TO - 147 $\mathrm{c}{\mathrm{m}}^{-1}$) and longitudinal optical (LO - 179 $\mathrm{c}{\mathrm{m}}^{-1}$) phonon modes. These modes exhibited exceptional thermal stability, while the LO mode intensity showed a selective, temperature-dependent quenching attributed to a weakening of the Fröhlich interaction. DFT phonon calculations for the thermodynamically stable cubic ${\mathrm{Na}}_2\mathrm{KSb}$ phase and the hexagonal ${\mathrm{NaK}}_2\mathrm{Sb}$/${\mathrm{Na}}_3\mathrm{Sb}$ polymorphs confirmed the experimental assignment of the cubic ${\mathrm{Na}}_2\mathrm{KSb}$ phase as the intrinsic structure of the as-grown film. However, the analysis revealed that high laser power density in Raman spectroscopy can induce a local phase transformation from the cubic structure to partially hexagonal ${\mathrm{NaK}}_2\mathrm{Sb}$ phase, highlighting the metastability of the material under nonequilibrium conditions. These findings provide fundamental understanding into the lattice dynamics and phase stability essential for the performance and operational limits of high-efficiency photocathodes.

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• Crystal structures
• Thin films
• Density functional calculations
• Raman spectroscopy