Eu${}_2$O${}_3$ role in the optical and photoluminescence properties of 50 SiO${}_2$ - 7 MgO - 20 ZnO - (23 - x) La${}_2$O${}_3$ - x Eu${}_2$O${}_3$ nano-crystalline thin films

50 SiO${}_2$ - 7 MgO - 20 ZnO - (23 - x) La${}_2$O${}_3$ - x Eu${}_2$O${}_3$(0.0≤x≤4 mol%.) termed as (Eu-SMZL) thin films have been synthesized by highly acidic sol–gel technique and calcined at 450 °C for 2 h. The nano-porous Eu-SMZL films showed the improved structure and morphology with increasing Eu${}_2$O${}_3$ content. The XRD and FTIR investigation proved an increase in the crystallinity and changes in the internal structure with the increase in Eu${}^{3+}$ level. The photoluminescence (Pl) result agrees with the XRD, proving the increase in the crystallinity. The transmittance (T) and absorbance (A) spectra have been measured within 300-2000 nm spectral range. The absorption coefficient (α) and refractive index (n) have been estimated. It is worth to mention that the increment of Eu${}_2$O${}_3$ at the expense of La${}_2$O${}_3$ shifts the absorption edge to the long wavelength side (red shift) which leads to a decrement in the band gap (E${}_{opt}$) values while the n values increases. This behavior of n is well discussed in terms of the glass density (G${}_d$), molar volume (V${}_m$), electronic polarizability (α${}_{el}$) and optical basicity (Λ). Furthermore, the single oscillator model helps us to investigate the static index of refraction (n(0)), energy of single oscillator (E${}_o$), and dispersion energy (E${}_d$).