Eu${}^{3+}$-doped SrGe${}_4$O${}_9$ red emitting phosphors. Structural, luminescence properties, theoretical calculation and applications

Eu${}^{3+}$-doped germanate based red emitting phosphors SrGe${}_4$O${}_9$: xEu${}^{3+}$ (0 ≤ x ≤ 0.5 mol%) were prepared by high-temperature solid-state procedure. Moreover, the Density Functional Theory (DFT) calculation, X-ray diffraction (XRD) Rietveld refinement, scanning electron microscopy (SEM), and photoluminescence (PL) measurement were discussed in detail to fully investigate the crystal structure and PL properties of SrGe${}_4$O${}_9$: xEu${}^{3+}$. Furthermore, the Judd–Ofelt (J–O) analysis demonstrated that the Eu${}^{3+}$ ions in the SrGe${}_4$O${}_9$ host possessed lower sites symmetry. In addition, the phosphor SrGe${}_4$O${}_9$: 0.35%Eu${}^{3+}$ displayed superior thermal stability with PL intensity at 423 K is around 90% of its initial value at 298 K. As a result, the white light emitting diodes (WLED) device based on SrGe${}_4$O${}_9$: 0.35%Eu${}^{3+}$ achieved an expected result of a correlated color temperature (CCT) of 4573 K and a color rendering index (CRI) of 80.8 under a voltage of 3 V and a driven current of 20 mA.