Unraveling the Lattice Dynamics and Inherent Emission of NaGdMgTeO₆ Double Perovskite Phosphor

Inherently emitting phosphors are gaining significant attention due to their minimal reabsorption, broad emission profiles, high efficiency, and low cost. In the present work, the inherent emission of the double perovskite tellurate NaGdMgTeO₆ is studied for the first time, and the underlying mechanism has been explored in detail. Under 334 nm excitation, the phosphor exhibits an unusual broad NIR emission spanning from 800  to 1200 nm, with a maximum at 948 nm. XPS analysis confirms the presence of Te⁴⁺ ions in the host, owing to the incomplete oxidation to Te⁶⁺, as well as the formation of oxygen vacancies. Upon annealing the phosphor under O₂ atmosphere, a reduction in the content of Te⁴⁺ and oxygen vacancies has been observed, which results in a decrease in the overall emission intensity. Temperature-dependent emission spectra reveal that the phosphor exhibits better thermal stability at low temperatures, but undergoes rapid thermal quenching at high temperatures, with an activation energy of 0.16 eV, obtained from the Arrhenius plot. Temperature-dependent Raman analysis indicates a decrease in the phonon lifetime from 49 ps (303 K) to 37 ps (773 K), resulting in an increase in the number of phonon decay channels, contributing to observed thermal quenching.