Luminescence of\nTriple Perovskite Ba<sub>3</sub>MgTa<sub>2</sub>O<sub>9</sub>:Dy<sup>3+</sup> Up to 1100 °C

The luminescence response of Dy<sup>3+</sup>-doped Ba<sub>3</sub>MgTa<sub>2</sub>O<sub>9</sub> (BMTO:Dy) between room temperature\nand 1100 °C was studied in the context of high-temperature thermosensitive\nphosphors. Single-phase BMTO:Dy was synthesized via solid-state reaction.\nPowder X-ray diffraction, diffuse reflectance spectroscopy, and density-functional\ntheory calculations were used to probe its crystal and electronic\nstructure. Variable-temperature time-resolved decays of the <sup>4</sup>I<sub>15/2</sub> → <sup>6</sup>H<sub>15/2</sub> transition\nof Dy<sup>3+</sup> (457 nm) were collected and the temperature dependence\nof the excited-state average lifetime was quantitatively analyzed\nwith two objectives: establish mechanisms driving luminescence thermal\nquenching and assess the potential of BMTO:Dy as a high-temperature\nluminescent thermometer. Quenching was observed above 450 °C\nand appeared to be driven by thermal ionization of the <sup>4</sup>I<sub>15/2</sub> excited state to the host’s conduction band,\nwith a temperature-dependent activation barrier in the 1.1–1.5\neV range. The occurrence of high-order (≈21) multiphonon relaxation\nto lower-lying 4<i>f</i> levels as an additional or alternative\nquenching pathway at high temperatures could not be ruled out. As\na luminescent thermometer, the operational temperature range of BMTO:Dy\nwas between 450 and 1100 °C and temperature sensitivities ranged\nbetween ≈2.1 % °C<sup>–1</sup> at 538 °C and\n≈0.9 % °C<sup>–1</sup> at 1100 °C. Findings\npresented herein expand the library of high-temperature thermosensitive\nphosphors to group V <i>d</i><sup>0</sup> refractory metalates,\na class of materials that offers the opportunity to tailor luminescence\nresponse through rational tuning of crystal, vibrational, and electronic\nstructure.