Dual-Mode Light Emission\nand Dynamic Studies of Er<sup>3+</sup>/Yb<sup>3+</sup>-Doped NaLa(MoO<sub>4</sub>)<sub>2</sub> Phosphor\nfor Optical Thermometry Operating from Cryogenic to above Room Temperat

Dual-mode light-emitting phosphors\nplay a crucial role in advanced\ntechnologies and serve as optical thermometers in both cryogenic and\nhigh-temperature environments. This study presents dual-mode light\nemission through Stokes and anti-Stokes emissions from Er<sup>3+</sup>-doped molybdate for optical thermometers over a broad temperature\nrange (100–543 K). A series of Er<sup>3+</sup>-doped and Yb<sup>3+</sup>/Er<sup>3+</sup>-codoped NaLa(MoO<sub>4</sub>)<sub>2</sub> phosphors were synthesized using a typical solid-state reaction\ntechnique. The Judd–Ofelt intensity parameters (Ω<sub>λ</sub>; λ = 2, 4, and 6) were calculated utilizing oscillator\nstrengths of the NaLa<sub>0.97</sub>Er<sub>0.03</sub>(MoO<sub>4</sub>)<sub>2</sub> phosphor, which predicts the nature of bonding, transition\nprobabilities, and branching ratios of the emission transitions. Under\n377 nm excitation, the sample exhibited intense green and weak red\nemission bands due to the Er<sup>3+</sup> electronic transitions from <sup>2</sup>H<sub>11/2</sub>/<sup>4</sup>S<sub>3/2</sub> → <sup>4</sup>I<sub>15/2</sub> and <sup>4</sup>F<sub>9/2</sub> → <sup>4</sup>I<sub>15/2</sub>, respectively, through the Stokes emission\nprocess. For anti-Stokes emission, Er<sup>3+</sup>- and Yb<sup>3+</sup>/Er<sup>3+</sup>-doped samples were excited under a 980 nm laser.\nFurthermore, dynamic studies of the Er<sup>3+</sup>- and Yb<sup>3+</sup>/Er<sup>3+</sup>-doped samples were conducted using suitable Stokes\n(377 nm) and anti-Stokes (980 nm) excitations to understand the emission\nthrough different modes. For application in contactless optical thermometers,\ntemperature-dependent emission spectra were analyzed using the luminescence\nintensity ratio (LIR) technique. For temperature sensing below room\ntemperature, the LIRs of the thermally coupled levels (<sup>4</sup>S<sub>3/2</sub> → <sup>4</sup>I<sub>15/2</sub> and <sup>2</sup>H<sub>11/2</sub> → <sup>4</sup>I<sub>15/2</sub>) and nonthermally\ncoupled levels (<sup>2</sup>H<sub>11/2</sub>/<sup>4</sup>S<sub>3/2</sub> → <sup>4</sup>I<sub>15/2</sub> and <sup>4</sup>F<sub>9/2</sub> → <sup>4</sup>I<sub>15/2</sub>) of Er<sup>3+</sup> ions were\nconsidered in the temperature-dependent Stoke emission spectra. The\nachieved maximum relative sensitivity was around 6.14% at a temperature\nof 100 K. Similarly, the LIR for the thermally and nonthermally coupled\nlevels of Er<sup>3+</sup> was considered from temperature-dependent\nanti-Stokes emission for temperature sensing at above room temperature.\nIn this case, the observed maximum relative sensitivity was approximately\n1.29% at a temperature of 303 K.