Dual-Mode Light Emission and Dynamic Studies of Er³⁺/Yb³⁺-Doped NaLa(MoO₄)₂ Phosphor for Optical Thermometry Operating from Cryogenic to above Room Temperatures

Dual-mode light-emitting phosphors play a crucial role in advanced technologies and serve as optical thermometers in both cryogenic and high-temperature environments. This study presents dual-mode light emission through Stokes and anti-Stokes emissions from Er3+-doped molybdate for optical thermometers over a broad temperature range (100–543 K). A series of Er3+-doped and Yb3+/Er3+-codoped NaLa(MoO4)2 phosphors were synthesized using a typical solid-state reaction technique. The Judd–Ofelt intensity parameters (Ωλ; λ = 2, 4, and 6) were calculated utilizing oscillator strengths of the NaLa0.97Er0.03(MoO4)2 phosphor, which predicts the nature of bonding, transition probabilities, and branching ratios of the emission transitions. Under 377 nm excitation, the sample exhibited intense green and weak red emission bands due to the Er3+ electronic transitions from 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively, through the Stokes emission process. For anti-Stokes emission, Er3+- and Yb3+/Er3+-doped samples were excited under a 980 nm laser. Furthermore, dynamic studies of the Er3+- and Yb3+/Er3+-doped samples were conducted using suitable Stokes (377 nm) and anti-Stokes (980 nm) excitations to understand the emission through different modes. For application in contactless optical thermometers, temperature-dependent emission spectra were analyzed using the luminescence intensity ratio (LIR) technique. For temperature sensing below room temperature, the LIRs of the thermally coupled levels (4S3/2 → 4I15/2 and 2H11/2 → 4I15/2) and nonthermally coupled levels (2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2) of Er3+ ions were considered in the temperature-dependent Stoke emission spectra. The achieved maximum relative sensitivity was around 6.14% at a temperature of 100 K. Similarly, the LIR for the thermally and nonthermally coupled levels of Er3+ was considered from temperature-dependent anti-Stokes emission for temperature sensing at above room temperature. In this case, the observed maximum relative sensitivity was approximately 1.29% at a temperature of 303 K.