Cr3+-doped CaMgGe2O6 phosphors: crystal field effects and the synergistic role of FIR and fluorescence lifetime in multi-mode optical thermometry†

Abstract

Luminescence thermometry has attracted growing interest for its potential in remote and non-contact temperature sensing. Among luminescent ions, Cr³⁺ is widely studied for optical thermometry using fluorescence intensity ratio (FIR), but its fluorescence lifetime (FL) thermometric potential remains largely undiscovered. In this study, we present a detailed spectroscopic investigation of Cr³⁺-doped CaMgGe₂O₆, demonstrating the complementary advantages of FIR and FL-based thermometry. X-Ray diffraction (XRD) confirms the monoclinic phase with high crystallinity, while diffuse reflectance spectroscopy provides insight into the crystal field strength (D_q/B ≈ 1.81) and optical band gap (E_g). Under 405 nm excitation, we systematically analyze the photoluminescence and temperature-dependent luminescence behavior. The multi-mode thermal sensing approach reveals that FIR (I₅₈₉/I₇₇₁) achieves a remarkable maximum sensitivity of 1.4% K⁻¹ at 390 K, with an exceptionally low temperature uncertainty (∼0.11 K at room temperature), establishing its reliability for precise temperature detection. Meanwhile, FL thermometry exhibits an even higher maximum sensitivity of 2.5% K⁻¹ at 478 K, underscoring its strong potential as an alternative or complementary technique. By integrating both methods, we achieve enhanced accuracy, broader temperature coverage, and improved adaptability to various sensing environments. This work highlights the first comprehensive demonstration of FL-based thermometry in Cr³⁺-doped phosphors, paving the way for optimized multi-mode luminescent thermal sensors and reinforcing the critical role of both the host matrix and advanced spectroscopic characterization.