Negative thermal expansion in Sc2Mo3O12:Sm3+ for white LEDs and unveiling the impact of phase transition on cryogenic luminescence

Abstract

Red-emitting phosphors are essential to achieve sustainable white-light-emitting diodes (WLEDs) for lighting and indoor plant growth. Materials with negative thermal expansion (NTE) can overcome the critical problem of the thermal quenching (TQ) of photoluminescence (PL). In this regard, we report a Sc₂Mo₃O₁₂:Sm³⁺ (SMO:Sm³⁺) reddish-orange emitting phosphor with no TQ up to 433 K. The intense charge transfer from the SMO matrix to the dopant (O²⁻ → Sm³⁺) reinforced the absorption of ultraviolet (UV) light, in addition to intra 4f–4f blue light excitation. The site occupation of Sm³⁺ was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy ruled out any contribution of Sm²⁺ in the PL process. Temperature-dependent XRD studies revealed strong NTE in SMO, which induced promising anti-TQ performance via intensifying the charge transfer absorption and improved structural rigidity. As a result, 591% of its intensity at room temperature was retained at 433 K resulting in a ∼6-fold enhancement in Sm³⁺ emission. Moreover, we demonstrated two prototypes for lighting and indoor plant growth by fabricating the SMO:Sm³⁺ phosphor onto UV (280 nm) and 410 nm LED chips, respectively. The WLED offers a high color rendering index (CRI) of 84, CIE (0.33, 0.32), and correlated color temperature (CCT) of 5408 K, with a high luminous efficacy of 113 lm W⁻¹. The LED emission bands overlap with the absorption of phytochrome, P_R, which is essential for plant growth. We further investigated the temperature-dependent cryogenic PL properties and its correlation with phase transition. These findings revealed that the lattice phase transition has minimal impact on the Sm³⁺ local structure and its luminescence profile. Interestingly, we discovered the green emission of the monoclinic SMO phase at liquid nitrogen temperature (−193 °C). The MoO₄²⁻ emission diminished on phase transition from monoclinic to orthorhombic SMO at room temperature. Our results demonstrate the potential of SMO:Sm³⁺ phosphors for applications in lighting and indoor plant growth LEDs with anti-TQ properties.