Negative Thermal Expansion in Sc2Mo3O12:Sm3+ for White LEDs and Unveiling the Impact of Phase Transition on Cryogenic Luminescence

Abstract

The red-emitting phosphors are essential for sustainable white-light-emitting diode (WLED) for lighting and indoor plant growth. The materials with negative thermal expansion (NTE) can overcome the critical problem of 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 SMO matrix to 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 photoluminescence (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 high luminous efficacy of 113 lm/W. The LED emission bands overlap with absorption of phytochrome, PR which is essential for plant growth. We further investigated temperature-dependent cryogenic PL properties and its correlation with phase transition. These findings revealed that lattice phase transition has minute impact on Sm3+ local structure and its luminescence profile. Interestingly, we unveil the green emission of monoclinic SMO phase at liquid nitrogen temperature (-193 oC). 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 lighting and indoor plant growth LEDs applications with anti-TQ properties.