Investigation of Ca2Al2SiO7: Pr3+/Gd3+ UVB upconverting phosphor and its optical thermometric properties

On basis of fluorescence intensity ratio (FIR) over certain narrow coupled levels of rare-earth ions, up-conversion of RE-doped materials perform excellent temperature-sensing abilities for thermometry applications. A series of Pr 3+ /Gd 3+ co-doped Ca 2 Al 2 SiO 7 phosphors (Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ ) were prepared by a conventional high-temperature solid-state reaction. Under excitation of blue ∼445 nm, the Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ phosphor yields intense photoluminescence (PL) bands around 489, 528, 537, 580, 604, 613, and 646 nm due to electronic transition of 3 P 0 → 3 H 4 , 3 P 0 → 3 H 5 , 3 P 1 → 3 H 5 , 3 P 1 → 3 H 6 , 1 D 2 → 3 H 4 , 3 P 0 → 3 H 6 , and 3 P 0 → 3 F 2 of Pr 3+ , respectively. Their relative intensities are sensitively determined by Pr 3+ contents, which, from 0.1 % to 5 %, tune the emission color from pink to warm white under excitation of 445 nm. It is of great interest that intense irradiation using a blue ∼445 laser does make efficient ultraviolet B (UVB) up-conversion at about 297, 306, and 313 nm, just stemming from the 6 P 3/2  →  8 S 7/2 , 6 P 5/2  →  8 S 7/2 , and 6 P 7/2  →  8 S 7/2 of Gd 3+ , respectively. A typical two-photon absorption process was proved to account for the UVB up-conversion, i.e. successive absorption of two blue ∼445 nm photons populating Pr 3+ 4f5d excited state, which further efficiently transfer its energy to the 6 I J / 6 P J states of Gd 3+ neighbor for UVB emission. Moreover, the thermally coupled levels (TCLs) of Gd 3+ : 6 P 3/2 - 6 P 7/2 , and Gd 3+ : 6 P 5/2 - 6 P 7/2 were thoroughly investigated for temperature sensing in 298–598 K. Maximum relative sensitivity of the 6 P 3/2 - 6 P 7/2 TCLs, i.e. S r (I 306 /I 313 ), is proved to be 0.96 % K −1 at 298 K, and that of the 6 P 5/2 - 6 P 7/2 TCLs, i.e. S r (I 297 /I 313 ), is 1 % K −1 at 373 K. The relevant thermal stabilities of the Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ thermometric candidate was well validated with 10 cycles of heating and cooling processes in 298–598 K. Such continuous explorations of Pr 3+ /Gd 3+ co-doping system will significantly promote the development of robust UVB up-converting phosphors as well as their practical applications in the field of high-resolution optical thermometry.