Thermally Induced Disorder–Order Phase Transition of Gd₂Hf₂O₇:Eu³⁺ Nanoparticles and Its Implication on Photo- and Radioluminescence

Abstract

Crystal structure has a strong influence on the luminescence properties of lanthanide-doped materials. In this work, we have investigated the thermally induced structural transition in Gd 2 Hf 2 O 7 (GHO) using Eu 3+ ions as the spectroscopic probe. It was found that complete phase transition from the disordered fluorite phase (DFP) to the ordered pyrochlore phase (OPP) can be achieved in GHO with the increase of annealing temperature from 650 → 1100 → 1300 °C. OPP is the more stable structural form for the GHOE nanoparticles (NPs) annealed at a higher temperature based on the energy calculation by density functional theory (DFT). The asymmetry ratio of the GHOE-650 NPs was the highest, whereas the quantum yield, luminescence intensity, and lifetime values of the GHOE-1300 NPs were the highest. Emission intensity of Eu 3+ ions increases significantly with the phase transition from the DFP to OPP phase and is attributed to the higher radiative transition rate (281 s –1 ) of the 5 D 0 level of the Eu 3+ ion in the environment with relatively lower symmetry ( C 2 v ) because of the increase of crystal size. As the structure changes from DFP to OPP, radioluminescence showed tunable color change from red to orange. The Eu 3+ local structure obtained from DFT calculation confirmed the absence of inversion symmetry in the DFP structure, which is consistent with the experimental emission spectra and Stark components. We also elucidated the host to dopant optical energy transfer through density of states calculations. Overall, our current studies present important observations for the GHOE NPs: (i) thermally induced order–disorder phase transition, (ii) change of point group symmetry around Eu 3+ ions in the two phases, (iii) high thermal stability, and (iv) tunability of radioluminescent color. This work provides fundamental understanding of the relationship between the crystal structure and photophysical properties of lanthanide-doped materials and helps design a strategy for advanced optoelectronic materials.

Bright aspects of defects and dark traits of dopants in the photoluminescence of Er2X2O7:Eu3+ (X = Ti and Zr) pyrochlore: an insight using EXAFS, positron annihilation and DFT

“…still present in Er 2 Zr 2 O 7 . This is also observed in Y 2 Sn 2Àx Zr x O 7 with Zr substitution as well as the thermally induced structural evolution in Gd 2 Hf 2 O 7 15,39. Similar short range order and the partial retention of bands attributed to pyrochlore are also observed in the supposedly fluorite-type rare earth hafnates of Dy and Yb under ambient conditions 40.…”

“…The phase transition from the pyrochlore to the defect-fluorite structure is an order-disorder transition that is very sensitive to chemical composition and electronic configuration and can be induced by heating or irradiation. 14,15 IP is normally formed when the radius ratio falls between 1.46 r r(A 3+ )/r(B 4+ ) r 1.78. On the other hand, when the radius ratio is less than 1.48, the DF phase is more likely to be stabilized.…”

“…Each structure has its own advantage; the IP structure is preferred for luminescence, thermal barrier coatings, and upconversion, whereas the DF structure is more suitable for nuclear waste hosts, catalysis, oxygen conductors in fuel cells, etc. 15 Moreover, the defect density will also be different in the two phases, which can have great implications on various properties such as magnetic, conductivity, and optical properties. Recently, we have explored Nd 2 Zr 2 O 7 and Gd 2 Zr 2 O 7 , both of which exist in the DF structure, for self-activated emission, luminescence host and speciation of uranium.…”

Composition‐dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La, Nd, Gd; B = Sn, Hf, Zr)

“…The good agreements ensure the reliability of our first-principles calculations. Furthermore, the calculated E g is smaller than the experimental values, 31,[41][42][43][44][45][46][47] which is the typical deviation of DFT calculations. 48 In RE 2 B 2 O 7 , there is only one configuration for the cation vacancies and cation antisites, but the interstitials and oxygen vacancies are not.…”

“…The good agreements ensure the reliability of our first‐principles calculations. Furthermore, the calculated E g is smaller than the experimental values, which is the typical deviation of DFT calculations …”