Understanding the structure-property relationship and optimizing properties of phosphors for use in lighting and scintillation fields is an important materials challenge. In this work, we investigated the effects of the pH value of the coprecipitating solution adjusted by the concentration of NHOH(aq) on the structure and optical properties of the obtained LaHfO nanoparticles (NPs). The obtained NPs stabilize in the ideal pyrochlore structure, but the extent of ordering increased with an increase in the pH value used. The NPs prepared at pH = 12.1 displayed the best optical performance owing to the balance of the crystallinity, agglomeration, and surface defects. On the basis of density functional theory (DFT) calculations, the origin of violet-blue emission in undoped LaHfO NPs was attributed to defect states in the electronic band gap arising due to oxygen defects. For the LaHfO:Eu NPs, the Eu dopants possess low symmetry and their occupancy is more favorable at the LaO site. DFT calculations further justify the complete host-to-dopant energy transfer and origin of the most intense red emission observed experimentally. Understanding the interplay of the experimental and theoretical results thus is a very useful general approach for improving the efficiency of luminescent materials.
High temperature annealed La2Hf2O7:Eu3+ nanoparticles favor tunneling of Eu3+ to symmetric sites showing orange emission, whereas low temperature annealed samples favor red emission.
Ln
3+
-ion-doped nanomaterials
possess excellent properties
because of their high color purity, longer excited state lifetime,
narrow emission, and large Stokes shifts. In this work, we studied
the correlation between the luminescence properties of La
2
Hf
2
O
7
:Eu
3+
pyrochlore nanoparticles
(NPs) synthesized by a molten salt synthesis (MSS) method at a relatively
low temperature and several MSS processing durations (from 1 to 12
h). We synthesized these NPs with different sizes just by changing
the MSS processing time without subjecting to high temperature. Raman
spectroscopy confirmed the stabilization of the ideal pyrochlore structure
of the La
2
Hf
2
O
7
:Eu
3+
NPs
at various MSS processing durations. The synthesized NPs exhibited
bright red emission under UV, visible, and X-ray excitations, highlighting
their potential applications as a red phosphor and scintillator. As
the MSS processing time was increased from 1 to 12 h, a spectral change
in the position of the charge transfer state in the La
2
Hf
2
O
7
:Eu
3+
NPs was observed. The
sample processed by the MSS with a duration of 3 h exhibited the highest
luminescence intensity, which was attributed to its optimum crystals
with least surface defects and less agglomeration. The obtained results
strongly and unambiguously indicate the brighter side of this new
type of pyrochlore-based NPs in the fast growing field of solid-state
lighting and scintillator materials.
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.
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