Europium-Doped Lanthanum Hafnate Nanoparticles: Structure, Photoluminescence, and Radioluminescence

Due to their novel physical properties, nanostructured phosphors are of interest for radiation-based imaging and therapeutics. Herein, the structural and luminescent properties of europium-doped lanthanum hafnate (La$_{\mathrm{2}}$Hf$_{\mathrm{2}}$O$_{\mathrm{7}}$:xmol{\%}Eu$^{\mathrm{3+}}$, x $=$ 0 - 35) nanoparticles are investigated for use as scintillators. X-ray diffraction, Raman spectroscopy, and scanning electron microscopy confirm samples prepared through a combined co-precipitation and low-temperature molten salt synthetic process homogenously form spherical nanocrystals of $\sim $ 36 nm in the ordered pyrochlore phase. Ultraviolet and X-ray excitation of these samples induce strong red emissions in the 580 - 590 and 612 - 630 nm range corresponding to the $^{\mathrm{5}}$D$_{\mathrm{0}}\to ^{\mathrm{7}}$F$_{\mathrm{1}}$ magnetic dipole and $^{\mathrm{5}}$D$_{\mathrm{0}}\to^{\mathrm{7}}$F$_{\mathrm{2}}$ electric dipole transitions of Eu$^{\mathrm{3+}}$. Optical response and quantum yield are optimized at 5{\%} Eu$^{\mathrm{3+}}$; a proposed trade-off between quenching mechanisms (defect-states/cross-relaxation) and dopant concentration is discussed. Owing to their high density, large effective atomic number, and bright luminescence, these La$_{\mathrm{2}}$Hf$_{\mathrm{2}}$O$_{\mathrm{7}}$:xmol{\%}Eu$^{\mathrm{3+}}$ nanoparticles warrant further investigation for scintillator applications.