Eu 3+ -doped gadolinium oxyhydroxide Gd 1−x Eu x OOH crystals were synthesized by the flux method. The X-ray diffraction data for the crystals were well refined assuming a monoclinic structure with the P2 1 /m space group. Gd 1−x Eu x OOH (x ≤ 0.2) crystals showed strong red emission, and the highest fluorescence quantum yield (Φ f ) was 0.27, obtained for x = 0.10. Φ f decreased rapidly as the Eu 3+ content x increased above 0.2, owing to concentration quenching. Analysis with a percolation model indicated three-dimensional energy transfer between the Eu 3+ ions.

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