We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF
3 and BaLaF
5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the
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We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF
3 and BaLaF
5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na
+ for La
3+ ions (97% of total Na
+ codopant) in the crystal system, and interstitial Na
+ (3% of detected Na
+) and La
3+ (3% of detected La
3+) present in (Na)LaF
3 and only direct substitution of Na
+ for Ba
2+ in Ba(Na)LaF
5. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08–0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E.
3+) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5–100 W/cm
2 range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm
3+ 3F
3 and
3H
4 states was observed and noted as a key initial condition for effective population of the
1D
2 and
1I
6 UV emitting states, via Tm-Tm cross-relaxation.
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