C6h8o7

To obtain the nHAp nanoparticles, microwave-stimulated hydrothermal technique based on the preparation methods described by P. Sobierajska and R.J. Wiglusz was applied [28 (link)]. The following reagents were used: Ca(NO₃)₂·4H₂O (Sigma-Aldrich, St. Louis, MO, USA, >99%), (NH₄)₂HPO₄, (Acros Organics, Geel, Belgium, 98+%) and C₆H₈O₇ (Alfa Aesar, Haverhill, MA, USA, 99+%). In the first step, 1.0577 g (4.48 mmol) of calcium nitrate tetrahydrate and 0.1721 g (0.08 mmol) of citric acid were dissolved in 25 mL of deionized water. Subsequently, water solution of 1.0647 g (8.06 mmol, 3-fold excess relative to the calcium ion concentration) diammonium hydrogen phosphate and 5 mL of ammonia (25%) were added, respectively. The obtained mixture was transferred to the Teflon vessel and put in a microwave reactor (Magnum II, ERTEC-Poland, Wrocław, Poland) under the pressure of 20–30 atm at 200 °C for 90 min. Afterwards, the obtained powder was washed with deionized water to obtain neutral pH and further dried in 70 °C for 24 h.

The powders of LiYO₂:1%Er³⁺, x% Yb³⁺ (x = 1, 2, 5, 10) nanocrystals were synthesized with a modified Pechini
method. Li₂CO₃ (99.9% purity, Chempur), Y₂O₃ (99.999% purity, Stanford Materials Corporation),
Er₂O₃ (99.99% purity, Stanford Materials Corporation),
Yb₂O₃ (99.99% purity, Stanford Materials Corporation),
C₆H₈O₇ (>99.5% purity, Alfa Aesar),
and H(OCH₂CH₂)_nOH,
(PEG-200, n = 200, Alfa Aesar) were used as starting
materials. Yttrium, ytterbium, and erbium oxides were dissolved in
deionized water with the addition of a small amount of HNO₃ (65% purity, Avantor) and then recrystallized three times to remove
the excess nitrogen. The 4-fold stoichiometric excess of lithium carbonate
was added to the water solution of nitrates. After that, anhydrous
citric acid and polyglycol were added to the mixture. The molar ratio
of citric acid to all metals was set up as 6:1. Meanwhile, PEG-200
and citric acid were used in a molar ratio of 1:1. Subsequently, the
obtained solution was dried for 3 days at 363 K until a resin was
formed. The produced resin of the samples with 1% Er³⁺ and x% Yb³⁺ (x = 1, 2, 5, 10) concentration
with respect to the number of Y³⁺ moles ions was annealed
in porcelain crucibles for 6 h in the air at a temperature of 1273
K.