Combustion synthesis of a Y₂O₃:Eu³⁺ phosphor
Europium-activated yttrium oxide was synthesised by combustion of the metal nitrates with urea, and the product was analysed by complexometric titration, X-ray diffraction and UV excitation at two wavelengths.
Context
This lab report was written for the inorganic chemistry laboratory course CHEM-C2223 at Aalto. Phosphors are materials in which excited electrons emit photons at visible wavelengths when the excitation is released, and they are used in, for example, watch faces and LED lamps. In lanthanide phosphors the emission comes from 4f → 4f transitions of an activator ion doped into a host lattice that does not itself emit visible light. In this work the host was yttrium oxide and the activator Eu³⁺, which emits red light at around 611 nm.
The phosphor was made by combustion synthesis, i.e. a mixture of metal nitrates as the oxidiser and urea as the fuel is heated until it ignites, and the exothermic reaction drives the temperature well above that of the furnace. Most of the reaction products leave as gas, so a fairly pure solid is left behind. The product was then analysed to see whether the synthesis had given the intended compound.
Approach
6.7394 g of Y(NO₃)₃·6H₂O, 0.3216 g of Eu(NO₃)₃·6H₂O and 3.0822 g of urea were dissolved in 25 cm³ of deionised water, and the crucible was placed in a muffle furnace at 500 °C. The reaction was observed after about ten minutes as a strong release of orange gas and a faint bang. After cooling, the product was ground with a pestle, crystallised at 1000 °C for one hour and left to cool in a desiccator for 30 minutes. From the amounts of the nitrates, the europium mole fraction of the product was calculated as 0.0394, i.e. (Y₀.₉₆₁Eu₀.₀₃₉)₂O₃, with a theoretical mass of 2.114 g and a theoretical rare earth content of 79.20 wt-%.
Three analyses were carried out. The luminescence was looked at under 365 nm and 254 nm UV light. The rare earth content was determined by complexometric titration of two parallel samples of about 0.1 g, which were dissolved in nitric acid, buffered to pH 5 and titrated with 0.0500 M EDTA using xylenol orange as the indicator. The molar mass of the product was calculated from the EDTA consumption and the rare earth content from the molar mass. Lastly, a powder diffractogram was measured with a PANalytical Aeris diffractometer using Cu Kα radiation over 2θ = 15–68°, and the d-values were compared with a reference card for (Y₀.₉₅Eu₀.₀₅)₂O₃.
Outcome
The product was a white, crumbly foam after the 500 °C step and a fine white powder after crystallisation. 2.135 g was weighed, which gives a yield of 101.0 %. The powder glowed red under both UV lamps, and the emission was considerably stronger under 254 nm. The titrations gave rare earth contents of 79.52 % and 79.56 %, against the theoretical 79.20 %. All 23 measured diffraction peaks had a match on the reference card, so no other crystalline phases were seen. Every measured d-value was slightly higher than the reference, which was attributed to the powder sitting slightly above the holder during the measurement.
The results have some weak spots. A yield above 100 % is not possible, and it was put down to moisture, since the nitrates are hygroscopic, their water content was not checked, and 30 minutes in the desiccator is too short to dry the product properly. The crucible also broke during crystallisation, which adds some uncertainty to the weighed mass. The report read the slightly high rare earth content as a hint of more europium than intended, but this does not hold up well. The titration counts all mass that is not oxygen as rare earth metal, so the same moisture blamed for the yield would raise the result in the same way. Explaining the measured molar mass of 234.3 g/mol with europium alone would need a europium fraction of about 0.067 instead of 0.039, which does not fit the weighed amounts. In any case, the difference of about 0.3 percentage points is close to the ±0.25 percentage point uncertainty coming from the EDTA concentration alone.
The explanation given for the stronger emission under 254 nm was also weak. The report argued that photons of higher energy cause more 4f → 4f transitions, but the energy of a single photon does not set how many ions are excited. The more likely reason, which the report mentions only in passing, is that 254 nm falls on the O²⁻ → Eu³⁺ charge transfer band, which absorbs much more strongly than the narrow f-f lines reached at 365 nm. The height displacement explanation for the XRD shift is also plausible but was not tested, for example with an internal standard.