The investigation of ultrafine and nanocrystalline oxide upconversion materials has been studied for fundamental research and potential applications in upconversion phosphors, detectors for infrared radiation, and fluorescent labels for sensitive detection [1]. Er³⁺ ion is the most popular and efficient for upconversion because the metastable levels ⁴I_9/2 and ⁴I_11/2 of Er³⁺ can be conveniently populated by low-cost commercial high-power 800 nm and 980 nm laser diodes [2]. The sensitization of rare earth ions doped materials with Yb³⁺ ions is a well-known method for increasing the optical pump efficiency because of the high absorption cross-section of Yb³⁺ ions around 980 nm and the efficient energy transfer from Yb³⁺ to other rare earth ions [3].
Different formulae describe erbium and ytterbium-doped strontium titanate nanopowders: (i) Sr1-3(0.01+x)/2Er0.01YbxTiO3 and (ii) Sr1-3(0.01+x)/2ErxYb0.01TiO3, where 0 < x < 0.07. Soft chemistry synthesis methods employ high-purity starting materials and can control the final characteristics, such as microstructure, chemical homogeneity, shape, etc. Precursor powders were synthesized using the sol-gel method. After annealing at 1000^oC for 3 hours, the powders show single-phase compositions for all the samples. A slight decrease in the average particle size and a higher aggregation tendency were observed with the increase in the dopants concentration.
SEM investigations of the ceramics sintered at 1400^oC for 4 hours indicate significant changes in the microstructural features due to dopant content and stoichiometry. A noticeable decrease in the average grain size was observed as the Er³⁺ content increased. On the other hand, more homogeneous and denser microstructures were observed for the ceramics with higher Yb³⁺ concentration than those with higher Er³⁺ content. 
The luminescence properties of the perovskites are improved by increasing the Ytterbium concentration.
This work was supported by a grant of the Ministry of Research, Innovation, and Digitization, CNCS – UEFISCDI, project number PN-III-P1-1.1-TE-2021-1624 within PNCDI III and Program NUCLEU - LAPLAS VII, contract 30N/2023.

References:
1.  Hai Guo et al. J. Alloys Compd., 415 (1–2)  (2006), pp 280-283
2. J.A. Capobianco et al., J. Phys. Chem. B, 106 (2002), pp. 1181-1187
3. F. Vetrone et al., J. Phys. Chem. B, 107 (2003), pp. 1107-1112