Nanocomposites and nanomaterials
Substitution limits and stability of LnxSc2–xSiO5 solid solutions to be used for multi-nanolayer films
Get’man E.I., Radio S.V.

Vasyl’ Stus Donetsk National University, Vinnytsia, Ukraine
e-mail: radio@donnu.edu.ua
Solid solutions synthesized at high temperatures are prone to decomposition and, upon cooling, can change their composition and properties uncontrollably. Therefore, before synthesizing and studying the dependence of properties on composition, it is necessary to know the limits of substitutions and the phase stability of solid solutions. Their experimental determination is hampered by the complexity of reaching an equilibrium due to low diffusion rate at low temperatures and solutions’ decay as a result of high diffusion rate at high temperatures. Therefore, we recommend using the calculation methods without the abovementioned disadvantages. The paper presents the calculation of substitution limits in solid solutions of oxyorthosilicates of rare-earth elements (REE) LnxSc2–xSiO5 (Ln = Tb–Lu, and Y), which can be used as a silicon-based light source with high optical amplification, since they have a higher Er concentration compared to Si-based Er-doped materials [1].
The calculation was carried out using the V.S. Urusov crystal-chemical method [2] within the approximation of regular solutions. It was established that in the systems containing REE from Tb to Lu the mixing energy (34.5–10.9 kJ/mol), critical decay temperatures (2060–650 K) and decomposition temperatures for compositions with x = 0.20, 0.10, 0.05, 0.03, and 0.01 are expectedly decreasing.
The areas of stability, instability and metastability of solid solutions were estimated. A diagram of thermodynamic stability was built (see Fig.), which allows to quickly determine the decomposition temperature by a given equilibrium limit of substitution (x), or the limit of substitution by a given temperature.	
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x = 0.01

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Figure.	

1. Najar A., Omi H., and Tawara T. Effect of structure and composition on optical properties of Er–Sc silicates prepared from multi-nanolayer films // OPTICS EXPRESS.-2015.–23, N 6.–P. 7021-7030.
2. Urusov V. Energetic theory of miscibility gaps in mineral solid solutions // Fortschr. Mineral.–1975.–52. –P. 141-150.