**4. Conclusions**

It was established that the L-alanine–L-serine system belongs to systems featuring eutectic behavior with very limited solid solutions (less than 10 mol.%) in the vicinity of the components. Solid solutions shown are the compositions (mol.%): L-ser/L-ala = 93/7 and 7/93. Using the physical mixture L-ser/L-ala = 90/10 as an example, it is demonstrated that the limits of solid solutions expand with increasing temperature. At a temperature of 175 ◦C, the aforementioned mixture is homogenized.

The heating of the system components L-ser and L-ala revealed a pronounced anisotropy of the thermal deformations of their crystal structures. This is reflected in the values of the parameters of the thermal deformation tensor of L-ala and L-ser and, respectively, in the figures of their thermal expansion coefficients. A multidirectional synchronous change in the CTE values with an intensive increase in the parameter *b* and a decrease in the parameter *a* (up to negative values) indicates a hinge mechanism of thermal deformations.

The results obtained add new insights in the structure-property relationships of amino acid systems as compounds of biological, geochemical, and industrial importance.

**Author Contributions:** Conceptualization, E.K. and H.L.; methodology, E.K., H.L., L.K. and R.S.; formal analysis, R.S. and L.K.; investigation, R.S., L.K. and E.K.; writing—original draft preparation, E.K.; writing—review and editing, E.K., H.L. and R.S.; visualization, E.K., R.S., L.K.; supervision, E.K. and H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the President of Russian Federation grant to leading scientific schools (NSh-2526.2020.5).

**Acknowledgments:** The investigations were performed using equipment of the Resource Centre "Centre for X-ray Diffraction Studies" of St. Petersburg State University and support of the MPI Magdeburg PCF lab. The authors thank M. G. Krzhizhanovskaya, A. I. Isakov and A. A. Zolotarev Jr. for collaboration.

**Conflicts of Interest:** The authors declare no conflict of interest.
