*2.2. Methods*

Crystals in the L-alanine–L-serine system were obtained by spontaneous crystallization from the aqueous solutions of 13 different L-ala/L-ser mixtures by reducing the temperature with the subsequent evaporation of the solvent. Mixtures with different contents of the components (mol.%) of L-ser/L-ala = 0/100, 7/93, 10/90, 15/85, 25/75, 35/65, 50/50, 65/35, 75/25, 85/15, 90/10, 93/7, and 100/0 were weighed on a Shimadzu AX200 scale (accuracy 0.0001 g). The solutions were prepared for a saturation temperature of 50 ◦C with the constant stirring of the solution using a magnetic stirrer, and afterwards were filtered through a microporous filter (pore size 1.2 μm). The solution was poured into a Petri dish and kept at room temperature for 5–7 days. The evaporation rate of the solvent was regulated by changing the position of the lid on the Petri dish. The resulting crystalline precipitate was removed from the solution and quickly dried on filter paper.

All the 13 samples obtained were investigated by means of Powder X-ray Diffraction (PXRD) using a Rigaku MiniFlex II diffractometer (Rigaku Co., Tokyo, Japan) with CuK<sup>α</sup> and CoK<sup>α</sup> irradiation and the 2θ range of 5–60◦. Three samples (L-ala, L-ser, and a sample of composition L-ser/L-ala = 90/10 mol. %) were studied by Temperature-Resolved Powder X-ray Diffraction (TRPXRD). These experiments were conducted in the atmospheric air using a Rigaku Ultima IV diffractometer (Rigaku Co., Tokyo, Japan) equipped with a high-temperature accessory and the following settings: CoK<sup>α</sup> irradiation, the 2θ range of 5–60◦, and the temperature range of 23–200 ◦C with the temperature pitch of 10 ◦C. The X-ray patterns were processed and the unit cell parameters in the whole temperature range were calculated using the PDXL 2.7 (Rigaku Co., Tokyo, Japan) and Topaz (Bruker AXS GmbH, Karlsruhe, Germany) software.

The temperature dependences of the orthorhombic unit cell parameters were used to calculate the thermal expansion coefficients (CTE) of the L-ala and L-ser crystal structures (α, 10−<sup>6</sup> ◦C<sup>−</sup>1) along the three orthogonal axes of the thermal deformation tensor (α11, α22, α33). The values obtained were utilized to plot the projections of the CTE figures onto the *ab*, *ac*, and *bc* planes of the corresponding orthorhombic crystal structures. In the orthorhombic syngony, the directions of the tensor axes coincide with those of the crystallographic axes. The TEV software (Thomas Langreiter and Volker Kahlenberg, Institute of Mineralogy and Petrography, Innsbruck, Austria) was used to calculate the thermal deformation tensor and plot the CTE figures.
