**4. Discussion**

A search of new phases of molecular crystals at extreme conditions is an important area of modern chemistry. In this work, a unique bending crystal of LLHM, which preserves its plasticity at 77 K, was studied at low temperatures and high pressure. Based on literature data and computational vibrational spectra, major band regions were assigned to follow their changes at extreme conditions. Raman spectra of LLHM were recorded down to 11 K, showing no phase transition. Based on this experimental fact, we sugges<sup>t</sup> the possibility to preserve LLHM plasticity below liquid nitrogen temperature. High pressure experiments showed different behavior of LLHM crystal in paraffin and PIP used as PTM. We found no phase transition using paraffin up to 2.95 GPa. Raman spectra and optical microscopy gave reasons to postulate the LLHM phase transition, phase transition cascade, or amorphization at high pressure, which results in crystal destruction at PIP PTM at 1.35 GPa. Additional XRPD or SCXRD using synchrotron radiation may be applied to highlight additional details of LLHM high pressure behavior in future studies.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/cryst11121575/s1, Figure S1: Gas phase calculated (a) Raman and (b) IR spectra of L-Leucine (Black), Maleic acid (Red), and L-Leucinium hydrogen maleate (Blue), Figure S2: Experimental LLHM Raman spectra at low temperatures, showing no phase transition on cooling. Detailed three different regions of Raman spectra, Figure S3: Raman spectra of LLHM at multiple pressures in PIP (left) and paraffin (right) PTM. Diamond peak from anvil cell is marked with \*.

**Author Contributions:** Conceptualization, D.A.R.; methodology, D.A.R.; validation, D.A.R. and K.D.S.; formal analysis, K.D.S.; investigation, D.A.R.; resources, D.A.R.; data curation, D.A.R. and K.D.S.; writing—original draft preparation, D.A.R.; writing—review and editing, D.A.R.; visualization, K.D.S.; supervision, D.A.R.; project administration, D.A.R.; funding acquisition, D.A.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** Computational part of this research was funded by the Russian Science Foundation, gran<sup>t</sup> number 21-73-00094.

**Data Availability Statement:** The data presented in this study are available in the Supplementary Information section.

**Acknowledgments:** Denis Rychkov acknowledge Elena Boldyreva for basic idea to study the crystals of amino acids and their salts, including L-Leucinium Hydrogen Maleate system, at low temperatures and high pressures. All the experiments were performed at REC-008 "Molecular design and ecologically safe technologies" of the Novosibirsk State University using its instruments: The crystals of L-Leucinium Hydrogen Maleate were grown by T. Nguyen (under joint guidance of S. Arkhipov and D. Rychkov), S. Arkhipov has collected and preliminary processed Raman spectroscopy data at low temperature. E. Losev performed high-pressure X-ray diffraction and Raman spectroscopy experiments and provided the optical microphotographs. S. Arkhipov and E. Losev have performed preliminary data analysis. The authors thank S. Arkhipov and E. Losev for fruitful discussions. The authors are also grateful to A. A. Iurchenkova for assistance in preparing figures. The Siberian Branch of the Russian Academy of Sciences (SB RAS) Siberian Supercomputer Center is gratefully acknowledged for providing supercomputer facilities. The authors also acknowledge the Supercomputing Center of the Novosibirsk State University (http://nusc.nsu.ru) for providing computational resources.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
