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Crystals
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Polymorphism and Phase Transitions in Crystal Materials
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Polymorphism and Phase Transitions in Crystal Materials

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystal Engineering".

Deadline for manuscript submissions: 31 March 2026 | Viewed by 2624

Special Issue Editor

Dr. Łukasz Szeleszczuk

E-Mail Website
Guest Editor
Department of Organic and Physical Chemistry, Faculty of Pharmacy, Medical University of Warsaw, Banacha 1 St., 02-093 Warsaw, Poland
Interests: NMR; solid state NMR; DFT; PXRD; calculations; GIPAW; CASTEP; FT-IR; API; drug; polymorphs; polymorphism; hydrates; solvates; CSP; crystal structure prediction
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Polymorphs differ in crystal packing, and thus have different structure-related properties such as melting point, solubility, stability, hardness, elasticity, and color. As a general phenomenon in nature, polymorphism brings challenges to the manufacturing of a desired solid form with consistent properties against the formation of a second solid form, and opportunities in the sense that a new crystal structure can provide solids with higher solubility, mechanical strength or stability. As a result, the comprehension of the solid-state landscape for molecule of interests by form screening and predictions has become an increasingly important topic in the development of pharmaceutical solids, agrochemicals, explosives, dyes and pigments.

It is also essential to characterize the thermodynamics and kinetics of polymorph formation and their transitions in order to control the crystallization outcome and avoid potential form change risks.

Although an unexpected form change is undesirable in most cases, phase transitions by design can provide unique opportunities for creative applications. For example, researchers have shown great interest in employing phase-change materials in energy storage, due to the latent heat during phase transitions and in optical data storage due to the electrical conductivity difference between crystalline and amorphous phases. In recent years, a growing number of crystals have been discovered that present thermosalient effects, that is, a crystal jumps as a result of a polymorphic transition, simultaneously converting heat to work, which is sometimes reversible. These discoveries, together with others not mentioned, have greatly broaden the applications of phase transitions in crystals, and emphasize the importance of continuing research on the characterization and understanding of these processes.

This Special Issue aims to present the recent progress in the discovery, characterization, control, prediction and application of polymorphs and their transformations using both experimental and computational tools. We welcome original research and review papers both on single-component and multi-component crystals, on natural and man-made crystals, and on inorganic and organic crystals.

Dr. Łukasz Szeleszczuk
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2100 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • polymorph
  • polymorphism
  • phase transition
  • form change
  • crystal
  • cocrystal
  • solid
  • pharmaceutical
  • solid state

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Published Papers (3 papers)

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Research

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15 pages, 3287 KB  
Article
Functionalized Polyphenols: Understanding Polymorphism of 2-Chloro-3′,4′-Diacetoxy-Acetophenone
by Roxana Angela Tucaliuc, Sergiu Shova, Violeta Mangalagiu and Ionel I. Mangalagiu
Crystals 2025, 15(9), 780; https://doi.org/10.3390/cryst15090780 - 30 Aug 2025
Viewed by 606
Abstract
We report here an in-depth study concerning the synthesis, NMR, and X-ray structure determination of two new polymorphs of 2-chloro-3′,4′-diacetoxy-acetophenone. A new, ecologically friendly method of synthesis in the solid phase, as well as a suitable method for protecting hydroxyl functionality, is presented. [...] Read more.
We report here an in-depth study concerning the synthesis, NMR, and X-ray structure determination of two new polymorphs of 2-chloro-3′,4′-diacetoxy-acetophenone. A new, ecologically friendly method of synthesis in the solid phase, as well as a suitable method for protecting hydroxyl functionality, is presented. The 1H- and 13C-NMR spectra as well as the single crystal X-ray diffraction studies proved unambiguously the structure of the compounds: the two polymorphs of 2-chloro-3′,4′-diacetoxy-acetophenone and 2-chloro-3′-hydroxy-4′-acetoxy-acetophenone. The polymorph I crystalizes in the monoclinic P21/c space group, while polymorph II crystalizes in the Sohnke P212121 space group of the orthorhombic system, with no interstitial solvate molecules. Significant differences were observed in the supramolecular interactions in the crystal structure of the two polymorphs. Polymorph I is characterized as a parallel packing of weakly interacting supramolecular layers oriented in the 1 1 0 plane. The crystal structure of polymorph II is much more complex: each molecule is interconnected through 12 (twelve) hydrogen bonds with 9 (nine) adjacent symmetry-related molecules. The monoacetoxy derivative 2-chloro-3′-hydroxy-4′-acetoxy-acetophenone 3 crystallizes in the monoclinic P21/c space group, with one molecule in the asymmetric unit. Full article
(This article belongs to the Special Issue Polymorphism and Phase Transitions in Crystal Materials)
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Review

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35 pages, 2008 KB  
Review
Isosymmetric Phase Transitions in Crystals: From Subtle Rearrangements to Functional Properties
by Anna Maria Mazurek, Monika Franczak-Rogowska and Łukasz Szeleszczuk
Crystals 2025, 15(9), 807; https://doi.org/10.3390/cryst15090807 - 13 Sep 2025
Viewed by 348
Abstract
Isosymmetric phase transitions (IPTs) represent a rare class of solid-state transformations in which substantial structural reorganization occurs without a change in crystallographic symmetry. These phenomena, though subtle, can have a profound impact on the physical and functional properties of materials, offering novel opportunities [...] Read more.
Isosymmetric phase transitions (IPTs) represent a rare class of solid-state transformations in which substantial structural reorganization occurs without a change in crystallographic symmetry. These phenomena, though subtle, can have a profound impact on the physical and functional properties of materials, offering novel opportunities for property tuning without chemical modification. This review provides a comprehensive overview of the experimental and computational methods used to detect and characterize IPTs, including single-crystal and powder X-ray diffraction, Raman and FT-IR spectroscopy, differential scanning calorimetry, and advanced simulation techniques such as density functional theory, molecular dynamics, and crystal structure prediction. Special emphasis is placed on correlating local structural rearrangements—such as hydrogen-bond reconfiguration, polyhedral tilting, and molecular fragment reorientation—with macroscopic thermodynamic signatures. A broad selection of examples from the literature is discussed, covering molecular crystals, coordination compounds, organic functional materials, simple salts, and inorganic oxides, with detailed tables summarizing pressure- and temperature-induced IPTs. The review also analyses the primary factors that trigger IPTs, particularly temperature and pressure, and examines their role in governing structural stability and transformation pathways. By combining structural, spectroscopic, and thermodynamic perspectives, this work aims to consolidate the understanding of IPT mechanisms and to highlight their significance for the design of responsive crystalline materials. Full article
(This article belongs to the Special Issue Polymorphism and Phase Transitions in Crystal Materials)
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38 pages, 6660 KB  
Review
Field-Effect Crystal Engineering in Proton–π-Electron Correlated Systems
by Sachio Horiuchi, Hiromi Minemawari, Jun’ya Tsutsumi and Shoji Ishibashi
Crystals 2025, 15(8), 736; https://doi.org/10.3390/cryst15080736 - 19 Aug 2025
Cited by 1 | Viewed by 1009
Abstract
Dielectric crystals with switchable electric polarizations represent the key functional materials utilized for a broad range of practical applications. They allow for academically intriguing platforms, where the use of a strong external electric field can potentially unveil hidden crystal phases. Proton–π-electron correlated bistable [...] Read more.
Dielectric crystals with switchable electric polarizations represent the key functional materials utilized for a broad range of practical applications. They allow for academically intriguing platforms, where the use of a strong external electric field can potentially unveil hidden crystal phases. Proton–π-electron correlated bistable systems turn out to be promising for exploring such electrically induced crystal polymorphisms, mainly because strong π-electronic polarization can be sensitively switched depending on mobile hydrogen locations. Pseudo-symmetry and hydrogen disorder are utilized as clues for the data mining of the Cambridge Structural Database in the search for molecular candidates with novel switchable dielectrics. The polarization hysteresis, electrostriction, and second harmonic generation of the candidates were experimentally evaluated, together with the re-inspection of crystal structure. This feature article highlights the rich variation and competition of some candidate polarization configurations and switching modes in close relation to high and efficient electrical energy storage/discharge, large electrostriction effects, polarization rotations, and multistage switching phenomena. The experimental findings are well-reproduced by the computational optimization of crystal structure and the simulation of the switchable polarization, piezoelectric coefficients, and relative stability for each of the real or hypothetical hydrogen-ordered crystal phases. Effective prediction and strategic design are thereby guaranteed by systematically understanding the appropriate integration of experimental, computational, and data sciences. Full article
(This article belongs to the Special Issue Polymorphism and Phase Transitions in Crystal Materials)
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