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1. How many more polymorphs of ROY remain undiscovered

   https://doi.org/10.1039/d1sc06074k  

   Beran, Gregory J.; Sugden, Isaac J.; Greenwell, Chandler; Bowskill, David H.; Pantelides, Constantinos C.; Adjiman, Claire S. (February 2022, Chemical Science)

   With 12 crystal forms, 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecabonitrile (a.k.a. ROY) holds the current record for the largest number of fully characterized organic crystal polymorphs. Four of these polymorph structures have been reported since 2019, raising the question of how many more ROY polymorphs await future discovery. Employing crystal structure prediction and accurate energy rankings derived from conformational energy-corrected density functional theory, this study presents the first crystal energy landscape for ROY that agrees well with experiment. The lattice energies suggest that the seven most stable ROY polymorphs (and nine of the twelve lowest-energy forms) on the Z′ = 1 landscape have already been discovered experimentally. Discovering any new polymorphs at ambient pressure will likely require specialized crystallization techniques capable of trapping metastable forms. At pressures above 10 GPa, however, a new crystal form is predicted to become enthalpically more stable than all known polymorphs, suggesting that further high-pressure experiments on ROY may be warranted. This work highlights the value of high-accuracy crystal structure prediction for solid-form screening and demonstrates how pragmatic conformational energy corrections can overcome the limitations of conventional density functionals for conformational polymorphs. 

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2. Reliable crystal structure predictions from first principles

   https://doi.org/10.1038/s41467-022-30692-y  

   Nikhar, Rahul; Szalewicz, Krzysztof (December 2022, Nature Communications)

   Abstract An inexpensive and reliable method for molecular crystal structure predictions (CSPs) has been developed. The new CSP protocol starts from a two-dimensional graph of crystal’s monomer(s) and utilizes no experimental information. Using results of quantum mechanical calculations for molecular dimers, an accurate two-body, rigid-monomer ab initio-based force field (aiFF) for the crystal is developed. Since CSPs with aiFFs are essentially as expensive as with empirical FFs, tens of thousands of plausible polymorphs generated by the crystal packing procedures can be optimized. Here we show the robustness of this protocol which found the experimental crystal within the 20 most stable predicted polymorphs for each of the 15 investigated molecules. The ranking was further refined by performing periodic density-functional theory (DFT) plus dispersion correction (pDFT+D) calculations for these 20 top-ranked polymorphs, resulting in the experimental crystal ranked as number one for all the systems studied (and the second polymorph, if known, ranked in the top few). Alternatively, the polymorphs generated can be used to improve aiFFs, which also leads to rank one predictions. The proposed CSP protocol should result in aiFFs replacing empirical FFs in CSP research. 

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3. Polymorphism and π Stacking Affect Thermal Expansion Behavior in Halogen-Bonded Cocrystals Based on 1,4-Diiodoperchlorobenzene

   https://doi.org/10.1021/acs.cgd.3c01435  

   George III, Gary C.; Karimi, Mahshad; Juneja, Navkiran; Unruh, Daniel K.; Groeneman, Ryan H.; Hutchins, Kristin M. (March 2024, Crystal Growth & Design)

   The thermal expansion behavior of a series of halogen-bonded cocrystals containing 1,4-diiodoperchlorobenzene as the donor is described. Two of the solids are polymorphs and contain 4-stilbazole as the acceptor, while the third solid contains 4-(phenylethynyl)pyridine as the acceptor, and this solid is isostructural with one of the polymorphs. All solids are sustained by I···N halogen bonds, and the least thermal expansion occurs along this direction in all solids. The polymorphs exhibit significant differences in π stacking, and we show that electronically similar face-to-face stacked rings undergo more expansion compared to electronically different stacked rings. Moreover, in the two polymorphs, the directions of moderate expansion and most expansion are reversed, demonstrating how cocrystal polymorphism can affect material properties. 

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4. Effect of different crystallographic properties on the electrical conductivity of two polymorphs of a spin crossover complex

   https://doi.org/10.1088/1361-648X/ad9a81  

   Mahbub, Rifat; McElveen, Kayleigh A; Zaz, M Zaid; Ekanayaka, Thilini K; Mishra, Esha; Bissell, Eric; Banerjee, Parag; Shapiro, David; Lai, Rebecca Y; Dowben, Peter A; et al (December 2024, Journal of Physics: Condensed Matter)

   Abstract In this study, the structure and transport properties of two polymorphs, nanoparticles and nanorods, of the iron(II) triazole [Fe(Htrz)₂(trz)](BF₄) spin crossover complex were compared. Conductive atomic force microscopy was used to map the electrical conductivity of individual nanoparticles and nanorods. The [Fe(Htrz)₂(trz)](BF₄) nanorods showed significantly higher conductivity compared to nanoparticles. This difference in electrical conductivity is partially associated to the different Fe–N bond lengths in each of the polymorphs, with an inverse relationship between Fe–N bond length and conductivity. Transport measurements were done on the nanorods for both high spin (at 380 K) and low spin (at 320 K) states under dark and illuminated conditions. The conductance is highest for the low spin state under dark conditions. In illumination, the conductance change is much diminished. 

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5. Influence of Polymorphism on the Electronic Structure of Ga ₂ O ₃

   https://doi.org/10.1021/acs.chemmater.0c02465  

   Swallow, Jack E.; Vorwerk, Christian; Mazzolini, Piero; Vogt, Patrick; Bierwagen, Oliver; Karg, Alexander; Eickhoff, Martin; Schörmann, Jörg; Wagner, Markus R.; Roberts, Joseph W.; et al (September 2020, Chemistry of Materials)

   The search for new wide-band-gap materials is intensifying to satisfy the need for more advanced and energy-efficient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure–electronic structure relationships. Valence and conduction electronic structure as well as semicore and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provides detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations. 

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