A new intermediate polymorph of 1-fluoro-adamantane and its second-order-like transition toward the low temperature phase
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Phase transitions of 1-fluoro-adamantane have been thoroughly investigated by temperature-resolved second harmonic generation (TR-SHG) and X-ray powder diffraction (XRPD). A new polymorph - an intermediate centrosymmetric phase (MT) - between the known orientationally disordered high temperature phase (HT, Fm3-m, Z = 4) and the low temperature phase (LT, P4-21c, Z = 2) was unveiled by TR-SHG. The crystal structure of MT was resolved by XRPD in the P42/nmc (Z = 2) space group, and it is related to the LT phase in a group-subgroup relation. No evidence of any solid-solid transition between these two phases by differential scanning calorimetry (DSC) or cold-stage microscopy could be obtained. Therefore, combing TR-SHG, XRPD, DSC, and cold-stage microscopy results, a second-order transition mechanism is proposed for the MT ¿ LT transition. Moreover, the critical exponent (ß) of the order parameter was calculated by fitting TR-SHG data to a critical power law. The obtained ß value (0.26) is close to the value from XRPD data (0.25).
CitationYuan, L., Clevers, S., Burel, A., Negrier, P., Del Barrio, M., Ben Hassine, B., Mondieig, D., Dupray, V., Tamarit, J. Ll., Coquerel, G. A new intermediate polymorph of 1-fluoro-adamantane and its second-order-like transition toward the low temperature phase. "Crystal growth and design", 7 Juny 2017, vol. 17, núm. 6, p. 3395-3401.