A new intermediate polymorph of 1-fluoro-adamantane and its second-order-like transition toward the low temperature phase

Abstract

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).