New Intermediate Polymorph of 1-Fluoro-adamantane and Its Second-Order-like Transition toward the Low Temperature Phase.

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, Fm3m, Z = 4) and the low temperature phase (LT, P421c, 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 (β) of the order parameter was calculated by fitting TR-SHG data to a critical power law. The obtained β value (0.26) is close to the value from XRPD data (0.25). [ABSTRACT FROM AUTHOR]