Phase transitions of 1-fluoro-adamantane have been thoroughly investigated by temperatureresolved 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, F ̅ , Z=4) and low temperature phase (LT, P ̅ , Z=2) was unveiled by TR-SHG. The crystal structure of MT was resolved by XRPD in the P mc (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 microscope could be obtained. Therefore, combing TR-SHG, XRPD, DSC and cold-stage microscope results, a second-order transition mechanism is proposed for MT LT transition. h (β) f w by fitting TR-SHG data to a critical w w. Th b β (0.26) h from XRPD data (0.25).

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