*4.1.4.2 Structure of the high-temperature solid phases (slightly lower than the melting temperature)*

High-temperature solid phases called "rotator phases" exist only in a few degrees between the transition temperature in the solid state and the melting temperature. These high-temperature phases maintain an organized structure. In these rotator phases noted, respectively, β-RI and α-RII, the disorder is caused by a 180° rotation of some chains around their axis, which allows them to take a balanced position [24].

The β0 (n-C2p+1) low-temperature phases of odd n-alkanes undergo a solid state transition accompanied by a significant enthalpic effect that characterizes the appearance of a new orthorhombic phase, noted b. Then, this phase passes into a state called rotator noted β-RI, which has been highlighted by heating only in odd n-paraffin for 9 ≤ n ≤ 25. This phase has an orthorhombic structure with the space group Fmmm, where two layers of molecules generate the periodicity in the direction perpendicular to the stacking plane according to the sequence …ABAB… In the RI state of the β phase, the parameters (a, b) of the mesh continuously change without changing the space group: this causes an evolution as a function of temperature, which certainly reflects a second-order transition.

The rotator phase noted, α-RII, was highlighted in the case of even and odd paraffin for 22 ≤ n ≤ 26. The increasing temperature study of the β-RI phase in odd n-alkanes (tricosane and pentacosane) identified the α-RII phase. The transition from the b-RI phase to the α-RII phase occurs when the mesh parameter ratio b/a is equal to √ \_\_ 3. A change in the stacking sequence accompanies this evolution with the succession … ABCABC… for the phase α-RII. The transition from the β-RI phase to the α-RII phase is a first-order transition, with a small enthalpy effect.

**Figure 3.** *Triclinic structure of even n-alkanes.*
