The methyl groups in hexamethylbenzene C6(CH3)6 become magnetically ordered at the molecular level below 118 K. This is also near the temperature at which the system structurally transitions from triclinic to a unique near-cubic phase. High-precision measurements of the near-static dielectric constant reveal that the structural phase transition actually comprises four successive transformations upon cooling at T1 = 110.7 K, T2 = 109.5 K, T3 = 109.1 K, and T4 = 107.8 K. In contrast to warming, only two transitions occur at T′4 = 119.2 K and T′1 = 120.9 K. The methyl groups in the near-cubic phase become slightly distorted according to existing neutron powder diffraction measurements. Analysis of the 26 = 64 possible spin orientation configurations of the methyl groups reveal a 20-fold ground state degeneracy presiding in each molecule rendering the system to become highly unstable. From such, it is interpreted that T1 and T2 are temperatures at which the molecules successively lower their symmetry to remov...

Dielectric Properties of Ammonium Halides

Citations

Magnetically Driven Structural Phase Transition in Hexamethylbenzene

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