Magnetically-driven structural phase transition in hexamethylbenzene

TL;DR

This study uncovers a complex, multi-stage magnetically-driven structural phase transition in hexamethylbenzene near 118 K, involving successive symmetry-lowering transformations and Jahn-Teller distortions linked to methyl group ordering.

Contribution

It reveals the detailed sequence of structural and magnetic transitions in hexamethylbenzene, highlighting the role of methyl group degeneracy and Jahn-Teller effects in low-temperature phase behavior.

Findings

Multiple successive phase transitions at 110.7 K, 109.5 K, 109.1 K, and 107.8 K.

Identification of a highly degenerate ground state involving methyl groups.

Proposal of Jahn-Teller distortions as a key mechanism in the phase transition.

Abstract

The methyl groups in hexamethylbenzene C$_6$(CH$_3$)$_6$ 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 is actually comprised of four successive transformations upon cooling at T$_1$=110.7 K, T$_2$=109.5 K, T$_3$=109.1 K and T$_4$=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 2$^6$=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 T$_1$ and T$_2$ are temperatures at which the molecules successively lower their symmetry to remove the energy degeneracy which involve methyl group elongation and further out-of-plane tilting. This triggers the system to phase transition into the near-cubic phase which involves a shearing of the molecular planes and partial rotation of the methyl groups at T$_3$ and T$_4$. We interpret the low temperature near-cubic phase to be a manifestation of Jahn-Teller distortions based on energy degeneracies of the orbital motion of protons in each molecule and suggest that the metastable nature of the phase transition originates from the methyl groups requiring a larger amount of energy to order than to disorder. Our findings help explain why many unusual structural phase transitions occur at low temperatures in other molecular crystals possessing periodic motion of protons.