English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

The methyl groups in hexamethylbenzene C$_6$(CH$_3$)$_6$ become magneticallyordered at the molecular level below 118 K. This is also near the temperatureat which the system structurally transitions from triclinic to a uniquenear-cubic phase. High-precision measurements of the near-static dielectricconstant reveal that the structural phase transition is actually comprised offour 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 transitionsoccur at T'$_4$=119.2 K and T'$_1$=120.9 K. The methyl groups in the near-cubicphase become slightly distorted according to existing neutron powderdiffraction measurements. Analysis of the 2$^6$=64 possible spin orientationconfigurations of the methyl groups reveal a 20-fold ground state degeneracypresiding in each molecule rendering the system to become highly unstable. Fromsuch, it is interpreted that T$_1$ and T$_2$ are temperatures at which themolecules successively lower their symmetry to remove the energy degeneracywhich involve methyl group elongation and further out-of-plane tilting. Thistriggers the system to phase transition into the near-cubic phase whichinvolves a shearing of the molecular planes and partial rotation of the methylgroups at T$_3$ and T$_4$. We interpret the low temperature near-cubic phase tobe a manifestation of Jahn-Teller distortions based on energy degeneracies ofthe orbital motion of protons in each molecule and suggest that the metastablenature of the phase transition originates from the methyl groups requiring alarger amount of energy to order than to disorder. Our findings help explainwhy many unusual structural phase transitions occur at low temperatures inother molecular crystals possessing periodic motion of protons.

Magnetically-driven structural phase transition in hexamethylbenzene