Incommensurate chirality density wave transition in a hybrid molecular framework

Abstract

Using single-crystal X-ray diffraction we characterise the 235\,K incommensurate phase transition in the hybrid molecular framework tetraethylammonium silver(I) dicyanoargentate, [NEt${}_4$]Ag${}_3$(CN)${}_4$. We demonstrate the transition to involve spontaneous resolution of chiral [NEt${}_4$]${}^{+}$ conformations, giving rise to a state in which molecular chirality is incommensurately modulated throughout the crystal lattice. We refer to this state as an incommensurate chirality density wave (XDW) phase, which represents a fundamentally new type of chiral symmetry breaking in the solid state. Drawing on parallels to the incommensurate ferroelectric transition of NaNO${}_2$ we suggest the XDW state arises through coupling between acoustic (shear) and molecular rotoinversion modes. Such coupling is symmetry-forbidden at the Brillouin zone centre but symmetry-allowed for small but finite modulation vectors $\mathbf{q}=[0,0,q_z{]}^{\ast }$. The importance of long-wavelength chirality modulations in the physics of this hybrid framework may have implications for the generation of mesoscale chiral textures, as required for advanced photonic materials.

5 pages, 3 figures

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Cited by in corpus (1)

• Room temperature spin-ice physics in cadmium cyanide