Using single-crystal x-ray diffraction we characterize the 235 K incommensurate phase transition in the hybrid molecular framework tetraethylammonium silver(I) dicyanoargentate, $[{\mathrm{NEt}}_4]{\mathrm{Ag}}_3(\mathrm{CN}{)}_4$. We demonstrate the transition to involve spontaneous resolution of chiral $[{\mathrm{NEt}}_4{]}^{+}$ 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 ${\mathrm{NaNO}}_2$, we suggest the XDW state arises through coupling between acoustic (shear) and molecular rotoinversion modes. Such coupling is symmetry forbidden at the Brillouin zone center but symmetry allowed for small but finite modulation vectors $\mathbf{q}=[0,0,q_z{]}^{*}$. 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.

中文翻译：

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杂化分子框架中不相称的手性密度波跃迁

使用单晶X射线衍射，我们表征了杂化分子框架四乙基铵银（I）二氰基Argentate中的235 K不相称的相变， $[{网}_4]{银}_3（\mathrm{CN}{）}_4$。我们展示了涉及手性自发拆分的过渡$[{网}_4{]}^{+}$构象，从而导致分子手性在整个晶格中受到不适当调节的状态。我们将此状态称为不相称的手性密度波（XDW）相，它表示从根本上打破了固态的新型手性对称性。利用平行于铁的不平衡的铁电跃迁${\mathrm{硝酸钠}}_{\mathrm{2个}}$，我们建议XDW状态是通过声学（剪切）和分子旋转反转模式之间的耦合产生的。这种耦合在布里渊区中心是对称禁止的，但是对称允许很小但有限的调制矢量$\mathbf{q}=[0，0，q_{ž}{]}^{*}$。在这种混合框架的物理学中，长波长手性调制的重要性可能会对中尺度手性纹理的产生产生影响，这是高级光子材料所要求的。