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arXiv 1706.01121cond-mat.mtrl-sci
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杂化分子框架中的无公度手征密度波跃迁

Incommensurate chirality density wave transition in a hybrid molecular framework

Joshua A. Hill, Kirsten E. Christensen, Andrew L. Goodwin

摘要:

用单晶X射线衍射方法研究了四乙基铵银(I)二氰合物[NEt $_4$ ]Ag $_3$(CN)$_4$的235 K无公度相变.我们证明的过渡涉及自发分辨率的手性[NEt $_4$ ] $^+$构象,从而产生一个国家,其中分子的手性是inadmissive调制整个晶格。我们把这种状态称为无公度手征密度波(XDW)相,它代表了固态中一种全新的手征对称性破缺。绘制平行的NaNO $_2$的无公度铁电转变,我们建议XDW状态通过声学(剪切)和分子旋转反转模式之间的耦合而产生。这样的耦合在布里渊区中心处是完全禁止的,但对于小但有限的调制矢量$\mathbf q=[0,0,q_z]^\ast$是完全允许的。长波长的手性调制的重要性,这种混合框架的物理可能会产生影响的中尺度手性纹理,先进的光子材料所需的。

英文摘要:

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.

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