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首页> 外文期刊>Crystal growth & design >Proton disorder in NH?N bonded [dabcoH]~+I~- relaxor: New insights into H-disordering in a one-dimensional H_2O ice analogue
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Proton disorder in NH?N bonded [dabcoH]~+I~- relaxor: New insights into H-disordering in a one-dimensional H_2O ice analogue

机译：NH？N键合的[dabcoH]〜+ I〜-弛豫子中的质子紊乱：一维H_2O冰类似物中H无序的新见解

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Protons in the NH~+?N hydrogen bonds linking the cations into strictly linear aggregates in the crystal of 1,4-diazabicyclo[2.2.2]octane hydroiodide ([C_6H_(13)N_2]~+?I ~-, dabcoHI) remain disordered, and the average crystal symmetry (space group P6m2) remains unchanged, down to 1.5 K, as evidenced by single-crystal neutron-diffraction, but against the expectation based on the third law of thermodynamics. This proton disorder reveals the role of the thermal-activation and proton-tunneling processes for the polarizability of hydrogen bonds and for the formation of polar nanodomains, resulting in the unique property of an anisotropic giant dielectric response in the chemically homogeneous compound. The dielectric constant exceeding several thousand in the direction of the crystal [z] axis between 150 and 300 K can be explained by the fluctuations of nanodomains, whereas the dielectric constant of about 100 at 10 K can be due to the residual contribution of tunneling protons in hydrogen bonds. The proton transfers disproportionate the dabco monocations into dications and neutral molecules and generate high crystal-field fluctuations. The consideration of possible crystal-symmetry changes shows that ferroelectric proton ordering would lead to the lowered symmetry of space group P3m1 within the hexagonal system, and antiferroelectric proton ordering would require a transformation to an orthorhombic phase; however, the proton-site correlations are too weak for these transformations to take place, and the resultant structure remains disordered. The crystal symmetry and the lack of Fowler-Bernal rules distinguish dabcoHI from the H_2O ice-I_h, and are essential for the dielectric properties of this substance.
机译：NH〜+？N氢键中的质子将阳离子连接到1,4-二氮杂双环[2.2.2]辛烷氢碘化物（[C_6H_（13）N_2]〜+？I〜-，dabcoHI）晶体中的严格线性聚集体。保持无序状态，平均晶体对称性（空间群P6m2）保持不变，低至1.5 K（单晶中子衍射证明），但与基于热力学第三定律的预期相反。这种质子紊乱揭示了热激活和质子隧穿过程对于氢键的极化性和极性纳米域的形成的作用，从而导致了化学均相化合物中各向异性的巨型介电响应的独特性质。介电常数在晶体[z]轴方向上超过几千，介于150和300 K之间，这可以通过纳米域的波动来解释，而在10 K处，介电常数约为100可以归因于隧穿质子的残留贡献在氢键中。质子转移不成比例的dabco单阳离子成指示剂和中性分子，并产生高的晶体场波动。对可能的晶体对称性变化的考虑表明，铁电质子有序化将导致六方晶系内空间群P3m1的对称性降低，反铁电质子有序化将需要转变为正交晶相。但是，质子-位点的相关性太弱，以至于无法进行这些转换，并且最终的结构仍然无序。晶体的对称性和Fowler-Bernal规则的缺乏使dabcoHI与H_2O ice-I_h区别开来，并且对于该物质的介电性能至关重要。

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