Proton strings and rings in atypical nucleation of ferroelectricity in ice
Proton strings and rings in atypical nucleation of ferroelectricity in ice
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DOI：
10.1073/pnas.2018837118
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发表时间：
2021-01-05
影响因子：
11.1
通讯作者：
Tosatti, E.
中科院分区：
文献类型：
--
作者：
Lasave, J.;Koval, S.;Tosatti, E.
普通冰具有质子无序相，它在动力学上是亚稳的，在低温下不能自发达到铁电(FE)基态，在那里仍然存在剩余的鲍林熵。在低温下光掺杂KOH后，发生了向Fe冰的转变，但其微观机制仍需澄清。我们引入了一个基于偶极相互作用加上一个竞争的、令人沮丧的项的晶格模型，该项实施了冰规则(IR)。在没有红外破缺缺陷的情况下，标准的蒙特卡罗(MC)模拟使该冰模型处于无序的质子环构型状态，并具有正确的Pauling熵。副本交换加速的MC采样策略在没有开放路径移动、界面或非晶格配置的情况下成功地平衡了这种无缺陷的冰，通过定义明确的一阶相变达到了其低温FE顺序。当模拟KOH杂质的质子空位被植入IR守恒晶格时，它们使得标准的MC模拟能够工作，揭示了冰在转变温度以下从质子无序向偏FE有序演化的动力学。每个杂质取代了普通的成核作用，打开了一个质子环，产生了一条线，这是一个实际的FE氢键线，随着时间的推移而膨胀。这让人想起那些关于自旋冰的描述，这些杂质诱导的弦被认为也存在于掺杂的水冰中，那里的红外光谱甚至更强。新出现的机制产生了长时间的FE有序分数与掺杂浓度和淬火温度的依赖关系，这与现实生活中KOH掺杂的冰中的已知结果相比是有利的。
Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.