Lasave J, Koval S, Laio A, Tosatti E
Instituto de Física Rosario, Consejo Nacional de Investigaciones Científicas y Técnicas and Universidad Nacional de Rosario, 2000 Rosario, Argentina.
International Center for Theoretical Physics, I-34151 Trieste, Italy.
Proc Natl Acad Sci U S A. 2021 Jan 5;118(1). doi: 10.1073/pnas.2018837118. Epub 2020 Dec 21.
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.
普通冰具有一个质子无序相，该相在动力学上是亚稳的，无法在低温下自发地达到铁电（FE）基态，在该基态下仍存在残余的鲍林熵。在低温下用氢氧化钾进行轻掺杂时，会发生向FE冰的转变，但其微观机制仍需阐明。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，该项强制执行冰规则（IR）。在没有违反IR的缺陷的情况下，标准蒙特卡罗（MC）模拟使这个冰模型陷入具有正确鲍林熵的无序质子环构型状态。一种复制交换加速MC采样策略成功地在不进行开放路径移动、界面或非晶格构型的情况下，使这种无缺陷的冰达到平衡，通过明确的一级相变达到其低温FE序。当将模拟氢氧化钾杂质的质子空位植入保持IR的晶格中时，它们使标准MC模拟能够起作用，揭示了冰在转变温度以下从质子无序到部分FE序的演化动力学。每个杂质取代普通成核，打开一个质子环，产生一条线性链，这是一条实际的FE氢键线，随时间扩展。让人联想到自旋冰中描述的那些，这些杂质诱导的链也被认为存在于掺杂的水冰中，在那里IR更强。新出现的机制产生了长时间FE序分数对掺杂剂浓度和淬火温度的依赖性，这与实际的氢氧化钾掺杂冰中已知的情况相当吻合。
