发表机构
Mineral Physics Institute, Stony Brook University（石溪大学矿物物理研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过高压实验与理论计算，发现非中心对称尿素在10 GPa以上转变为中心对称的V'相，并揭示中间X相存在质子隧穿与电荷转移，挑战了传统化学直觉。
AI 中文摘要
尿素是一种简单的原型超分子晶体，由于N-H-O氢键的断裂与恢复，在低压下表现出丰富的多晶型现象。非中心对称尿素的高压多晶型（V'相）结晶于中心对称结构，因其潜在的外来结构（类似于对称的冰X相）而成为引人关注的研究目标。利用粉末X射线衍射、红外和拉曼光谱、二次谐波产生（SHG）测量（最高达20 GPa）以及基于约束进化方法的从头算晶体结构预测（CSP），研究了尿素的压力诱导多晶型现象。在10 GPa的转变压力下，SHG信号的显著下降表明高压多晶型确实具有中心对称性，这进一步由晶格振动模式中观察到的选择定则所证实，与非中心对称尿素的化学直觉相反。由X射线衍射、SHG和CSP计算获得的结构演化序列如下：相I（P421m；Z=2）从0到0.5 GPa，相III（P212121；Z=4）从0.5到5.2 GPa，以及相V'（P21/m；Z=6）超过10.0 GPa，该相在能量上与理论预测的相V（Pnma；Z=4）竞争。在5.2至10.0 GPa之间形成具有独特光谱和衍射特征的X相，这可解释为相III和V'之间的量子无序中间态，归因于在极度压缩环境下难以破坏氢键网络。N-H振动的软化以及与氢键相关振动强度的变化为X相中的质子隧穿和电荷转移相互作用提供了证据。
英文摘要
Urea is a simple prototype supramolecular crystal that exhibits rich polymorphism at low pressure due to broken and restored N-H-O hydrogen bonds. The high pressure polymorph (phase V') of acentric urea crystallizes in a centrosymmetric structure, which presents an appealing target because of its potential exotic structure, analogous to the symmetric ice phase X. The pressure-induced polymorphism of urea was studied using powder X-ray diffraction, infrared and Raman spectroscopy, second harmonic generation (SHG) measurements up to 20 GPa and ab initio crystal structure prediction (CSP) based on the constrained evolutionary approach. A strong decrease of the SHG signal at the transition pressure 10 GPa reveals that the high-pressure polymorph is indeed centrosymmetric, further confirmed by the selection rules observed in the lattice vibration modes, in contrast to chemical intuition for acentric urea. The structural evolution sequence obtained from X-ray diffraction, SHG and CSP calculations is as follows: phase I (P421m; Z=2) from 0 to 0.5 GPa, Phase III (P212121; Z=4) from 0.5 to 5.2 GPa, and phase V' (P21/m; Z=6) beyond 10.0 GPa which is energetically competitive with the theoretically predicted phase V (Pnma; Z=4). A phase X with distinct spectral and diffraction features forms between 5.2 and 10.0 GPa, which could be explained by a quantum disorder intermediate state between phase III and V', that is ascribed to the difficulty to disrupt the H-bonding network under extremely compressed environment. The softening of N-H vibrations and the change in intensity of the vibrations associated with the hydrogen bonding provide evidence for proton tunneling and charge-transfer interaction in phase X.
