通过分子动力学和机器学习技术揭示ZIF-4热非晶化和ZIF-zni熔化的微观机制,Journal of Materials Chemistry A

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通过分子动力学和机器学习技术揭示ZIF-4热非晶化和ZIF-zni熔化的微观机制
Journal of Materials Chemistry A ( IF 10.7 ) Pub Date : 2024-01-24 , DOI: 10.1039/d3ta07361k
Emilio Méndez ₁ , Rocio Semino ₁

Affiliation

_{我们研究了两种分子式为 Zn(C 3} H ₃ N ₂ ) ₂的沸石咪唑酯骨架的热致环境压力有序-无序相变的微观机制：多孔 (ZIF-4) 和致密无孔 (ZIF) -zni）通过数据科学和计算机模拟方法相结合的多晶型。分子动力学模拟是通过 nb-ZIF-FF 力场在原子水平上进行的，该力场结合了配体-金属反应性，并依靠虚拟原子在 Zn ²⁺中心周围重现正确的四面体拓扑。力场能够再现 ZIF-4、ZIF-zni 以及这些晶体材料加热时分别产生的非晶相 (ZIF_a) 和液相 (ZIF_liq) 的结构。在四个相的结构数据库上计算的对称函数被用作训练神经网络的输入，该神经网络以 90% 的准确度预测属于局部 Zn ²⁺水平的四个相中的每一个的概率。我们应用这种方法来跟踪 ZIF-4 的非晶化和 ZIF-zni 的熔化沿着一系列分子动力学轨迹的时间演化。我们首先计算转变温度并确定相关的热力学状态函数。随后，我们研究了其中的机制。这两个过程均由两个步骤组成：(i) 对于 ZIF-4，首先形成低密度非晶相，然后形成最终的 ZIF_a 相，而 (ii) 对于 ZIF-zni，在形成 ZIF_a 相之前形成类似 ZIF_a 相。液相。^{这些过程涉及中心 Zn 2+}阳离子周围第一相邻配体的连接性变化。我们发现ZIF-4的非晶化是一个非各向同性的过程，并且我们将这种各向异性行为的根源追溯到配位键的密度和不稳定性。

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Microscopic mechanism of thermal amorphization of ZIF-4 and melting of ZIF-zni revealed via molecular dynamics and machine learning techniques

We investigate the microscopic mechanism of the thermally induced ambient pressure ordered–disordered phase transitions of two zeolitic imidazolate frameworks of formula Zn(C₃H₃N₂)₂: a porous (ZIF-4) and a dense, non-porous (ZIF-zni) polymorph via a combination of data science and computer simulation approaches. Molecular dynamics simulations are carried out at the atomistic level through the nb-ZIF-FF force field that incorporates ligand–metal reactivity and relies on dummy atoms to reproduce the correct tetrahedral topology around Zn²⁺ centres. The force field is capable of reproducing the structure of ZIF-4, ZIF-zni and the amorphous (ZIF_a) and liquid (ZIF_liq) phases that respectively result when these crystalline materials are heated. Symmetry functions computed over a database of structures of the four phases are used as inputs to train a neural network that predicts the probabilities of belonging to each of the four phases at the local Zn²⁺ level with 90% accuracy. We apply this methodology to follow the time-evolution of the amorphization of ZIF-4 and the melting of ZIF-zni along a series of molecular dynamics trajectories. We first computed the transition temperature and determined the associated thermodynamic state functions. Subsequently, we studied the mechanisms. Both processes consist of two steps: (i) for ZIF-4, a low-density amorphous phase is first formed, followed by the final ZIF_a phase while (ii) for ZIF-zni, a ZIF_a-like phase precedes the formation of the liquid phase. These processes involve connectivity changes in the first neighbour ligands around the central Zn²⁺ cations. We find that the amorphization of ZIF-4 is a non-isotropic process, and we trace back the origins of this anisotropic behaviour to the density and lability of coordination bonds.

更新日期：2024-01-24

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