Pressure-induced amorphization, mechanical and electronic properties of zeolitic imidazolate framework (ZIF-8)

Highlights

• Pressure induced transitions of ZIF-8 in a wide pressure-range was investigated by using ab initio simulations.

• Crystalline-crystalline and crystalline-amorphous phase transitions were observed.

• Mechanical and electronic properties were accurately estimated.

• The compression and tension strengths of the framework were found significantly different, as in compact bones.

Abstract

Ab initio molecular dynamics (AIMD) simulations are carried out to probe the high-pressure behavior of ZIF-8 over wide pressure-range. Under compression, the enormous distortions in the ZnN₄ tetrahedral units lead to a crystal-to-amorphous phase transition at around 3 GPa. During the amorphization process, the Zn-N coordination is retained. No other phase change but a possible fracture of the system is proposed above 10 GPa. Depending on released pressures, amorphous states with different densities are recovered. Yet when the applied pressure is released just before the amorphization, the rotations of imidazolate linkers (swing effect) cause an isostructural crystal-to-crystal phase transition, in agreement with experiments. In the tensile regime, no phase transition is perceived up to −2.75 GPa at which point the structural failure is observed. The crystal-amorphous phase transitions are also discovered at around 4 GPa under uniaxial compressions. The amorphous structures formed under uniaxial stress are about 20% denser than the one formed under the hydrostatic pressure. The average Young's modulus and Poisson's ratio of ZIF-8 are estimated to be around 5.6 GPa and 0.4, respectively. Interestingly, the tensile strength of ZIF-8 is found to be about 50% greater than its compressive strength. This paper shows that the experimentally observed phase transitions can be successfully reproduced with a clear explanation about the transition mechanism(s) at the atomistic level and all mechanical properties can be accurately calculated for a given ZIF structure by using AIMD simulations.

Introduction

Pressure-induced amorphization (PIA), a transition between crystal and amorphous phases, is attracting a widespread of interest due to its importance in materials science, physics and chemistry [1,2]. It is also considered as an alternative method to produce novel amorphous solids that may be structurally different from those fabricated by the commonly used methods, i.e. the rapid quenching of the melts [2]. The PIA phenomenon was first observed in ice (H₂O) [3], and shortly afterwards discovered in quartz [4] and coesite forms of SiO₂ [5]. The existence of PIA in various types of materials [2] has been demonstrated in later studies. So far, several suggestions have been proposed to explain the underlying mechanisms of PIA, including the breakdown of Born stability conditions [6], kinetic hindrance of phase transitions to a thermodynamically stable high-pressure phase [7], poly-tetrahedral packing [8] and so on [1]. However, there is no generally accepted picture on the driving force(s) of such a solid-state amorphization.

Zeolitic imidazolate frameworks (ZIFs), a sub-class of metal-organic frameworks (MOFs), are currently attracting considerable interest by reason of their relatively high chemical and thermal properties [9]. They also offer promising potentials for gas storage [[10], [11], [12], [13]], catalysis [[14], [15], [16]] and electrochemical [[17], [18], [19]] applications. They possess zeolite topologies, wherein each tetrahedral metal nodes (M = Zn(II) or Co(II)) coordinates to imidazolate-based linkers (Im⁻ = C₃H₃N₂⁻) to form (M(Im)₂) neutral open framework structure [12,20]. Specifically, the M-Im-M coordination linkage in ZIFs subtends an angle of around 145° at Im ring center, equivalent to the Si-O-Al angle in aluminosilicate zeolites [12,21]. ZIF-8 (C₈H₁₀N₄Zn), zinc tetrahedral bridged 2-methylimidazolate (mim), has a high symmetry sodalite (SOD) topology and crystallizes in the cubic I4̅3 m space group (a = 16.992 Å) [21]. The pores with a diameter around 12 Å connected by 3.5 Å diameter six-ring apertures with the 4-ring yield a large pore volume, ~2400 ų [22].

Pressure-induced transition (PIT) investigations on MOFs are generally categorized into two groups in the literature: adsorption-induced transitions (AIP), and mechanical-stress induced transitions (MSIT) [23,24]. AIP studies have mainly focused on clarifying the relationship between structural distortions and adsorbent (framework) –adsorbate (e.g. N₂) interactions during the adsorption process [[25], [26], [27], [28]]. The common technique for monitoring phase transitions in the MOFs is the diamond anvil cell (DAC) experiment, in which the frameworks are exposed to a hydrostatic pressure by means of pressure-transmitting medium (PTM) [22,29]. However, because of the pores being filled with PTM molecules, the framework changes to a more resilient structure, which causes a delay in amorphization pressure [30]. In addition, the results may be controversial due to framework-PTM molecules interactions and changes in the physical properties of the carrier fluid under high pressure [31]. So far, in situ TEM compression method was proposed to investigate the direct amorphization process of a single crystal MOF and its mechanical properties [31]. However, it is improbable that these experiments are widely available for now because of the equipment infrastructure required.

As a promising engineering material family, it is vital to determinate the mechanical properties and critical transitions pressure/temperature of the MOFs. A key problem with much of literature on determination of them for MOFs is use of gas molecules or liquids as PTM in both experimental and computational studies. Although these reports provide crucial information for some applications (such as gas adsorption, separation), it is not possible to reveal the mechanical properties of the frameworks reliably by using this method. Since the aforementioned experimental methods are not widely available for now, the most appropriate method is computational material tools. To date, the mechanical properties of ZIF-8 have been studied with both classical molecular dynamics (MD) and density functional theory (DFT) simulations. But as far as we know, such a wide pressure range has never been studied before. So in this work, we reported the high-pressure behavior of desolvated ZIF-8 over a wide compression and tension stresses ranging from −2.75 GPa to 50 GPa using ab initio molecular dynamics (AIMD) simulations, inspired by Suslick's [31] in-situ TEM compression study and Hu's [30] DAC experiment. We witnessed a crystal-amorphous phase transition, a possible fracture of the material on compression, and a crystal-crystal phase change upon decompression through the simulations. The amorphization took place under uniaxial compression as well. Under tension, the framework was failed at −2.75 GPa. The mechanical and electronic properties of the framework were also investigated.