Hybrid glasses from strong and fragile metal-organic framework liquids

Abstract

Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density 'perfect' glass, similar to those formed in ice, silicon and disaccharides. This order-order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order-disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and opens up the possibility of 'melt-casting' MOF glasses.

Figure 1. Phase transitions of ZIF-4 on heating.

(a) Highlighting the rings and imidazolate linkages in zeolitic topologies, in the ordered structure of crystalline ZIF-4 (left) and ZIF-8 (center), and the disordered HDA phase (right) obtained by Molecular Dynamics modelling (Supplementary Methods). Zn, orange; N, blue; C, green; and H, grey. (b) Thermogravimetric analysis and C_p plots for ZIF-4 and ZIF-8, showing, for the former (inset), exothermic collapse to the LDA phase (1) which is closely followed by (2) endothermic formation of the HDA phase, and (3) recrystallization (exothermic). Endothermic melting (4) then follows before thermal degradation. (c) X-ray PDF data D(r) measured for the MQG (green), ZIF-4 (broken black) and the HDA phase (broken pink). The X-ray total scattering data S(q) is presented in Supplementary Fig. 1. Inset: optical images of (left) ZIF-4 (right) MQG, showing the typical fracture pattern of a non-metallic bulk glass. Scale bars, 100 μm.

Figure 2. SAXS/WAXS data on ZIF-4 (top) and ZIF-8 (bottom).

(a) I(q)_SAXS profiles of ZIF-4, with Lorentzian fits (Supplementary Methods) and three-dimensional plot (inset), highlighting the emergence of a peak between 618 and 663 K (Supplementary Fig. 2f). (b) WAXS data shows the major loss of Bragg diffraction on collapse at ca. 642 K. (c) I(q)_SAXS profiles with Lorentzian fits and three-dimensional plot of the SAXS results for ZIF-8. (d) WAXS data show the retention of crystallinity across the entire temperature range studied.

Figure 3. Dynamics of ZIF-4 amorphization, polyamorphic glass transitions and coexistence

(a) Sequence of DSC up-scans on ZIF-4 at 10 K min⁻¹ starting with ZIF-4 (black), showing: solvent release (A), collapse to LDL phase (D–F), followed by the LLT to HDL (F–H). The jump in the isobaric heat capacity (C_p) through the LLT (E–G) is 0.33 J g⁻¹ K⁻¹. ΔC_p is the difference in C_p from glass to liquid at T_g, being 0.11 and 0.16 J g⁻¹ K⁻¹ for LDA and HDA phases, respectively. The endotherms in successive scans (2–red, 3–green) relate to HDA phase. (b) DSC second up-scans on the same samples at different rates right after cooling, yielding T_g and m for HDA. (c) The change in integrated SAXS , showing the increase of the peak temperature (T_peak) for different heating rates, giving T_g and m for the LDA phase. Inset: dependences of the Maxwell viscosity η=G_∞.τ, where G_∞ and τ are the adiabatic shear modulus (2 GPa) and structural relaxation time ∼1/heating rate, respectively. (d) DSC up-scans preheated to temperatures A(529 K), B (563 K), C(578 K), D(588 K), E(601 K), F(608 K), G(613 K), H(673 K), cooled back to room temperature, and then reheated to 673 K—all at 10 K min⁻¹. Arrows indicate T_g HDA increasing and T_g LDA decreasing with increases in initial scan temperature. Temperature at 588 K reveals coexistence of LDA and HDA. With double scans (d), amorphization stages occur 20 K lower than for single scans (a).

Figure 4. Fragilities and critical point of ZIF-4 polyamorphs, projecting T_m from T_g, and PEL schematic of ZIF-4 amorphization, melting and quenching routes.

(a) Angell plot showing the fragility of LDL and HDL ZIF-4 (Fig. 3b,c), alongside other glass-forming liquids including the silica with <20 p.p.m. hydroxyl and <60 metallic impurities. Solid lines are fits to the measured viscosity-temperature relation of the model derived in previous literature. (b) T–P phase diagrams obtained from the limiting thermobaric amorphization parameters for ZIF-4 P₁, P₂, T₁ and T₂, which extrapolate to a critical point C at negative pressure T_c (659 K) and P_c (–0.063 GPa). P_A and T_A refer to 50% amorphization points under pressure (RP) and temperature (RT), respectively. (c) 2/3's Law (T_g versus T_m) for different glass-forming systems, including ZIF-4 and ZIF-8 compared with DAF-2 and sodalite, respectively. The thermal degradation temperature separating the locations of the two amorphized ZIFs is shown. (d) Schematic of the PEL for ZIF-4, informed from DSC experiments from Figs 1b and 3a. The adjacent LDA and HDA minima bear resemblance to the two states for water, different in density and topology, recently identified in modelling ST2 water.