Transition from isotropic positive to negative thermal expansion by local Zr₆O₈ node distortion in MOF-801

INTRODUCTION

Thermal expansion, as an inherent physical property, significantly influences the performance and lifetime of materials. Consequently, designing smart materials with tunable or switchable thermal expansion properties is a significant yet challenging pursuit for materials scientists. The emergence of negative thermal expansion (NTE) materials presents a promising prospect^[1,2]. Generally, the coefficients of thermal expansion (CTEs) can be modified by precisely controlling the chemical substitution^[3]. For instance, substituting Sc³⁺ with isovalent but smaller radius cations such as Ga³⁺ or Fe³⁺ can induce local distortions, resulting in a continuous transition from NTE to positive thermal expansion (PTE)^[4]. In the (Sc,Ti)Fe₂-based intermetallic, a crossover from typical PTE observed in ScFe₂ and TiFe₂ to a pronounced NTE in (Sc_0.4Ti_0.6)Fe₂ was noted, attributed to the unconventional magnetovolume effect^[5]. Furthermore, NTE materials can be composited with positive ones, bringing about a series of multifunctional materials. For example, the composite of Mn_0.6Fe_0.4NiGe_0.5Si_0.5 with In not only exhibits tunable thermal expansion but also demonstrates excellent machinability and heat transfer performance^[6].

Metal-organic frameworks (MOFs), a class of innovative porous materials constructed from inorganic metal nodes and organic linkers, have recently been recognized as an ideal platform for studying NTE^[7-9]. The structural design flexibility and ease of decoration characteristics are expected to expand the range of NTE materials, offering additional strategies for tailoring CTEs^[10]. To date, several MOFs have reported to exhibit NTE, such as IRMOF-1^[11], HKUST-1^[12], UiO-66(Hf)^[13], MIL-68(In)^[14], NU-1000(Zr)^[15], Cd_sq^[16], Cd(trz)Cl^[17], and so on. A common characteristic of these MOFs is their construction using units with high symmetry, such as linear, triangular or square configurations. This structural feature imparts the framework with significant quantities of low-frequency phonons^[18,19]. Upon heating, the transverse vibration, rotation or flipping tends to drag the connected nodes closer, resulting in the manifestation of NTE^[20]. The close relationship between the ligand vibration and NTE suggests that introducing guest solvents or gases can effectively tailor NTE due to the steric hindrance effect^[21-23]. Additionally, steric hindrance groups can be directly decorated as side groups of the ligands^[24,25]. In addition to decorating organic ligands, the inorganic nodes, serving as pivotal elements within the framework, have been suggested to play a crucial role in determining lattice vibration behaviors. For example, the spin state of hexanuclear Fe^III₄Fe^II₂ significantly influences the geometric configuration of the framework and the spin crossover can induce a colossal thermal expansion^[26]. Recently, Chen et al. and Platero-Prats et al. discovered that the 8-connected Zr₆O₈ nodes can undergo local distortion upon heating, which can also contribute to the NTE^[15,27].

In this research, we chose to investigate the thermal expansion property of MOF-801, which is composed of “zig-zag” type dicarboxylate ligands and Zr₆O₈ nodes^[28]. Considering the widespread applications of the MOF-801 in gas separation and water adsorption^[29-32], a detailed understanding of how the lattice and structure change upon heating (and cooling) will be useful for modifying its actual performance.

MATERIALS AND METHODS

MOF-801 samples were prepared and activated according to synthesis procedures as reported^[28,33,34]. A mixture of ZrOCl₂·8H₂O (0.23 g, 0.70 mmol) and fumaric acid (0.081 g, 0.70 mmol) was dissolved in N,N-Dimethylformamide (DMF) within a 50-mL screw-capped glass jar, and 5.3 mL formic acid was added into the solution; then, the mixture was heated at 393 K for 24 h. The powder product was washed at least twice using water and alcohol and then dried at 383 K to obtain a white powder for subsequent experiment measurements. Prior to variable temperature X-ray powder diffraction (XRD), the powder sample was in situ dried at 383 K for 2 h to completely remove the adsorbed water molecules. Temperature-dependent X-ray diffraction (Cu-K_α = 1.54056 Å) data was collected using a PANalytical X’Pert PRO. All Rietveld refinements were performed by the GSAS and EXPGUI program^[35]. Raman spectra were acquired with LabRAM HR Evolution, Horiba. In situ X-ray total-scattering data suitable for pair distribution function (PDF) and diffraction analyses were collected at the beamline of BL08W at the Super Photon ring-8 (SPring-8) facility in Japan. The samples, in powder form, were loaded into 1 mm-diameter capillaries and were tested over a temperature range of 100 to 500 K with 25 K intervals under atmospheric conditions. The PDF patterns, denoted as G(r), were extracted from X-ray total scattering data (λ = 0.6597 Å) using xPDFsuite up to a Q_max value of 20 Å^-1[36].

RESULTS AND DISCUSSION

MOF-801, Zr₆O₈(fumarate)₆, is constructed by Zr₆O₈ nodes and fumaric acid ligands [Figure 1A-C]. The Zr₆O₈ node contains six zirconium atoms with μ₃-O moieties coordinating at each face. The metal coordination is further completed by 12 carboxylate donor groups, which bridge the metal atoms along the octahedral edges. Furthermore, these 12-connected metal clusters are connected by fumarate ligands to form a three-dimensional cubic lattice with the fcu (face-centered cubic) topology (P n-3, a = 17.83480 Å). Structure arrangement in MOF-801 bears a resemblance to archetypal members of the extensively studied iso-reticular series of “UiO” MOFs, which also feature the Zr₆O₈ node but use different dicarboxylate ligands^[37]. Considering the broad temperature range of MOF-801 potential applications, there is considerable interest in understanding its thermal expansion characteristic.

Figure 1. (A) Illustration of the crystal structure of MOF-801 viewed along the c-axis. (B) The structure of 12-connected Zr₆O₈ node in MOF-801 (Zr: blue, O: orange, C: grey, H: light green). (C) Chemical structure of the fumaric acid ligand.

To investigate the thermal expansion behavior of MOF-801, in situ variable temperature X-ray diffraction analyses were used [Supplementary Figure 1]. As shown in Figure 2A, upon heating from 100 to 500 K, an obvious peak shift (particularly in the high temperature range) toward high angles was observed, inferring lattice contraction according to the Bragg equation. Subsequently, when cooling from 500 to 100 K, a peak shift toward a low angle value can also be observed. To precisely reveal the lattice changes, Rietveld refinements were performed and the results are shown in Figure 2B and Supplementary Figure 2. From 100 to 375 K, the lattice parameter a monotonously expands with the coefficient α_a = +4.30(7) MK^-1. The contradictory result between the Rietveld refinements and observed peak shifts is due to the height of sample stage changes. Evidently, the thermal expansion behavior is different from previously reported Zr₆O₈-based analogs. Those MOFs constructed from linear dicarboxylates, such as 1,4-benzenedicarboxylic acid and 4,4′-(ethyne-1,2-diyl)dibenzoic acid, tend to show monotonous NTE owing to the transverse vibration of ligands^[13,38], while, here, we found that the MOF-801 shows normal PTE. This may be caused by the unique “zig-zag” configuration of the fumarate ligand that can frustrate the correlated vibrations. Noticeably, in the high temperature range (375-500 K), a dramatically contracts with the coefficient α_a = -31.6(1) MK^-1, which exceeds most of the reported NTE materials and is close to the value of UiO-66(Hf) (α_a = -32.3 MK^-1,Supplementary Table 1)^[13]. In contrast to UiO-66(Hf), the transition between NTE and PTE in MOF-801 still exists in the subsequent cooling treatment. Moreover, the second cycling variable temperature measurement showed a similar trend of changes, although the magnitude of change was much smaller. This indicates that the transition between PTE and NTE is reversible to some extent. Noticeably, through the entire in situ variable temperature diffraction measurements, no discernible appearance or emergence of peaks was observed, except for slight shifts. This suggests the absence of any long-range phase transitions. Moreover, all the Rietveld refinements fit well based on the previously reported structure [Supplementary Figure 2]. The weakened magnitude of NTE for the subsequent cooling and cycling measurements may be caused by the defect-induced thermal densification similar to the UiO-66(Hf)^[13].

Figure 2. (A) The zoomed temperature-dependent X-ray diffraction profiles of MOF-801. (B) The unit cell parameters of MOF-801 with temperatures. Color code: first cycle, red; second cycle, blue. The symbols corresponding to the heating and cooling processes are shown as filled and empty scatters, respectively.

For framework materials, the NTE is generally believed to be associated with low frequency phonons^[18,39,40]. To gain a deeper insight into the vibration modes of MOF-801, in situ variable temperature Raman spectroscopy investigation was conducted across a temperature range of 125-475 K with 25 K internals [Figure 3A]. Throughout the entire temperature range, no noticeable blue shift was observed for the low frequency bands (< 200 cm^-1, Figure 3B). The high frequency vibration bands corresponding to the carboxylate stretching ν_as(COO), ν_s(COO) and -CH₂ wagging also show similar trends [Supplementary Figure 3]^[41], indicating that the anomalous PTE to NTE transition may not be attributed to the transverse vibration of ligands.

Figure 3. (A) In situ variable temperature Raman spectra of MOF-801. (B) Representative low-frequency Raman band shifts of MOF-801 vs. temperature.

Both the in situ powder X-ray diffraction and Raman spectroscopy indicated the absence of a phase transition in the long-range of the structure. With the concern of how the local structure changes, we performed PDF analysis based on the high-resolution X-ray total scattering. The PDF patterns predominantly feature atom-atom correlations, primarily focusing on the strongly scattering Zr atoms [Figure 4A]. This includes the Zr-O bond at distance of ~2.2 Å (path 1, Figure 4B), the Zr···Zr distances as the edges of the Zr₆ octahedron at distance of ~3.5 Å (path 2) and as the two diagonal sites at distance of ~5.0 Å (path 3). The observed single Zr···O distances involve contributions from overlapping Zr-carboxylate and Zr-oxo distances. The singular Zr···Zr distance at 3.5 Å corresponds to the 12 equivalent Zr···Zr distances within the regular octahedron, highlighting the structural intricacies of MOF-801 at the local level. Before 375 K, no obvious peak changes, including peak intensity and position, can be observed, while a noticeable alteration in the local structure becomes evident when the temperature exceeds 375 K. As shown in Figure 4C, the initially singular peak at ~3.5 Å that corresponds to the Zr···Zr pair splits into two peaks at 3.3 and 3.8 Å with an intensity ratio of nearly 2:1. The peak at ~5 Å also broadens, indicating a wider distribution of Zr···Zr distances and disorder. These changes of Zr···Zr pairs suggest that the originally regular Zr₆ octahedron become distorted Figure 4D, consistent with a recently reported 8-connected Zr₆O₈ node in NU-1000 that capped with H₂O or OH^[15]. As discussed by Chen et al., this type of distortion behavior possesses relatively lower reversibility, which may be the origin of difference between the first and second cycling measurements^[15]. To quantify the distortion extent in the local structure, the change in the crucial Zr···O distance was evaluated. As shown in Supplementary Figure 4, the change of Zr···O distance aligns with the trend observed in the CTEs depicted in Figure 2B. So, we have reason to believe that the distorted Zr₆O₈ nodes possess a smaller volume, giving rise to observed volume contraction at high temperatures.

Figure 4. (A) PDF patterns of MOF-801 at 300 K (path 1: Zr-O atom pair, path 2: Zr···Zr atom pair, path 3: Zr···Zr atom pair). (B) Schematic diagram of the Zr₆O₈ node, showing paths 1, 2 and 3, respectively. (C) Temperature-dependent PDF patterns in the range of 1.5-5.5 Å. (D) Illustration of the local Zr₆O₈ node distortion with temperature.

CONCLUSIONS

In summary, we investigated the thermal expansion behavior of MOF-801, a significant representative of typical Zr-based MOFs. In the low temperature range (< 375 K), it exhibits normal PTE due to the transverse vibration of dicarboxylate ligands being frustrated by the “zig-zag” configuration of fumarate ligands. Intriguingly, a distinctive transition from PTE to NTE was identified in the temperature range of 375~500 K. Detailed analyses through in situ powder X-ray diffraction and Raman spectroscopy analysis excluded the occurrence of long-range phase transitions. The PDF study unveiled a temperature-induced local distortion of the Zr₆O₈ node, contributing to the occurrence of NTE. This sheds light on the dynamics of the local structure within a specific temperature range, enhancing our understanding of NTE. The current observation of NTE in MOF-801 holds promise for exploring diverse potential applications.