Photon Factory Activity Report 2016 #34 (2017)

BL-9A/2015U001, BL-12C/2012G017

Amorphous-amorphous transition in a porous coordination polymer
Hiroyoshi Ohtsu1*, Thomas D. Bennett2, Tatsuhiro Kojima3, David A. Keen4,5, Yasuhiro Niwa6 andMasaki Kawano1*
1
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
2
University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK
3Osaka University, Toyonaka, Osaka, 560-0043, Japan
4
Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire OX11 0QX, UK
5
Oxford University, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
6Photon Factory, 1-1 Oho, Tsukuba, 305-0801, JapanThe amorphous state plays a key role in porous coordination polymer and metal-organic framework phase transitions. We
investigate a crystalline-to-amorphous-to-amorphous-to-crystalline (CAAC) phase transition in a Zn based coordination
polymer, by X-ray absorption fine structure (XAFS) and X-ray pair distribution function (PDF) analysis. We show thatthe system shows two distinct amorphous phases upon heating. The first involves a reversible transition to a desolvatedform of the original network, followed by an irreversible transition to an intermediate phase which has elongated Zn-I
bonds.
1 Introduction
Polyamorphism i.e. the existence of several amorphous
phases of the same material, is of interest in inorganic
materials and glasses as functional properties may vary
drastically between several amorphous states. Suchpolyamorphism is rare in the field of coordination
polymer/metal-organic frameworks and no structuralcharacterization exists. Here we report the firstobservation and structural characterization of multiple
amorphous phases for the same PCP. Amorphizationfrom a crystalline state in this case is followed byrecrystallization to a more stable coordination network.
The amorphous phases are a crucial intermediate in final
crystalline network formation.
2 Experiment
We examined the phase transition of the coordination
polymer, [(ZnI2)3(TPT)2] (TPT = 2,4,6-tris(4-pyridyl)1,3,5-triazine).1 Upon heating phase 1, it became
amorphous phase (phase 2) at 550 K, and then, by further
heating at 600 K, it became a 1D crystalline phase (phase4) which has permanent porosity. In this study, weclosely investigated this phase transition. Zn K-edge
XAFS (X-ray absorption fine structure) were measured in
BL-12C/BL-9A beamline in KEK.
3 Results and Discussion
Thermal analysis revealed that one more amorphous
phase (phase 3) existing before reaching to phase 4. We
isolated two amorphous phases, phase 3 and phase 4, andinvestigate its structure by XAFS and X-ray PDF (pairdistribution function) analysis. From XAFS analysis(Fig.1), The first amorphous phases has the same
geometry around Zn with crystalline phase 1 and 4,indicating phase 2 formation is coming from desolvationof the solvent inside the pore. However, phase 3 has
totally different PDF (Fig.2) from the other phases,
indicating significant change in the Zn coordination

environment accompanied with Zn-I elongation from thenormal geometry. This is the first example of thecharacterization of subsequent solvent collapsed andamorphous states in MOF/PCPs.

Fig. 1: (a) k3-weighted XAFS spectra of each phase (b)Fourier transformed k3-weighted XAFS spectra

Fig. 2: PDF function of each phase
References[1] H. Ohtsu, T. D. Bennett, T. Kojima, D. A. Keen, Y.
Niwa, M. Kawano, Chem. Commun. 53, 7060 (2017).
* ohtsu@chem.titech.ac.jp, mkawano@chem.titech.ac.jp