Elucidation of the Mechanism of Phase Transition in a Zinc Formate Framework Templated by a Diammonium Cation—Structural, Phonon and Dielectric Studies

Aneta Ciupa-Litwa,Jan Janczak,Adam Sieradzki,Paulina Peksa

doi:10.3390/cryst11020213

Abstract

In this paper we present the synthesis method and a detailed description of the crystal structure, as well as thermal, dielectric and phonon properties, of the [CH3NH2CH2CH2NH2CH3][Zn2(HCOO)6] (dmenH2-Zn) metal organic framework. The negative charge of the anionic framework ([Zn2(HCOO)6]2-) is balanced by N,N′-dimethylethylenediamine (dmenH22+) ions located in the voids of the framework. Thermal analysis revealed that dmenH2-Zn underwent a reversible structural phase transition at around room temperature (Tc~300 K). The single-crystal X-ray diffraction showed that dmenH22+ templates were dynamically disordered at 295 K, since N-H…O bonds were too weak to surmount their thermally activated motions. Reduction in the temperature resulted in ordering of the dmenH22+ cations as a consequence of freezing of their reorientational movements. This behavior caused a symmetry change from P-31c (trigonal) to C 2/c (monoclinic). The mechanism of the observed phase transition of dmenH2-Zn compound was also investigated by temperature-dependent IR measurements. These spectroscopic studies showed that the ordering of the dmenH22+ ions also resulted in the distortion of the anionic framework. Dielectric investigations revealed the occurrence of the dipolar relaxation process clearly defined in the monoclinic phase. The asymmetric shape of the studied process, which indicated a non-Debye-like relaxation, was analyzed using the Havriliak–Negami relaxation function, leading to an Ea value of approximately 0.36 eV.

Highlights

The family of dense hybrid metal–organic framework (MOF) materials based on formate ions as ligands has gained interest due to the variety of their achievable structures and promising applications in many areas such as optics and electronics [1,2]
Studied [(CH3)2NH2][M(HCOO)3] materials that exhibit dielectric, magnetic, magnetoelectric or even multiferroic properties belong to this group [4,5,6,7,8,9]
Within this class of materials, the occurrence of phase transitions between 160 K and 185 K is associated with the order–disorder of (CH3)2NH2+ cations located in the channel of the anionic framework, while the magnetic properties arise from the presence of metal ions inside the framework skeletons [4,5,6,7,8,9,10,11]

Summary

Introduction

The family of dense hybrid metal–organic framework (MOF) materials based on formate ions as ligands has gained interest due to the variety of their achievable structures and promising applications in many areas such as optics and electronics [1,2] These compounds are characterized by modular structures that can be freely varied using different organic cations as guest molecules and metal ions [3]. The copper formate framework shows lower symmetry and structural difference compared to other materials templated by dmenH22+ ions; that is, it belongs to the space group of C2/c. This behavior arises from the occurrence of the Jahn–Teller effect on the Cu2+ ion [14]. The good agreement between the experimental powder XRD and the calculated one based on the single-crystal structure implied the correctness of the found structure in the direct-space method (Figure S1)

Single-Crystal X-ray Data Collection

Dielectric Properties

Raman and IR Studies

Structural Studies

Thermal Properties

Assignment of Modes

Temperature-Dependent IR Studies