Structure, Phonon Properties, and Order–Disorder Transition in the Metal Formate Framework of [NH₄][Mg(HCOO)₃]

Abstract

We report the synthesis, crystal structure, thermal, dielectric, IR, and Raman studies of [NH4][Mg(HCOO)3] formate. Single-crystal X-ray diffraction shows that it crystallizes in the hexagonal space group P6322, with orientationally disordered NH4(+) ions located in the cages of the network. Upon cooling, [NH4][Mg(HCOO)3] undergoes a phase transition at around 255 K to the ferroelectric P63 structure. Raman and IR spectra show a strong increase in intensity of the N-H stretching bands as well as narrowing of the bands related to the NH4(+) ions upon cooling. These changes indicate that the phase transition is due to orientational ordering of the NH4(+) ions. Analysis of the Raman data show, however, that the rotational and translational motions of NH4(+) do not freeze completely at the phase transition but exhibit further slowing down below 255 K, and the motional freezing becomes nearly complete below 140 K.

Comparative studies of vibrational properties and phase transitions in metal-organic frameworks of [NH4][M(HCOO)3] with M=Mg, Zn, Ni, Fe, Mn

“…The observation of the good dielectric hysteresis loop below T c confirmed that the low-temperature structure of these materials is ferroelectric [18]. It was also reported that the phase transition has an order disorder character [18][19][20]. Interestingly, X-ray diffraction data of Mg analog showed that ordering of NH 4 + cations occurs in two steps [19,20].…”

“…They exhibit electric order at 191-255 K, associated with decrease of symmetry to P6 3 (C 6 6 ) [18][19][20]. The observation of the good dielectric hysteresis loop below T c confirmed that the low-temperature structure of these materials is ferroelectric [18].…”

“…It was also reported that the phase transition has an order disorder character [18][19][20]. Interestingly, X-ray diffraction data of Mg analog showed that ordering of NH 4 + cations occurs in two steps [19,20]. Another interesting feature of this family of compounds is negative thermal expansion below T c [19,20] and negative linear compressibility in the c direction [26].…”

“…Compounds with the smallest ammonium cation, NH 4 + and M = Mg, Zn, Mn, Ni, Co, Fe, were shown to possess chiral framework of 4 9 Á 6 6 topology with space group P6 3 22 (D 6 6 ) [18][19][20]25]. They exhibit electric order at 191-255 K, associated with decrease of symmetry to P6 3 (C 6 6 ) [18][19][20].…”

A multiferroic material [NH2-CH+-NH2]Co(HCOO)3 of metal-organic frameworks with weak ferromagnetism and dielectric relaxation

“…[17][18][19][20] It was proved by Xu, et al that the magnetic [NH 4 ][M(HCOO) 3 ] had electric ordering and could be a new type of MOF-based multiferroics. [21][22][23] The property of ferroelectricity induced by magnetic order was also found in the metal-organic framework magnet [CH 3 NH 3 ][Co(HCOO) 3 ]. 9,24 Recently, Ren-Gen Xiong and his team reported that the para-toferroelectricity occurred in range of 3 ] series, which the disorder-order transition of (CH 3 ) 2 NH 2 + template could be the origin of electric ordering.…”