title: 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.
authors.affiliation: Institute of Low Temperature and Structure Research , Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław 2, Poland.
authors.forename: Mirosław
authors.initials: M
authors.lastname: Mączka
authors.forename: Adam
authors.initials: A
authors.lastname: Pietraszko
authors.forename: Bogusław
authors.initials: B
authors.lastname: Macalik
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Hermanowicz