Temperature- and pressure-induced phase transitions in the niccolite-type formate framework of [H3N(CH3)4NH3][Mn2(HCOO)6]

Abstract

We report the synthesis, crystal structure, thermal, pyroelectric, Raman, infrared and magnetic properties of [NH₃(CH₂)₄NH₃][Mn₂(HCOO)₆] niccolite. Our results show that this compound crystallizes in a trigonal structure (space group P1c) with dynamically disordered [NH₃(CH₂)₄NH₃]²⁺ cations. It undergoes a phase transition near T_c = 350 K. The low-temperature structure is polar (space group Cc) and pyroelectric measurements confirm that it exhibits ferroelectric properties. Detailed analysis of the structural changes shows that both the spatial arrangement of the [NH₃(CH₂)₄NH₃]²⁺ dipole moments and distortion of the manganese formate framework contribute to the spontaneous polarization within the (a, c) plane. Based on Raman and IR data, assignment of the observed modes to the respective vibrations of atoms is also proposed. Dynamic disorder of organic cations in the high-temperature phase manifests in the vibrational spectra through very large width of bands corresponding to vibrations of the NH₃ groups. Ordering of these cations is clearly observed in the spectra through a pronounced decrease in their bandwidths below the phase transition temperatures. Low-temperature magnetic studies show that this compound is a weak ferromagnet below 9.0 K. We also report high-pressure Raman scattering studies of this compound, which reveal the presence of two pressure-induced phase transitions between 0.5 and 0.9 GPa and between 1.3 and 1.9 GPa.