Perovskite metal formate framework of [NH2-CH +-NH2]Mn(HCOO)3]: Phase transition, magnetic, dielectric, and phonon properties

Abstract

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH+-NH 2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3-] is counterbalanced by formamidinium (FMD+) cations located in the cavities of the framework. These cations form extensive N-H···OO hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̄c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD+ cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH 2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD+ cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD+ cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K. © 2014 American Chemical Society.