ic402425n_si_001.pdf (1.83 MB)
Order–Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn)
journal contribution
posted on 2014-01-06, 00:00 authored by Mirosław Mączka, Anna Gągor, Bogusław Macalik, Adam Pikul, Maciej Ptak, Jerzy HanuzaWe report the synthesis,
crystal structure, thermal, dielectric,
Raman, infrared, and magnetic properties of hydrogen and deuterated
divalent metal formates, [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3], where M = Ni, Mn. On the basis
of Raman and IR data, assignment of the observed modes to respective
vibrations of atoms is proposed. The thermal studies show that for
the Ni compounds deuteration leads to a decrease of the phase transition
temperature Tc by 5.6 K, whereas it has
a negligible effect on Tc in the Mn analogues.
This behavior excludes the possibility of proton (deuteron) movement
along the N–H···O (N–D···O)
bonds as the microscopic origin of the first-order phase transition
observed in these crystals below 190 K. According to single-crystal
X-ray diffraction, the dimethylammonium (DMA) cations are dynamically
disordered at room temperature, because the hydrogen bonds between
the NH2 (ND2) groups and the metal-formate framework
are disordered. The highly dynamic nature of hydrogen bonds in the
high-temperature phases manifests in the Raman and IR spectra through
very large bandwidth of modes involving vibrations of the NH2 (ND2) groups. The abrupt decrease in the bandwidth and
shifts of modes near Tc signifies the
ordering of hydrogen bonds and DMA+ cations as well as
significant distortion of the metal-formate framework across the phase
transition. However, some amount of motion is retained by the DMA+ cation in the ferroelectric phase and a complete freezing-in
of this motion occurs below 100 K. The dielectric studies reveal pronounced
dielectric dispersion that can be attributed to slow dynamics of large
DMA+ cations. The low-temperature studies also show that
magnetic properties of the studied compounds can be explained assuming
that they are ordered ferrimagnetically with nearly compensated magnetic
moments of Ni and Mn. IR data reveal weak anomalies below 40 K that
arise due to spin-phonon coupling. Our results also show that due
to structural phase transition more significant distortion of the
metal-formate framework occurs for the deuterated samples.