The Role of Order–Disorder Transitions in the Quest for Molecular Multiferroics: Structural and Magnetic Neutron Studies of a Mixed Valence Iron(II)–Iron(III) Formate Framework

Neutron
diffraction studies have been carried out to shed light
on the unprecedented order–disorder phase transition (ca. 155
K) observed in the mixed-valence iron­(II)–iron­(III) formate
framework compound [NH2(CH3)2]n[FeIIIFeII(HCOO)6]n. The crystal structure at 220
K was first determined from Laue diffraction data, then a second refinement
at 175 K and the crystal structure determination in the low temperature
phase at 45 K were done with data from the monochromatic high resolution
single crystal diffractometer D19. The 45 K nuclear structure reveals
that the phase transition is associated with the order–disorder
of the dimethylammonium counterion that is weakly anchored in the
cavities of the [FeIIIFeII(HCOO)6]n framework. In the low-temperature
phase, a change in space group from P3̅1c to R3̅c occurs,
involving a tripling of the c-axis due to the ordering
of the dimethylammonium counterion. The occurrence of this nuclear
phase transition is associated with an electric transition, from paraelectric
to antiferroelectric. A combination of powder and single crystal neutron
diffraction measurements below the magnetic order transition (ca.
37 K) has been used to determine unequivocally the magnetic structure
of this Néel N-Type ferrimagnet, proving that the ferrimagnetic
behavior is due to a noncompensation of the different FeII and FeIII magnetic moments.