title: 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.
abstract: 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 [NH(2)(CH(3))(2)](n)[Fe(III)Fe(II)(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 [Fe(III)Fe(II)(HCOO)(6)](n) framework. In the low-temperature phase, a change in space group from P31c to R3c 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 Fe(II) and Fe(III) magnetic moments.
authors.affiliation: Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, C/Pedro Cerbuna 12, E-50009, Zaragoza, Spain.
authors.forename: Laura
authors.initials: L
authors.lastname: Cañadillas-Delgado
authors.forename: Oscar
authors.initials: O
authors.lastname: Fabelo
authors.forename: J Alberto
authors.initials: JA
authors.lastname: Rodríguez-Velamazán
authors.forename: Marie-Hélène
authors.initials: MH
authors.lastname: Lemée-Cailleau
authors.forename: Sax A
authors.initials: SA
authors.lastname: Mason
authors.forename: Emilio
authors.initials: E
authors.lastname: Pardo
authors.forename: Francesc
authors.initials: F
authors.lastname: Lloret
authors.forename: Jiong-Peng
authors.initials: JP
authors.lastname: Zhao
authors.forename: Xian-He
authors.initials: XH
authors.lastname: Bu
authors.forename: Virginie
authors.initials: V
authors.lastname: Simonet
authors.forename: Claire V
authors.initials: CV
authors.lastname: Colin
authors.forename: Juan
authors.initials: J
authors.lastname: Rodríguez-Carvajal