ABSTRACT
A niccolite series of [bnH2 (2+) ][M(HCOO)3 ]2 (bnH2 (2+) =1,4-butyldiammonium) shows four kinds of metal-dependent phase transitions, from high temperature para-electric phases to low-temperature ferro-, antiferro-, glass-like, and para-electric phases. The conformational flexibility of bnH2 (2+) and the different size, mass, and bonding character of the metal ion lead to various disorder-order transitions of bnH2 (2+) in the lattice and relevant framework modulations, thus different phase transitions and dielectric responses. The magnetic members display a coexistence or combination of electric and magnetic orderings in the low-temperature region.
ABSTRACT
We report the synthesis, crystal structures, IR, and thermal, dielectric, and magnetic properties of a new series of ammonium metal formate frameworks of [HONH(3)][M(II)(HCOO)(3)] for M = Mn, Co, Ni, Zn, and Mg. They are isostructural and crystallize in the nonpolar chiral orthorhombic space group P2(1)2(1)2(1), a = 7.8121(2)-7.6225(2) Å, b = 7.9612(3)-7.7385(2) Å, c = 13.1728(7)-12.7280(4) Å, and V = 819.27(6)-754.95(4) Å(3). The structures possess anionic metal formate frameworks of 4(9)·6(6) topology, in which the octahedral metal centers are connected by the anti-anti formate ligands and the hydroxylammonium is located orderly in the channels, forming strong O/N-H···O(formate) hydrogen bonds with the framework. HONH(3)(+) with only two non-H atoms favors the formation of the dense chiral 4(9)·6(6) frameworks, instead of the less dense 4(12)·6(3) perovskite frameworks for other monoammoniums of two to four non-H atoms because of its small size and its ability to form strong hydrogen bonding. However, the larger size of HONH(3)(+) compared to NH(4)(+) resulted in simple dielectric properties and no phase transitions. The three magnetic members (Mn, Co, and Ni) display antiferromagnetic long-range ordering of spin canting, at Néel temperatures of 8.8 K (Mn), 10.9 K (Co), and 30.5 K (Ni), respectively, and small spontaneous magnetizations for the Mn and Ni members but large magnetization for the Co member. Thermal and IR spectroscopic properties are also reported.
ABSTRACT
We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.
