Magnetic layered perovskites of [CH₃C(NH₂)₂]₂[M(HCOO)₄] (M = Co²⁺ and Ni²⁺): synthesis, structures and properties

We present two layered perovskites of the formula [CH₃C(NH₂)₂]₂[M(HCOO)₄] (M = Co and Ni). The two materials are isostructural and crystallize in the orthorhombic space group Pccn. The structures consist of anionic 〈100〉-oriented perovskite or square metal-formate layers and interlayer [CH₃C(NH₂)₂]⁺ cations. In each layer octahedral M²⁺ ions are linked by equatorial anti–anti HCOO⁻ bridges and are further coordinated by apical monodentate HCOO⁻ groups. The interlayer acetamidinium cations form N–H⋯O H-bonds with the anionic layers to support the 3D structure. As the directions of most vibrations of the interlayer atoms are approximately along the a-axis, the two materials exhibit nearly zero thermal expansion (a few (MK)⁻¹) along the b-direction perpendicularly to the layers and positive thermal expansion along the a- (∼40 (MK)⁻¹) and c- (∼20 (MK)⁻¹) axes parallel to the layers. They are both weak ferromagnets, and the Néel temperatures are 6.0 K and 17.0 K for M = Co and Ni, respectively. The Co compound exhibited high spontaneous magnetization of 444 cm³ G mol⁻¹ under a field of 10 Oe and an unusually low coercive field of 38 Oe at 2 K. IR spectroscopic, thermal and dielectric properties were investigated.