Perovskite-Like Polar Lanthanide Formate Frameworks of [NH₂NH₃][Ln(HCOO)₄] (Ln = Tb-Lu and Y): Synthesis, Structures, Magnetism, and Anisotropic Thermal Expansion

A series of isostructural hydrazinium lanthanide (Ln) formate framework compounds of [NH₂NH₃][Ln(HCOO)₄] for Ln³⁺ ions from Tb³⁺ to Lu³⁺ and Y³⁺ have been successfully prepared by utilizing NH₂NH₃⁺. The compounds crystallize in orthorhombic polar space group Pca2₁, with cell parameters at 180 K of a = 18.2526(7)-18.1048(5) Å, b = 6.5815(2)-6.5261(2) Å, c = 7.6362(3)-7.5044(2) Å, and V = 917.33(6)-886.67(4) ų, showing the effect of lanthanide contraction. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the cavities of the NaCl-like framework, in which the Ln³⁺ ions in a bicapped trigonal prism are connected by anti-anti and syn-anti formate groups. The N-H···O_formate hydrogen-bonding interactions are between the hydrazinium cations and the anionic framework. One anti-anti formate group is frustrated by the competitive N-H···O_formate hydrogen-bonding interactions. It thus twists or flips upon warming, resulting in large anisotropic thermal expansion and negative thermal expansion below 180 K. A comparison with the transition metal and magnesium analogues revealed that the structural compactness, tighter binding of the hydrazinium cation by the framework, and symmetrically better match between the framework and ammonium cation for Ln compounds could inhibit the occurrence of phase transition in the series. The IR spectroscopic, thermal, and magnetic properties are investigated.