[NH2NH3][M(HCOO)3] (M = Mn2+, Zn2+, Co2+ and Mg2+): structural phase transitions, prominent dielectric anomalies and negative thermal expansion, and magnetic ordering

Abstract

We report here a new class of ammonium metal–formate frameworks of [NH₂NH₃][M(HCOO)₃] (M = Mn²⁺, Zn²⁺, Co²⁺ and Mg²⁺) incorporating hydrazinium as the cationic template and component. The perovskite Mn and Zn members possess anionic 4¹²·6³ metal–formate frameworks with cubic cavities occupied by the NH₂NH₃⁺ cations, while the Co and Mg members have chiral 4⁹·6⁶ metal–formate frameworks, with chiral hexagonal channels accommodating NH₂NH₃⁺ cations. On heating, the Mn and Zn members undergo phase transitions around 350 K. The structures change from low temperature (LT) polar phases in Pna2₁ to high temperature (HT) non-polar phases in Pnma, due to the thermally activated librational movement of the NH₂ end of the NH₂NH₃⁺ in the cavity and significant framework regulation. The Co and Mg members in LT belong to non-polar P2₁2₁2₁, are probably antiferroelectric, and they show phase transitions at 380 K (Co) and 348 K (Mg), and the structures change to polar HT phases in P6₃, triggered by the order–disorder transition of the cation from one unique orientation in LT to three of trigonally-disorder state in HT. Accompanying the phase transitions, which are ferro- to para-electric for Mn and Zn members while antiferro- to ferro-electric for Co and Mg, prominent anisotropic thermal expansions including negative ones, and dielectric anomalies, are observed. The spontaneous polarization values are estimated at 3.58 (Mn, 110 K), 3.48 (Zn, 110 K), 2.61 (Co, 405 K) and 3.44 (Mg, 400 K) μC cm⁻², respectively, based on the positive and negative charge separations in the polar structures. The structure–property relevance is established based on the order–disorder transitions of NH₂NH₃⁺ and the conformity and adaptability of the metal–formate frameworks to match such order–disorder alternations. The Mn and Co members show spin-canted antiferromagnetic long-range-ordering, with Néel temperatures of 7.9 K and 13.9 K, respectively. Therefore, the two members show coexistence of electric and magnetic orderings in the low temperature region, and they are possible molecule-based multiferroics.