Irreversible Structural Phase Transition in [(9‐triptycylammonium) ([18]crown‐6)][Ni(dmit)2]: Origin and Effects on Electrical and Magnetic Properties

Two salts, [(9-triptycylammonium)([18]crown-6)][Ni(dmit)₂] (1) and [(9-triptycylammonium)([15]crown-5)][Ni(dmit)₂] (2) were prepared and characterized. [Ni(dmit)₂]⁻ formed dimers in 1 but was arranged in zigzag stacks in 2. At 381 K, 1 underwent an irreversible phase transition originating from the rearrangement of [Ni(dmit)₂] dimers and therefore experienced a drastic change in magnetic and electrical properties.

Abstract

Materials exhibiting irreversible phase transitions, leading to changes in their properties, have a potential for novel application in electronic components such as a non-rewritable high-security memory. Here, we focused on the two salts, [(9-triptycylammonium)([18]crown-6)][Ni(dmit)₂] (1) and [(9-triptycylammonium)([15]crown-5)][Ni(dmit)₂] (2), which featured 2D sheet structures with alternately stacked cation and anion layers. Both salts exhibit similar cation arrangements, however, their anion arrangements differ significantly. The temperature-dependent magnetic susceptibilities of 1 and 2 were well reproduced by the alternating chain model (J _AC1/k _B=−306(8), J _AC2/k _B=−239(3) K) and the Curie-Weiss model (θ=−3.9(1) K), respectively. 1 experience a reversible phase transition around 40–60 K, causing anomalies in magnetic behavior. Moreover, an irreversible single-crystal-to-single-crystal phase transition to 1′ undergo at ~381 K, inducing a rearrangement of [Ni(dmit)₂]⁻ anions and a resistivity decrease from 6.5×10⁶ to 6.5×10² Ω cm. The susceptibility curve of 1′ was reproduced by a combination of the Curie-Weiss and dimer models (J _dimer/k _B=−407(5), θ=−26.7(5) K). The irreversible transition of 1 is the first example for such supramolecule and [Ni(dmit)₂]⁻ system to our knowledge, in opening potential new-type materials.

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