The compound [4'-CF3bzPy][Ni(mnt)2] (1) (where 4'-CF3bzPy = 1-(4'-(trifluoromethyl)benzyl)pyridinium and mnt(2-) = maleonitriledithiolate) was synthesized and displays a magnetic bistability with a surprisingly large thermal hysteresis loop (~49 K). X-ray crystallographic studies reveal that in the high-temperature (HT) phase the anions and cations form mixed stacks, with alternating anion dimers (AA) and cation dimers (CC) in an ...AACCAACC... fashion along the crystallographic a + b direction, and disordered CF3 groups in the cations are aligned into a molecular layer parallel to the crystallographic (001) plane. However, in the low-temperature (LT) phase, the c-axis length of the unit cell is roughly doubled, and the asymmetric unit switches from one [4'-CF3bzPy][Ni(mnt)2] pair in the HT phase to two [4'-CF3bzPy][Ni(mnt)2] pairs. Most interestingly, the CF3 group in the cations becomes ordered, and the conformation of one of two crystallographically different cations changes significantly. A dislocation motion between the neighboring molecular layers emerges as well. The analyses of the magnetic susceptibilities and the density functional theory calculations suggest that the antiferromagnetic exchange interaction within one of two types of [Ni(mnt)2]2(2-) dimers in the LT phase is much stronger than that within the [Ni(mnt)2]2(2-) dimer in the HT phase. The lattice reorganization during this phase transition is proposed to be responsible for the wide thermal hysteresis loop.
Two families of lanthanide metal-organic frameworks (Ln-MOFs), namely {[Ln2L2(DMF)4]·xDMF·yH2O}n (Ln = Ce(1), Pr(2), Sm(3), Eu(4), Gd(5); x = 0,1; y = 0,1), and {[Ln2L2(H2O)2.5]}n (Ln = Dy(6), Ho(7)), have been...
The second polymorph, the β-crystal, of the nickel-bis-dithiolene compound [4'-CF3bzPy][Ni(mnt)2], where 4'-CF3bzPy = 1-(4'-trifluoromethylbenzyl)pyridinium and mnt(2-) = maleonitriledithiolate, was obtained. The variable-temperature single crystal structures, magnetic behavior in 1.8-300 K and dielectric nature in 123-373 K have been investigated for the β-crystal. This polymorph experiences two hysteretic magnetic phase transitions in a narrow temperature region (190-217 K) with the thermal hysteresis loops ca. 6 K and ca. 11 K. The two hysteretic magnetic phase transitions are coupled with two isostructural phase transitions (IPTs), respectively, which are driven by the novel step-wise dynamic orientation motion of the anion and cation in the β-crystal. There is an absence of a dielectric anomaly in the structural transformation temperature interval. However, a dielectric relaxation, related to the dipole motion of polar CF3 groups in the cations under an ac electrical field, emerges in the high-temperature phase.
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