In this study, we comparatively analyzed the variable-temperature crystal structures for two isomorphous salts, [1-benzyl-4-aminopyridinium][M(mnt)
2] (M = Ni or Cu; mnt
2− = maleonitriledithiolate; labeled as
APy-Ni or
APy-Cu). Both salts crystallize in the triclinic
P–1 space group at
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In this study, we comparatively analyzed the variable-temperature crystal structures for two isomorphous salts, [1-benzyl-4-aminopyridinium][M(mnt)
2] (M = Ni or Cu; mnt
2− = maleonitriledithiolate; labeled as
APy-Ni or
APy-Cu). Both salts crystallize in the triclinic
P–1 space group at 296 K, comprising linear [M(mnt)
2]
− (M = Ni or Cu) tetramers. A magnetostructural phase transition occurs at T
C~190 K in S = ½
APy-Ni at ambient pressure, with a conversion of paramagnetic tetramers into nonmagnetic spin-paired dimers. The discontinuous alteration of cell parameters at T
C signifies the characteristic of first-order phase transition in
APy-Ni. No such transition appears in the nonmagnetic
APy-Cu within the same temperature vicinity, demonstrating the magnetic interactions promoting the structural phase transition in
APy-Ni, which is further reinforced through a comparison of the lattice formation energy between
APy-Ni and
APy-Cu. The phase transition may bear a resemblance to the mechanisms typically observed in spin-Peierls systems. We further explored the magnetic and phase transition properties of
APy-Ni under varying pressures. Significantly, T
C shows a linear increase with rising pressure within the range of 0.003–0.88 GPa, with a rate of 90 K GPa
−1, manifesting that the applied pressure promotes the transition from tetramer to dimer.
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