Influence of isotope substitution on lattice and spin-peierls-type transition features in one-dimensional nickel bis-dithiolene spin systems

Four new 1D spin-Peierls-type compounds, [D₅]1-(4'-R-benzyl) pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R-Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non-deuterated compounds, 1-(4'-R-benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R-Py; R=F, I, CH₃, and NO ₂). Compounds [D₅]R-Py and R-Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high-temperature phase and triclinic space group P1 with dimerized stacks of anions and cations in the low-temperature phase. Similar to the non-deuterated R-Py compounds, a spin-Peierls-type transition occurs at a critical temperature for each [D ₅]R-Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F-Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin-gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R-Py compound, the cell volume is almost unchanged for [D₅]F-Py and shows slight expansion for [D₅]R-Py (R=I, CH₃, and NO₂) as well as an increase in the spin-Peierls-type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin-Peierls-type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution. In a spin: In four one-dimensional spin-Peierls-type compounds, 1-(4'-R-benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R=F, I, CH₃, and NO ₂), replacing pyridinium by [D₅]pyridinium in the countercation gives rise to a significant isotope effect on the magnetic transition (see picture).