Influence of isotope substitution on lattice and spin-Peierls-type transition features in one-dimensional nickel...

Four new 1D spin-Peierls-type compounds, [D(5)]1-(4'-R-benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D(5)]R-Py; R=F, I, CH(3), and NO(2)), 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, CH(3), and NO(2)). Compounds [D(5)]R-Py and R-Py (R=F, I, CH(3), and NO(2)) crystallize in the monoclinic space group P2(1)/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 [D(5)]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 [D(5)]F-Py and shows slight expansion for [D(5)]R-Py (R=I, CH(3), and NO(2)) as well as an increase in the spin-Peierls-type transition temperature for all of these 1D compounds in the order of F>I≈CH(3)≈NO(2). The large isotopic effect of nonmagnetic countercations on the spin-Peierls-type transition critical temperature, T(C), can be attributed to the change in ω(0) with isotope substitution.