Variable magnetic interactions in an organic radical system of (m-N-methylpyridinium alpha -nitronyl nitroxide)

Abstract

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium \ensuremath{\alpha}-nitronyl nitroxide (m-${\mathrm{MPYNN}}^{+}$) is a spin-1/2 organic radical. The simple salt, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-${\mathrm{MPYNN}}^{+}$ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ions are in the organic layer, joining the m-${\mathrm{MPYNN}}^{+}$ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-${\mathrm{MPYNN}}^{+}$ makes a crystalline solid-solution system, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}(${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$${)}_{\mathit{x}}$\ensuremath{\cdot}${\mathrm{I}}_{1\mathrm{\ensuremath{-}}\mathit{x}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone) (0\ensuremath{\le}x\ensuremath{\le}1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction ${\mathit{J}}_{1}$ forming a triplet state and a weak antiferromagnetic interdimer interaction ${\mathit{J}}_{2}$ which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that ${\mathit{J}}_{2}$ quickly weakens with an increase in x, while ${\mathit{J}}_{1}$ shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagom\'e antiferromagnet at very low temperatures.