VARIABLE MAGNETIC-INTERACTIONS IN AN ORGANIC RADICAL SYSTEM OF (M-N-METHYLPYRIDINIUM ALPHA-NITRONYL NITROXIDE).X- - A POSSIBLE KAGOME ANTIFERROMAGNET

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium alpha-nitronyl nitroxide (m-MPYNN+) is a spin-1/2 organic radical. The simple salt, m-MPYNN+.ClO4-.1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ClO4- ions are in the organic layer, joining the m-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-MPYNN+ makes a crystalline solid-solution system, m-MPYNN+.(ClO4-)x.I1-x-.1/3 (acetone) (0 less-than-or-equal-to x less-than-or-equal-to 1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ClO4- 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 J1 forming a triplet state and a weak antiferromagnetic interdimer interaction J2 which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that J2 quickly weakens with an increase in x, while J1 shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagome anti-ferromagnet at very low temperatures.