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 frustrati on. m-N-methylpyridinium alpha-nitronyl nitroxide (m-MPYNN+) is a spin -1/2 organic radical. The simple salt, m-MPYNN+.ClO4-.1/3 (acetone), c rystallizes in a trigonal P3c1 space group, where the m-MPYNN+ molecul es exist as a dimer and the dimer units form a two-dimensional (2D) tr iangular lattice. One-third of the ClO4- ions are in the organic layer , joining the m-MPYNN+ molecules, and the remainder is between the lay ers, 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 s olid-solution system, m-MPYNN+.(ClO4-)x.I1-x-.1/3 (acetone) (0 less-th an-or-equal-to x less-than-or-equal-to 1), which also belongs to the t rigonal system. Both the a and c axes are slightly lengthened with inc reasing 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 tempera ture dependence of the magnetic susceptibilities of the solid solution s can be well interpreted in terms of a strong ferromagnetic intradime r interaction J1 forming a triplet state and a weak antiferromagnetic interdimer interaction J2 which is expected to give rise to spin frust ration among the triplet spin species on each side of the triangles. I t is found that J2 quickly weakens with an increase in x, while J1 sho ws little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagome anti-ferromagnet at very low t emperatures.