Structural phase transition in two-dimensional kagomé antiferromagnet m-N-methylpyridinium α-nitronyl nitroxide·BF ₄·1/3 (acetone)

We found for the first time a structural phase transition in two-dimensional kagomé antiferromagnet m-N-methylpyridinium α-nitronyl nitroxide (m-MPYNN)·BF₄·1/3 (acetone). As, below T_s = 128.7 K, the intense satellite reflections are clearly observed at the modulation wave vector of q_s = (1/3,1/3,0), the unit cell of the low-temperature phase is enlarged to √3a × √3a × c. In the low-temperature phase, the space group is determined to be P31c. The orientation of acetone molecules orders below T_s, leading to the lack of the three-fold axis. To confirm whether the low-temperature phase maintains a kagomé-like network structure or not, we estimate the overlap integrals between the singly occupied molecular orbital wave functions of the MPYNN molecules by the extended Hückel method. The result is that all the overlap integrals are modified below T_s, suggesting that the system below T_s cannot be regarded as the uniform S = 1 kagomé antiferromagnet.