Novel Phase Transition in Spin-1/2 Linear Chain Systems: NaTiSi2O6 and LiTiSi2O6

We synthesized a well-characterized powder sample of NaTiSi 2 O 6 , which had been missing as a spin-1/2 compound in one-dimensional (1D) magnets of the pyroxene family, and measured the magnetic susceptibility, lattice parameters and specific heat as functions of temperature. We observed a typical behavior of magnetic susceptibility in a spin-1/2 1D magnet, followed by a spin-Peierls-like transition at 210 K. The most significant feature of the transition is that it occurs at a temperature higher than that at the maximum point of the Bonner–Fisher curve. This means that the short-range magnetic correlations within the chain are not fully developed and intrinsic magnetoelastic instability of a 1D system cannot be considered as a driving force for the transition. We propose an important role of the orbital degree of freedom, that is, the orbital order, based on the structural feature that the 1D chain is made up of skew edge-sharing Ti 3+ O 6 octahedra. A similar transition was observed at 230 K in LiTiSi ...