Spin gap in Tl₂Ru₂O₇ and the possible formation of Haldane chains in three-dimensional crystals

The spin gap in Tl₂Ru₂O₇ and the formation of haldane chains in three-dimensional crystals were analyzed. The compound formed in a cubic pyrochlore structure with the space group for Fd3m and a=10.17936 Å. When it had undergone the metal-insulator transition, the crystal structure transformed the cubic pyrochlore structures to the orthorhombic structure. A drastic drop in bulk susceptibility at the meta-insulator transition suggested that the transition should be accompanied by a strong change in spin spectrum. The metal-insulator transition in Tl ₂Ru₂O₇ accompanied by a structural transition with the formation of a spin gap is found to be the first case of the spontaneous formation of 1D Haldane chain with the spin gap out of a 3D cubic structure.