Observation of Phase Transition from Metal to Spin-Singlet Insulator in MgTi2O4 with S=1/2 Pyrochlore Lattice

We have synthesized well-defined powder samples of MgTi 2 O 4 , that are expected to realize the pyrochlore lattice with S =1/2, and measured the magnetic susceptibility, lattice parameters and resistivity as a function of temperature. We found a phase transition from the metal to the spin-singlet insulator at around 260 K, which was accompanied by a structural transformation from cubic to tetragonal. The transition is a kind of Peierls transition in the three-dimensional lattice, which suggests a close relationship to the internal low dimensionality embedded in the spinel structure. The ground state may be spin-singlet dimers theoretically predicted to be the ground state of the S =1/2 Heisenberg antiferromagnet on the pyrochlore lattice.