Destruction of the Mott insulating ground state of Ca2RuO4 by a structuraltransition

We report a first-order phase transition at T-M = 357 K in single-crystal C a2RuO4, an isomorph to the superconductor Sr2RuO4. The discontinuous decrea se in electrical resistivity signals; the near destruction of the Mott insu lating phase and is triggered by a structural transition from the low-tempe rature orthorhombic to a high-temperature tetragonal phase. The magnetic su sceptibility, which is temperature dependent but not Curie-like decreases a bruptly at T-M and becomes less temperature dependent. Unlike most insulato r to metal transitions, the system is not magnetically ordered in either ph ase, though the Mott insulator phase is antiferromagnetic below T-N = 110 K .