Destruction of the Mott insulating ground state of Ca2RuO4 by a structural transition

We report a first-order phase transition at T_M=357 K in single-crystal Ca₂RuO₄, an isomorph to the superconductor Sr₂RuO₄. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low-temperature orthorhombic to a high-temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at T_M and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.