Destruction of the Mott insulating ground state of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ by a structural transition

We report a first-order phase transition at $T_M=357\mathrm{}\mathrm{K}$ in single-crystal ${\mathrm{Ca}}_2{\mathrm{RuO}}_4,$ an isomorph to the superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$ 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 $T_N=110\mathrm{}\mathrm{K}.$