Negative volume thermal expansion via orbital and magnetic orders in Ca₂Ru_1-xCr_xO₄(0<x<0.13)

Ca₂RuO₄ undergoes a metal-insulator transition at T_MI=357K, followed by a well-separated transition to antiferromagnetic order at T_N=110K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at T_MI and induces ferromagnetic behavior at T_C. The lattice volume V of Ca ₂Ru_1-xCr_xO₄ (0<x<0.13) abruptly expands with cooling at both T_MI and T_C, giving rise to a total volume expansion ΔV/V≈1%, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between T_C and T_MI represents an Invar effect. The two phase transitions, which surprisingly mimic the classic freezing transition of water, suggest an exotic ground state driven by an extraordinary coupling between spin, orbit, and lattice degrees of freedom.