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Ca2Ru1-xCrxO4 (0 < x < 0.13): Negative volume thermal expansion via orbital and magnetic orders

Ca2RuO4 undergoes a metal-insulator transition at TMI = 357 K, followed by a well-separated transition to antiferromagnetic order at TN = 110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at TMI and induces ferromagnetic behavior at TC. The lattice volume V of Ca2Ru1-xCrxO4 (0 < x < 0.13) abruptly expands with cooling at both TMI and TC, giving rise to a total volume expansion ΔV/V {\simeq} 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 TC and TMI represents an Invar effect. The two phase transitions suggest an exotic ground state driven by an extraordinary coupling between spin, orbit and lattice degrees of freedom.