  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 {\Delta}V/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.
