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 ferroma
gnetic 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.
BaIrO3 is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure readily restores an insulating state characterized by a three-orde
r-of-magnitude increase of resistivity. Since pressure generally increases orbital overlap and broadens energy bands, a pressure-induced insulating state is not commonplace. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO3.
