  The orthorhombic perovskite SrIrO3 is a semimetal, an intriguing exception in
iridates where the strong spin-orbit interaction coupled with electron
correlations tends to impose a novel insulating state. We report results of our
investigation of bulk single-crystal Sr0.94Ir0.78O2.68 or Ir-deficient,
orthorhombic perovskite SrIrO3. It retains the same crystal structure as
stoichiometric SrIrO3 but exhibits a sharp, simultaneous antiferromagnetic
(AFM) and metal-insulator (MI) transition at 185 K. Above it, the basal-plane
resistivity features an extended regime of almost linear-temperature dependence
up to 800 K but the strong electronic anisotropy renders an insulating behavior
in the out-of-plane resistivity. The Hall resistivity undergoes an abrupt sign
change and grows below 40 K, which along with the Sommerfeld constant of 20
mJ/mole K2 suggests a multiband effect. All results including our
first-principles calculations underscore a delicacy of the metallic state in
SrIrO3 that is in close proximity to an AFM insulating state. The contrasting
ground states in isostructural Sr0.94Ir0.78O2.68 and SrIrO3 illustrate a
critical role of even slight lattice distortions in rebalancing the ground
state in the iridates. Finally, the observed simultaneous AFM and MI
transitions reveal a direct correlation between the magnetic transition and
formation of a charge gap in the iridate, which is conspicuously absent in
Sr2IrO4.
