  The two-dimensional layered perovskite Sr2IrO4 was proposed to be a
spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified
on a narrow J_{eff} = 1/2 band due to strong spin-orbit coupling. On the other
hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains
metallic. To understand the physical origin of the metallic state and possible
transitions to insulating phases, we construct a tight-binding model for
SrIrO3. The band structure possesses a line node made of J_{eff} = 1/2 bands
below the Fermi level. As a consequence, instability towards magnetic ordering
is suppressed and the system remains metallic. This line node, originating from
the underlying crystal structure, turns into a pair of three-dimensional nodal
points on the introduction of a staggered potential or spin-orbit coupling
strength between alternating layers. Increasing this potential beyond a
critical strength induces a transition to a strong topological insulator,
followed by another transition to a normal band insulator. We propose that
materials constructed with alternating Ir- and Rh-oxide layers along the (001)
direction, such as Sr2IrRhO6, are candidates for a strong topological
insulator.
