  Using detailed synchrotron diffraction, magnetization, thermodynamic and
transport measurements, we investigate the relationship between the mixed
valence of Ir, lattice strain and the resultant structural and magnetic ground
states in the geometrically frustrated triple perovskite iridate
Ba$_{3}$NaIr$_{2}$O$_{9}$. We observe a complex interplay between lattice
strain and structural phase co-existence, which is in sharp contrast to what is
typically observed in this family of compounds. The low temperature magnetic
ground state is characterized by the absence of long range order, and points
towards the condensation of a cluster glass state from an extended regime of
short range magnetic correlations.
