Frustration, strain and phase co-existence in the mixed valent hexagonal iridate Ba$_{3}$NaIr$_{2}$O$_{9}$

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.