  We have synthesized and studied a new iridate, Ba13Ir6O30, with unusual Ir
oxidation states: 2/3 Ir6+(5d3) ions and 1/3 Ir5+(5d4) ions. Its crystal
structure features dimers of face-sharing IrO6 octahedra, and IrO6 monomers,
that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless,
Ba13Ir6O30 exhibits two transitions at TN1 = 4.7 K and TN2 = 1.6 K. This
magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K
below TN2, signaling a coexisting frustrated/disordered state persisting down
to at least 0.05 K. This iridate hosts unusually large Jeff=3/2 degrees of
freedom, which is enabled by strong spin-orbit interactions (SOI) in the
monomers with Ir6+ ions and a joint effect of molecular orbitals and SOI in the
dimers occupied by Ir5+ and Ir6+ ions. Features displayed by the magnetization
and heat capacity suggest that the combination of covalency, SOI and large
effective spins leads to highly frustrated ferrimagnetic ordering, possibly
into a skyrmion crystal, a novelty of this new high-spin iridate.
