Ground state in the novel dimer iridate Ba 13 Ir 6 O 30 with Ir 6+ (5$d^3$) ions
We have synthesized and studied an iridate, Ba 13 Ir 6 O 30 , with unusual Ir oxidation states: 2/3 Ir 6+ (5$d^3$) ions and 1/3 Ir 5+ (5$d^4$) ions. Its crystal structure features dimers of face-sharing IrO 6 octahedra, and IrO 6 monomers that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, Ba 13 Ir 6 O 30 exhibits two transitions at $T_{\text{N}1}$ = 4.7K and $T_{\text{N}2}$ = 1.6 K. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K 2 below $T_{\text{N}2}$, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large $J_{\text{eff}}$ = 3/2 degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with Ir 6+ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by Ir 5+ and Ir 6+ 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, an interesting feature of this high-spin iridate.