O molecules on a pyrochlore lattice

4d ruthenium compounds are a platform for exotic electronic states such as unconventional superconductivity, excitonic magnetism or molecular orbital crystal. The ground states in these systems are governed by the interplay of correlations, spin-orbit coupling, bandwidth and crystal field. We present a newly discovered pyrochlore ruthenate In₂Ru₂O₇, where yet another factor, namely the bonding character of the constituent non-magnetic “A-site” ions - the covalency of In-O bonds - plays a pivotal role in its unique ground state. While other pyrochlore ruthenates A₂Ru₂O­₇ (A: trivalent cation) with ionic A-O bonds (A = Y, Eu…) undergo long-range magnetic order, In₂Ru₂O₇ exhibits multiple structural transitions with decreasing temperature and forms a nonmagnetic ground state with semi-isolated Ru₂O units. Dominant hopping within the Ru₂O “molecules” via the Ru-O-Ru path leads to molecular orbital formation. Such molecular orbital formation is unique as it involves the O^2- anions unlike the transition metal dimers found in systems with edge- and corner-sharing octahedra. We argue that the bond disproportionation of the covalent In-O bonds plays a key role in the structural transitions and the distinct molecular orbital formation found in In₂Ru₂O₇.