Nearly linear orbital molecules on a pyrochlore lattice.

The interplay of spin-orbit coupling with other relevant parameters gives rise to the rich phase competition in complex ruthenates featuring octahedrally coordinated Ru ⁴⁺ . While locally, spin-orbit coupling stabilizes a nonmagnetic J _eff = 0 state, intersite interactions resolve one of two distinct phases at low temperatures: an excitonic magnet stabilized by the magnetic exchange of upper-lying J _eff = 1 states or Ru ₂ molecular orbital dimers driven by direct orbital overlap. Pyrochlore ruthenates A ₂ Ru ₂ O ₇ ( A = rare earth, Y) are candidate excitonic magnets with geometrical frustration. We synthesized In ₂ Ru ₂ O ₇ with covalent In─O bonds. This pyrochlore ruthenate hosts a local J _eff = 0 state at high temperatures; however, at low temperatures, it forms a unique nonmagnetic ground state with nearly linear Ru─O─Ru molecules, in stark contrast to other A ₂ Ru ₂ O ₇ compounds. The disproportionation of covalent In─O bonds drives Ru ₂ O molecule formation, quenching not only the local spin-orbit singlet but also geometrical frustration.