  Magnetic materials are composed of the simple building blocks of magnetic
moments on a crystal lattice that interact via short-range magnetic exchange
interactions. Yet from these simple building blocks emerges a remarkable
diversity of magnetic states. Some of these, such as ferromagnetism, are
familiar in our everyday lives, while others reveal the deep quantum mechanical
origins of magnetism. A prime example of the latter are quantum spin liquid
(QSL) states in which -- unlike in a ferromagnet where magnetic moments are
driven by their exchange interactions to adopt long-range order -- magnetic
moments remain disordered at low temperatures but are simultaneously correlated
over long length scales through quantum entanglement. A particularly promising
theoretical model of a QSL is the Kitaev model, composed of unusual
bond-dependent exchange interactions between magnetic moments on a honeycomb
lattice. However, the Kitaev QSL is extremely challenging to realise
experimentally as it is unstable to competing exchange interactions and crystal
lattice perturbations that inevitably arise in real materials. This makes it
essential to understand the relationship between the structure and interactions
that may give rise to Kitaev interactions in new candidate materials. Here we
show that the material requirements for the Kitaev QSL survive for an extended
pseudo-edge-sharing superexchange pathway of Ru3+ 4d5 octahedra within the
honeycomb layers of the inorganic framework solid, RuP3SiO11. Through materials
synthesis and structural characterisation, resonant inelastic X-ray and neutron
scattering experiments, we confirm the requisite jeff = 1/2 state of Ru3+ in
RuP3SiO11 and resolve the hierarchy of exchange interactions that provide
experimental access to an otherwise unexplored region of the extended Kitaev
phase diagram.
