Ab Initio Derivation of Magnetic Interactions in RuP3SiO11 and Substitutional Analogs: A Separated Face-Parallel Octahedra Kitaev Spin Liquid Candidate Family

RuP3SiO11 was recently identified as a Kitaev spin liquid candidate via neutron scattering. To evaluate its proximity to the Kitaev limit, we performed ab initio calculations of magnetic interactions. Our computed Kitaev and off-diagonal exchanges agree well with experiment, but we find a ferromagnetic Heisenberg exchange instead of the experimentally fitted antiferromagnetic one. We also reveal a sizable antiferromagnetic out-of-plane exchange absent from previous analyses. Extending our study to eight substitutional analogs in the MT3QO11 family (M = Ru, Os; T = P, As; Q = Si, Ge), we predict strong variations in magnetic interactions, highlighting the tunability of this materials family. Finally, by comparing idealized models of edge-sharing, separated edge-parallel, and separated face-parallel octahedra, we show that face-parallel octahedra exhibit distinct magnetic behavior, pointing to new strategies for approaching the Kitaev limit.