RIXS interferometry and the role of disorder in the quantum magnet Ba3Ti3−xIrxO9

Motivated by several claims of spin-orbit-driven spin-liquid physics in hexagonal ${\mathrm{Ba}}_{3}{\mathrm{Ti}}_{3\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{O}}_{9}$ hosting ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ dimers, we report on resonant inelastic x-ray scattering (RIXS) at the Ir ${L}_{3}$ edge for different $x$. We demonstrate that magnetism in ${\mathrm{Ba}}_{3}{\mathrm{Ti}}_{3\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{O}}_{9}$ is governed by an unconventional realization of strong disorder, where cation disorder affects the character of the local moments. RIXS interferometry, studying the RIXS intensity over a broad range of transferred momentum $\mathbf{q}$, is ideally suited to assign different excitations to different Ir sites. We find pronounced Ir-Ti site mixing. Both ions are distributed over two crystallographically inequivalent sites, giving rise to a coexistence of quasimolecular singlet states on ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ dimers and spin-orbit-entangled $j=1/2$ moments of $5{d}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{4+}$ ions. RIXS reveals different kinds of strong magnetic couplings for different bonding geometries, highlighting the role of cation disorder for the suppression of long-range magnetic order in this family of compounds.