We report x-ray and optical absorption spectroscopy as well as hard x-ray photoemission spectroscopy on hexagonal ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ in which a Cu $3d$ spin-orbital liquid state is suggested from absence of the Jahn-Teller distortion of ${\mathrm{Cu}}^{2+}{\mathrm{O}}_6$ octahedra and of the magnetic ordering. The experimental results and their cluster model analysis indicate that O $2p$ holes play a crucial role in realizing the spin-orbital liquid state of hexagonal ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$. These oxygen holes appear due to the “reaction” ${\mathrm{Sb}}^{5+}\to {\mathrm{Sb}}^{3+}+$ two O $2p$ holes, with these holes being able to attach to Cu ions. The present work opens avenues towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

• Received 23 March 2021
• Accepted 1 July 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.075002