We investigated spin and orbital magnetic moments of a divalent nickelate \({\text{Na}}_{3}{\text{Ni}}_{2}{\text{BiO}}_{6}\) using the Ni L2,3-edge x-ray absorption spectroscopy (XAS) measurements with magnetic circular dichroism (MCD) as well as linear dichroism (LD). In spite of the angular momentum L = 0 ionic ground state of Ni2+ (t2g6eg2; 3A2), the sum-rule estimation on the MCD spectrum surprisingly yields the orbital to spin moment ratio of \({m}_{orb}/{m}_{spin}\approx 0.2\) , including the existence of a considerable unquenched orbital magnetic moment \({m}_{orb}\) at the Ni site. Using the many-body full multiplet cluster model analyses on both the MCD and LD spectra, we successfully determined \({m}_{spin}=1.8\pm 0.1 {\mu }_{\text{B}}\) , and \({m}_{orb}=0.35\pm 0.03 {\mu }_{\text{B}}\) , which agree well with the saturated moment \({m}_{sat}=2.14 \pm 0.03 {\upmu }_{\text{B}}\) determined from the magnetization measurement. We found that the considerable unquenched \({m}_{orb}\) results from spin–orbit coupling (SOC) mixing the \({\text{L}}_{\text{eff}}=1\) excited states of 3T2 (t2g5eg3) and 1T1 (t2g5eg3) and further increases by ~ 10% with the trigonal distortion.
