O_6^4),
\label{eq.cef}$$ where $O_k^q$’s represent Stevens operator equivalence. Thereby the degenerate levels are split into one doublet $\Gamma_6$ and two quartets $\Gamma_8^{(1)}$ and $\Gamma_{8}^{(2)}$. The level scheme has been analyzed by the inelastic neutron scattering, which yields an estimate of CEF parameters as $B_4 = -3.03 \times 10^{-2}$ meV and $B_6 = 2.36 \times 10^{-4}$ meV. [@Amoretti92] The lowest levels are given by the $\Gamma_8^{(2)}$, which is separated about 55 meV from another quartet $\Gamma_{8}^{(1)}$. Diagonalizing Eq. (\[eq.cef\]), we obtain the bases of the lowest quartet as $$\begin{aligned}
\left| + \uparrow \right\rangle
& = & c_{1}  \left| + \frac{9}{2} \right\rangle   
                     + c_{2} \left| + \frac{1}{2} \right\rangle   
                     + c_{3} \left| - \frac{7}{2} \right\rangle, 
\label{eq.g81} \\
\left| + \downarrow \right\rangle
& = & c_{1}  \left| - \frac{9}{2} \right\rangle   
                     + c_{2} \left| - \frac{1}{2} \right\rangle   
                     + c_{3} \left| + \frac{7}{2} \right\rangle, 
\label{eq.g82} \\
\left| - \uparrow \right\rangle
& = & c_{4}  \left| + \frac{5}{2} \right\rangle   
                     + c_{5} \left| - \frac{3}{2} \right\rangle,  
\label{eq.g83} \\
\left| - \downarrow \right\rangle
& = & c_{4}  \left| - \frac{5}{2} \right\rangle   
                     + c_{5} \left| + \frac{3}{2} \right\rangle, 
\label{eq.g84}  \end{aligned}$$ with $c_1=0.2757, c_2=-0.4483, c_3=0.8503, c_4=-0.9751$ and $c_5=0.2216$. State $|m \rangle$ denotes the eigenstate with $J_z=m$. Symbols $\tau$ ($=\pm$) and $\sigma$ 