Crystalline-electric field of Pr ( Os_{1 - x} Ru_x )₄ Sb₁₂

High-quality single crystals of Pr ( Os_{1 - x} Ru_x )₄ Sb₁₂ were grown by the Sb self-flux method. Magnetic susceptibility χ ( T ) was measured precisely. The characteristic temperature T_max, where χ ( T ) exhibits a maximum, varies as a function of Ru-content x but its functional form changes at around x = 0.6 where competition of two types of superconductivity is suggested. Since T_max is thought to be relevant to the splitting energy between a ground state and a first excited state, we analyzed χ ( T ) considering the crystalline electric field (CEF) of the T_h symmetry. χ ( T ) is well fitted assuming a Γ₁ singlet ground state for all x values. As expected from T_max, the energy of the first excited state ( Γ₄^{( 2 )}) changes its functional form at around x = 0.6. Furthermore, the energies of the second and third excited states ( Γ₄^{( 1 )} and Γ₂₃) exhibit a minimum at almost the same value of x = 0.6. These results suggest that the CEF is related to the superconductivity in Pr ( Os_{1 - x} Ru_x )₄ Sb₁₂.