Non-Kramers doublet ground state in a quaternary cubic compound PrRu₂In₂Zn₁₈ investigated by ultrasonic measurements

We performed ultrasonic measurements on a quaternary cubic compound PrRu₂In₂Zn₁₈ to explore the ground state properties derived from non-Kramers Γ₃ doublet of Pr³⁺. PrRu₂In₂Zn₁₈ is a quaternary derivative of the ternary compound PrRu₂Zn₂₀ that exhibits a structural phase transition at T_S = 138 K. In PrRu₂In₂Zn₁₈, the Zn atoms at the 16c site in PrRu₂Zn₂₀ are selectively replaced by In atoms. A monotonic increase was observed in the temperature dependence of elastic constants C_L = (C₁₁ + 2C₁₂ + 4C₄₄)/3 and C_T = (C₁₁–C₁₂ + C₄₄)/3 in the temperature range around T_S to which an elastic softening was observed in (C₁₁ – C₁₂)/2 for PrRu₂Zn₂₀. The disappearance of the softening indicates that the structural transition in PrRu₂Zn₂₀ is suppressed by the substitution of Zn ions by In ones with a larger ionic radius. Alternatively, the C_T of PrRu₂In₂Zn₁₈ exhibits a precursor Curie-type elastic softening toward low temperatures being responsible for the non-Kramers Γ₃ ground state. We discuss the ground state and the evolution of the elastic properties of the different single-crystal samples of PrRu₂In₂Zn₁₈ grown under different conditions.