A possible Hund’s metal and unusual second-order metal insulator transition in perovskite La_1-xPr_xRuO₃

4d perovskite LaRuO₃ is a paramagnetic metal whereas PrRuO₃ is a paramagnetic insulator. Although Ru⁴⁺ ruthenates have shown very rich physics due to the subtle interplay of Coulomb repulsive potential U, Hund’s rule coupling J, the crystal-field splitting Δ, and the spin-orbit coupling (SOC), limited information of Ru³⁺ perovskite RRuO₃ (R=rare earth metals) family has been obtained since these perovskites have to be synthesized under high pressure. We report a successful synthesis of perovskite La_1-xPr_xRuO₃ compounds by spark plasma sintering. A thorough characterization on La_1-xPr_xRuO₃ system has been made by measurements of magnetic and transport properties and specific heat. The samples La_1-xPr_xRuO₃ (0.2 < x < 0.8) exhibit a second-order metal-insulator transition as temperature decreases without any sign of magnetic order in the insulator phase. Due to the reduction of Ru-O-Ru bond angle, substituting La³⁺ ions with smaller Pr³⁺ ions reduces the bandwidth so as to lead to an insulator phase at low temperatures. The strong SOC and Hund’s coupling on low spin 4d⁵ Ru³⁺ are responsible for the anomalous metallic phase found in LaRuO₃ and the absence of magnetic ordering in the insulator phase in La_1-xPr_xRuO_3.