The filled skutterudite compound PrOs₄Sb₁₂ exhibits superconductivity below a critical temperature T_c=1.85K that develops out of a nonmagnetic heavy Fermi liquid with an effective mass m^* \approx 50 m_e, where m_e is the free electron mass. Analysis of magnetic susceptibility, specific heat, electrical resistivity and inelastic neutron scattering measurements within the context of a cubic crystalline electric field yields a Pr³⁺ energy level scheme that consists of a  \Gamma₃ nonmagnetic doublet ground state that carries an electric quadrupole moment, a low lying  \Gamma₅ triplet excited state at \sim 10 K, and \Gamma ₄ triplet and  \Gamma₁ singlet excited states at much higher temperatures. The superconducting state appears to be unconventional and to consist of two distinct superconducting phases. An ordered phase of magnetic or quadrupolar origin occurs at high fields and low temperatures, suggesting that the superconductivity may occur in the vicinity of a magnetic or electric quadrupolar quantum critical point.

PACS numbers: 71.20.Eh, 71.27.+a, 74.70.Tx, 75.30.Mb