•Ralf Vollmer¹, Gernot Goll¹, Christian Pfleiderer¹, M. Brian Maple^1,2, Hilbert v. Löhneysen^1,3 und Paul C. Canfield^4
1Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsruhe, Germany
²Institute for Pure and Applied Physical Sciences, University of California San Diego, La Jolla, CA 92093, USA
³Institut für Festkörperphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe
⁴Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50010, USA

PrPb₃ exhibits antiferroquadrupolar order as seen by magnetization measurements [1] and neutron scattering [2]. The previously observed increase of the antiferroquadrupolar ordering temperature T_AFQ = 0.39 K in zero magnetic field up to T_AFQ = 0.66 K in 6 T is confirmed by specific heat measurements. In addition to this second-order transition, we find a first order transition - as evidenced by strong hysteresis effects - at T₁ = 0.35 K for 3 T and 0.49 K for 4.5 T. Surprisingly, both phase transitions are independent of hydrostatic pressure p up to p = 6.5 kbar. Measurements of the magnetocaloric effect are under way to complete the anomalous phase diagram of this cubic compound.