Rattling of Pr in heavy-fermion superconductor PrOs4Sb12 revealed by neutron powder diffraction

Neutron powder diffraction experiments were carried out on the heavy-fermion superconductor PrOs4Sb12 down to 7.7 K in order to reveal the thermal displacement of Pr and investigate the possible existence of off-center potential minima for Pr in the Sb cage. Rietveld analysis with the structure in which Pr is located at the center of the Sb cage well reproduces the observed patterns. A large thermal displacement was obtained for Pr of U-Pr = 0.0344 angstrom(2) at 295.1 K, roughly corresponding to a mean thermal displacement of 0.18 angstrom. This value is 5 times larger than those of other constituents, U-Os and U-Sb. U-Pr remains large, U-Pr = 0.0062 angstrom(2), even at 7.7 K, which is twice as large as U-Os and U-Sb. Neither structural transition nor significant change of the Sb cage accompanied this change in U-Pr. The large U-Pr and its strong temperature dependence indicate the rattling of Pr under a shallow potential in the Sb cage. An equal distribution of Pr at 12e sites (x00) was introduced to approximate the off-center potential minima. The R factors were not improved by the introduction of the off-center site.