Structural changes induced by Ce filling in partially filled skutterudites

The structural properties of the Ce delta Fe4-xNixSb12 series with 0.06 <= delta <= 0.72 have been determined by neutron powder diffraction in the temperature range 10-300 K. Large isotropic atomic displacement parameters are observed for the cerium atom throughout the series, however with two distinct origins. The first term, purely dynamic, is well described as an Einstein oscillator and has essentially no dependence with the filling fraction 8 and the lattice parameter. A vibrational frequency of 55(2) cm(-1) fitted the experimental data in the entire phase diagram. The second contribution is essentially static and well evidenced by a large residual displacement parameter extrapolated to 0 K. This latter term does depend strongly on 3 and is maximum at around delta = 0.5, representing almost 50% of the room temperature value. A number of structural changes of the Sb sublattice are correlated with the disorder of Ce as well as disorder on the Fe/Ni sublattice.