Complex vibrations in arsenide skutterudites and oxyskutterudites

Abstract

The local structure of two skutterudite families -CeM4As12 (M = Fe, Ru, Os) and LnCu3Ru4O12 (Ln = La, Pr, and Nd) -have been studied using the Extended X-Ray Absorption Fine Structure (EXAFS) technique with a focus on the lattice vibrations about the rare earth "rattler atoms", and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair-distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the M and As atoms in CeM4As12 and by Cu, O, and Ru atoms in LnCu3Ru4O12. Although some of the bonds within the cage are quite stiff (Correlated Debye temperatures, θcD, are ∼ 500 K for CeM4As12 and above 800 K for LnCu3Ru4O12) we show the structure is not completely rigid. For the rattler atom the nearest neighbor pairs have a relatively low Einstein temperature, θE; ∼ 100-120 K for Ce-As and ∼ 130 K for Ln-O. Surprisingly, the behavior of the second neighbor pairs are quite different; for CeM4As12 the second neighbor pairs (Ce-M have a weaker bond while for LnCu3Ru4O12 the Ln-Ru second neighbor pair has a stiffer effective spring constant than the first neighbor pair. In addition, we show that the As4 or CuO4 rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as "rattlers" and help suppress thermal conductivity. In general neither the rigid cage approximation nor the simple reduced mass approximation are sufficient for describing rattler behavior.

Modeling correlated motion in filled skutterudites

“…However, the second neighbor Ln-Ru bonds are substantially stiffer than the first neighbor Ln-O bonds which is opposite the results for the filled skutterudites. To estimate the effective spring constant providing the restoring forces for motion in a particular direction, we use the quantity K xy cos 2 (θ) where θ is the angle between the bond direction and the direc- 12 for the As skutterudite CeRu4As12 and oxyskutterudite NdCu3Ru4O12 using the inverse slope of the σ 2 plot. The spring constants for the cage atoms are significantly stiffer than for the springs connected to the rattler atoms.…”

“…Recent extended x-ray absorption fine structure (EX-AFS) studies of CeMAs 12 and LnCu 3 Ru 4 O 12 propose that the As 4 and CuO 4 squares are nearly rigid units and that their suspension within the skutterudite structure is anisotropic 12 . The squares are suspended in the unit cell via strong Ru-O or M-As bonds which are quite stiff, but are nearly perpendicular to the square; thus there are strong restoring forces only for motion perpendicular to the squares.…”

“…For this model we calculate the phonon dispersion curves; plots of these dispersion curves illustrate where avoided crossings occur and how they move with changes in parameters. We also calculate σ 2 (T) for each pair, including the contributions for each mode, and compare the results with recent EXAFS results 12 . The effective spring constant for each pair (Sec.…”

Hard x-ray spectroscopy of the itinerant magnets RFe4Sb12 (R=Na, K, Mounssef1, Cantarino2, Bittar3 et al. 2019Phys. Rev. B6090View full textAdd to dashboardCite

“…In Fig. 5(a)-(f ) we present the results of our EXAFS investigation at T = 10 K. The application of EXAFS to the skutterudites contributed valuable information concerning bond disorder and the vibrational dynamics in this class of materials [35,39,40]. In Fig.…”

“…Fe-Sb for distinct fillers. However, the RFe 4 Sb 12 skutterudites with light fillers present an unusual vibrational dynamics, as investigated in detail [37,41], which will affect the correlated Debye-Waller factors [35], rendering the comparison across the whole series inadequate.…”