Complex vibrations in arsenide skutterudites and oxyskutterudites

Article

关键词: CORRELATED ELECTRON PHENOMENA;    ACU(3)RU(4)O(12);    VALENCE;    TRANSITION;    SCATTERING;    CA;    SR;    LA;   

DOI  :  10.1103/PhysRevB.91.014109

来源: SCIE

【 摘 要 】

The local structure of two skutterudite families-CeM4As12 (M = Fe, Ru, Os) and LnCu(3)Ru(4)O(12) (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 LnCu(3)Ru(4)O(12). Although some of the bonds within the cage are quite stiff (correlated Debye temperatures, theta(cD), are similar to 500 K for CeM4As12 and above 800 K for LnCu(3)Ru(4)O(12)), we show that the structure is not completely rigid. For the rattler atom the nearest-neighbor pairs have a relatively low Einstein temperature, theta(E): similar to 100-120 K for Ce-As and similar to 130 K for Ln-O. Surprisingly, the behaviors of the second-neighbor pairs are quite different: for CeM4As12 the second-neighbor pairs (Ce-M) have a weaker bond while for LnCu(3)Ru(4)O(12) the Ln-Ru second-neighbor pair has a stiffer effective spring constant than the first-neighbor pair. In addition, we show that the As-4 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.

【 授权许可】

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