Unusual local disorder in NdOs4Sb12and PrOs4Sb12skutterudites

Abstract

We present a temperature-dependent, extended x-ray absorption fine structure (EXAFS) analysis of the filled skutterudite compounds NdOs${}_{4}$Sb${}_{12}$, PrOs${}_{4}$Sb${}_{12}$, and EuOs${}_{4}$Sb${}_{12}$. Although the interpretation of recent ultrasonic measurements suggested off-center displacements for Nd and Pr in NdOs${}_{4}$Sb${}_{12}$ and PrOs${}_{4}$Sb${}_{12}$, the EXAFS analysis shows that the Nd-Sb, Pr-Sb, and Sb-Sb peaks are well ordered. Surprisingly, however, the second-neighbor Nd-Os and Pr-Os peaks are highly disordered, even at low temperatures, and the Os-Os peak also has some disorder in the Nd and Pr systems. In contrast to the anomalous results for the Pr and Nd samples, neither the Eu-Sb, Eu-Os, nor the Os-Os peak is disordered at low temperature for EuOs${}_{4}$Sb${}_{12}$. For all three systems within estimated errors, the rare-earth atom is on-center inside the Sb cage. We propose that for the Nd and Pr compounds, the Os cage distorts, with some of the Os atoms moving either towards or away from the Nd or Pr atoms, such that the Nd-Sb, Pr-Sb, and Sb-Sb pair distances have very little disorder. Some possible distortion models are suggested and the possible ramifications for thermoelectric and transport properties are discussed.