Physical properties of skutterudites Yb_xM₄Sb₁₂, M = Fe, Co, Rh, Ir

Abstract:

A series of compounds Yb_xM₄Sb₁₂, were synthesised by reaction sintering. From Rietveld refinements isotypism was determined in all cases with the LaFe₄P₁₂-(skutterudite)-type, space group ${\mathop{\rm Im}\nolimits} \overline 3$ - No. 204. These refinements also served to derive the Yb-content in the samples. There is a systematic trend for the Yb-occupancy in the parent lattice M₄Sb₁₂, revealing a gradual decrease of the Yb-content from x =0.8 (M=Fe), x =0.5 (FeCo), x =0.2 (Co), x =0.1 (Rh) to $x \approx 0.02$ (Ir). This dependency seems to correlate with the thermal stability of the ternary compounds: a true ternary compound forms for M =Fe, whilst for M = C_o stable binary skutterudite compounds MSb₃ already exist. Measurements of various bulk properties revealed the absence of any long range magnetic order in this series of compounds. While the samples rich in Yb behave metallic like, the Rh and Ir based skutterudites show a semiconducting-like resistivity which at lower temperatures is characterised by variable range hopping in the presence of strong Coulomb interaction. Although Yb_0.1Rh₄Sb₁₂ exhibits a Seebeck coefficient up to about 150 μV/K, figures of merit ZT generally are below 0.1 near room temperature, primarily due to the large resistivities of the sintered material.