Abstract
The temperature dependence of anion ordering in the skutterudites CoGe1.5Q1.5 (Q=S, Te) has been investigated by powder neutron diffraction. Both materials adopt a rhombohedral structure at room temperature (space group R3¯ ) in which the anions are ordered trans to each other within Ge2Q2 rings. In CoGe1.5S1.5, anion ordering is preserved up to the melting point of 950°C. However, rhombohedral CoGe1.5Te1.5 undergoes a phase transition at 610°C involving a change to cubic symmetry (space group Im3¯). In the high-temperature modification, there is a statistical distribution of anions over the available sites within the Ge2Te2 rings. The structural transition involves a reduction in the degree of distortion of the Ge2Te2 rings which progressively transform from a rhombus to a rectangular shape. The effect of this transition on the thermoelectric properties has been investigated.
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