High-temperature order–disorder transitions in the skutterudites CoGe1.5Q1.5 (Q=S, Te)

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 R  3¯ ) 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 Im  3¯). 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.

Powder neutron diffraction reveals that the skutterudite CoGe1.5Te1.5 undergoes a phase transition at 610 °C, involving the disordering of the anions within the Ge2Te2 rings.Figure optionsDownload as PowerPoint slideHighlights
► CoGe1.5S1.5 retains an ordered skutterudite structure up to 950 °C.
► CoGe1.5Te1.5 undergoes an order–disorder phase transition at 610 °C.
► Below 610 °C, anions are arranged trans to each other within Ge2Te2 rings.
► Above 610 °C, anions are statistically distributed within the Ge2Te2 rings.
► The effect of the phase transition on the thermal conductivity is discussed.