Rational Synthesis of The Metastable Compounds BixCo4-xFexSb12

The synthesis of many targeted ternary compounds using conventional synthesis approaches has been unsuccessful because the ternary compounds are thermodynamically unstable with respect to disproportionation to a mixture of binary compounds. Typically these compounds have been synthetic targets based on predictions of enhanced properties. The ternary skutterudites with formula M'xM4Sbl2 (where M' = La, Lu, Y) are prime examples. Compounds with this structure have been found to be very promising thermoelectric materials having both good electrical properties and low thermal conductivities. Inserting heavy M' atoms into this crystal structure has been predicted to further decrease the thermal conductivity by increased phonon scattering. Attempts to make the title compound using conventional synthesis approaches failed due the formation of binary compounds as reaction products. Using modulated elemental reactants we were able to prepare the title compound at 160°C. Annealing at temperatures above 500°C resulted in exothermic decomposition into binary compounds. The Bi containing compounds are therefore thermodynamically unstable with respect to disproportionation at all temperatures. The amount of Bi can be varied by varying the composition of the starting modulated elemental reactant. Preliminary measurements to determine the variation of electrical conductivity and Seebeck coefficient as a function of bismuth content are presented.