The present invention allows optimum filling of void spaces typically found in skutterudite type crystal lattice structures associated with various semiconductor materials. Selective filling provides semiconductor materials which are particularly beneficial for use in fabricating thermoelectric devices for electrical power generation and/or cooling applications. By selectively filling a portion of the void spaces associated with skutterudite type crystal lattice structure, reductions in thermal conductivity of the resulting semiconducting material may be optimized while concurrently minimizing any reduction in electrical properties of the resulting semiconductor materials, thus maximizing the thermoelectric figure of merit for the associated thermoelectric device. The present invention allows optimizing both the size and type of atoms and/or molecules used to fill void spaces in skutterudite type crystal lattice structure and to optimize the number of void spaces which are filled by such atoms and/or molecules.