By converting waste heat into electricity, thermoelectric generators could be an important part of the solution to today's energy challenges. The compound Zn_4Sb_3 is one of the most efficient thermoelectric materials known. Its high efficiency results from an extraordinarily low thermal conductivity in conjunction with the electronic structure of a heavily doped semiconductor. Previous structural studies have been unable to explain this unusual combination of properties. Here, we show through a comprehensive structural analysis using single-crystal X-ray and powder-synchrotron-radiation diffraction methods, that both the electronic and thermal properties of Zn_4Sb_3 can be understood in terms of unique structural features that have been previously overlooked. The identification of Sb^(3-) ions and Sb_2^(4-) dimers reveals that Zn_4Sb_3 is a valence semiconductor with the ideal stoichiometry Zn_(13)Sb_(10). In addition, the structure contains significant disorder, with zinc atoms distributed over multiple positions. The discovery of glass-like interstitial sites uncovers a highly effective mechanism for reducing thermal conductivity. Thus Zn_4Sb_3 is in many ways an ideal 'phonon glass, electron crystal' thermoelectric material.
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