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Certain substances allow heat to be pumped from one place to another using electricity. Such thermoelectric materials also allow electricity to be generated from heat. Yet the promise of solid-state refrigerators and air conditioners has never been fulfilled, in large part because the efficiency of these materials is comparatively low. Thus thermoelectrics are restricted to specialized applications: thermocouples, radioisotope power generators and thermoelectric coolers for image sensors, for example. The fundamental problem is that a good thermoelectric must have high electrical conductivity and low thermal conductivity, but in most solids these two physical properties go hand in hand. A promising solution is to use semiconducting clathrates. These compounds have cage-like crystal structures in which the spaces are filled with atoms that can effectively rattle around. This motion interferes with the conduction of heat but not electricity, making them ideal candidates for the next generation of thermoelectrics.