Reactivity of Copper in Molten Polytelluride Salts. K4Cu8Te11, A3Cu8Te10 (A: Rb, Cs), AA′2Cu8Te10 (A,A′: K, Rb, Cs), and A2BaCu8Te10 ( A: K, Rb, Cs): Novel Solids Based on Endohedrally Occupied (Cu8Te12) Dodecahedral Cage‐Clusters.