Quaternary Rb2Cu2SnS4, A2Cu2Sn2S6 (A=Na, K, Rb, Cs), A2Cu2Sn2Se6 (A=K, Rb), K2Au2SnS4, and K2Au2Sn2S6. Syntheses, structures, and properties of new solid-state chalcogenides based on tetrahedral [SnS4]4- units