Quaternary rubidium copper tin sulfides (Rb2Cu2SnS4, A2Cu2Sn2S6 (A = Na, K, Rb, Cs), A2Cu2Sn2Se6 (A = K, Rb), potassium gold tin sulfides, K2Au2SnS4, and K2Au2Sn2S6. Syntheses, structures, and properties of new solid-state chalcogenides based on tetrahedral SnS44- units