Expanding crystal chemistry of anhydrous copper oxysulfates: synthesis and crystal structure of K _1.5 (Pb,Na,□) _0.5 [Cu ₅ O ₂ ](SO ₄ ) ₄

Abstract A new anhydrous sulfate K 1.5 (Pb,Na,□) 0.5 [Cu 5 O 2 ](SO 4 ) 4 ( 1 ) was synthesized in an evacuated silica ampoule. 1 is triclinic, P 1 ̅ $̅{1}$ , a  = 8.7357(2), b  = 9.4633(3), c  = 10.0243(3) Å, α  = 107.080(2), β  = 97.326(2), γ  = 105.831(2)°, V  = 742.42(4) Å 3 and R 1  = 0.027. The crystal structure represents a new structure type based on strongly corrugated [Cu 5 O 2 ] 6+ chains. This feature can be attributed to the flexibility of the [Cu 5 O 2 ] 6+ chain in accommodating ‘lone-pair’ Pb 2+ cations at one of the A sites. The crystal chemistry of known fumarolic minerals and synthetic compounds structurally based on [ M 5 O 2 ] n + chains is discussed.