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

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.