The crystal chemistry of duftite, PbCuAsO ₄ (OH) and the beta -duftite problem

Duftite, PbCu(AsO ₄ )(OH) is orthorhombic, space group P2 ₁ 2 ₁ 2 ₁ with a = 7.768(1), b = 9.211(1), c = 5.999(1) Aa, Z = 4; the structure has been refined to R = 4.6% and Rw = 6.5% using 640 observed reflections [F>2sigma (F)]. The structure consists of chains of edge-sharing CuO ₆ “octahedra”, parallel to c; which are linked via AsO ₄ tetrahedra and Pb atoms in distorted square antiprismatic co-ordination to form a three dimensional network. The CuO ₆ “octahedra” show Jahn-Teller distortion with the elongation running approximately along {627}. The hydrogen bonding network in the structure was characterized using bond valence calculations. “beta -duftite” is an intermediate in the duftite-conichalcite series, which has a modulated structure based on the intergrowth of the two structures in domains of approximately 50 Aa. The origin of the modulation is thought to be associated with displacements in the oxygen lattice and is related to the orientation of the Jahn-Teller distortion of CuO ₆ “octahedra”. Approximately half of the strips show an elongation parallel to {627} while the other strips are elongated parallel to [010]. This ordering results in an increase in the b cell repeat compared to duftite and conichalcite.