Proton location and hydrogen bonding in the hydrous lead copper sulphates linarite, PbCu(SO₄)(OH)₂, and caledonite, Pb₅Cu₂(SO₄)₃CO₃(OH)₆

Neutron, time-of-flight, Laue diffraction has been carried out on natural single crystals of linarite, PbCu(SO4)(OH)2, and caledonite, Pb5Cu2(SO4)3CO3(OH)6, at 293 K. The structural refinements converged to Rw(F) = 0.085 on the basis of 869 observed reflections for linarite and Rw(F) = 0.066 from 1605 observed reflections for caledonite. The locations of all the hydrogen atoms within these structures were extracted from the refinements, enabling calculation of the hydrogen bonding. Linarite is monoclinic, space group P21/m, with a 9.682(2), b 5.646(1), c 4.683(6) Å, and β 102.66(1)°. It comprises chains of edge-sharing Jahn–Teller-elongated CuO6 square bipyramids running parallel to the [010] direction, bound together by hydrogen bonds. Between these layers are double PbO8 and SO4 layers. The two independent hydrogen atoms have been located and are bound as hydroxyl groups to the oxygen atoms at the corners of the CuO4 square plane. The H(4) atom has conventional hydrogen bonding. The donor–acceptor distance is 2.924(5) Å, and the hydrogen bond is nearly linear, with an angle of 176.1(6)°. The O(4)–H(4) hydroxyl bond distance is 0.952(7) Å, and the H(4) ... O(2) hydrogen bond has a length of 1.973(9) Å. The H(5) hydrogen atom, however, forms a stronger hydrogen bond, with a donor–acceptor distance of 2.706(5) Å and an angle of 171.8(6)°. The O(5)–H(5) hydroxyl bond is lengthened to 1.025(8) Å, and the H(5) ... O(4) hydrogen bond is a short 1.687(8) Å. Caledonite is orthorhombic, space group Pmn21 with a 20.085(3) Å, b 7.141(1) and c 6.563(1) Å; like linarite, it comprises chains of edge-sharing Jahn–Teller-elongated CuO6 square bipyramids, although these chains run parallel to the [001] direction. There are three independent Pb sites, in which Pb is coordinated to nine oxygen ligands, with a range of Pb–O distances of 2.352(4) to 3.571(6) Å. Caledonite has three independent hydrogen atoms that similar to those in the linarite structure: they are bound as hydroxyl groups to the oxygen atoms at the corners of the square plane of the axially elongate CuO6 square bipyramids. The hydrogen bonds in caledonite are all significantly bent, with angles between 160 and 170°; otherwise, the H(1) and H(2) atoms have fairly conventional hydrogen bonding. The donor–acceptor distance for O(1) ... O(4) and O(2) ... O(4) are 2.887(7) and 2.776(6) Å, respectively; although the O(1)–H(1) hydroxyl bond at 0.95(1) Å is shorter than the O(2)–H(2) hydroxyl bond at 0.98(1) Å, the H(1) ... O(4) hydrogen bond, 1.97(1) Å in length, is slightly longer than the H(2) ... O(4) hydrogen bond at 1.84(1) Å . The O(3)–H(3) hydroxyl bond is 0.946(8) Å in length, and the H(3) ... O(8) hydrogen bond is slightly shorter than expected at 1.777(9) Å. With the O(3) ... O(8) donor–acceptor distance of 2.711(5) Å and the 20° deviation from linearity, this hydrogen bond may be strained