H-bonding in lazulite: a single-crystal neutron diffraction study at 298 and 3 K

Abstract

The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, Dawson mining district, Yukon, Canada) was investigated by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction at 298 and 3 K. Its empirical formula, based on EMPA data, is: (Mg_0.871Fe_0.127)_Σ0.998Al_2.030(P_1.985Ti_0.008Si_0.007O₄)₂(OH)₂. The neutron diffraction experiments at room and low T proved that the H-free structural model of lazulite previously reported, on the basis of X-ray structure refinement, is correct: the building unit of the lazulite structure consists of a group of three face-sharing (Al-octahedron) + (Mg,Fe-octahedron) + (Al-octahedron), connected to the adjacent one via a corner-shared OH-group and two corner-shared oxygen sites of the P-tetrahedron, to form a dense 3D-edifice. Only one crystallographically independent H site occurs in the structure of lazulite, forming a hydroxyl group with the O5 oxygen, with O5–H = 0.9997 Å at room temperature (corrected for riding motion effect). The H-bonding scheme in the structure of lazulite is now well defined: a bifurcated bonding scheme occurs with the O4 and O2 oxygen sites as acceptors. The two H-bonds are energetically different, as shown by their bonding geometry: the H-bond with the O2 site as acceptor is energetically more favorable, being O5–H···O2 = 152.67(9)°, O5···O2 = 3.014(1) Å and H···O2 = 2.114(1) Å, whereas that with O4 as acceptor is energetically more costly, being O5–H···O4 = 135.73(8)°, O5···O4 = 3.156(1) Å and H···O4 = 2.383(1) Å, at room temperature. No T-induced phase transition occurs within the T-range investigated. At low temperature, the O5–H···O2 bond is virtually identical to the room-T one, whereas the effects of T on O5–H···O4 are more pronounced, with significant differences of the O_donor···O_acceptor and H···O_acceptor distances. The experimental findings of this study do not support the occurrence of HPO₄ or H₂PO₄ units into the structure of lazulite, recently reported on the basis of infrared and Raman spectra.