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

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: (Mg0.871Fe0.127)Σ0.998Al2.030(P1.985Ti0.008Si0.007O4)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) + (Aloctahedron), 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 Odonor···Oacceptor and H···Oacceptor 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.