Single-crystal neutron diffraction and Raman spectroscopic study of hydroxylherderite, CaBePO4(OH,F)

The crystal structure, H bonding and chemical composition of hydroxylherderite from the Bennett pegmatite, Buckfield, Oxford County, Maine, USA were investigated by single-crystal X-ray diffraction and neutron Laue diffraction, electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectrometry and polarized Raman spectroscopy [Ca(Na0.01Ca1.01) ∑1.02 Be(Be0.98Li0.01) ∑0.99 P(Si0.03P0.98) ∑1.01O4(OH0.67F0.33) ∑1, Z = 4, a = 9.7856(5), b = 7.6607(5), c = 4.8025(3) Å, β = 90.02(3)°, V= 360.02(4) Å 3, space group P2 1/a]. The neutron-structure refinement converged with fully anisotropic displacement parameters to give a final agreement index R 1 = 0.0363 for 85 refined parameters and 1614 unique reflections with Fo >4σ(Fo). The structure refinement was used to determine the H position and geometry of H bonding in the structure. One H site was found on the O5 anion with an O-H interatomic distance, corrected for "riding motion", of 0.996(2) Å. The H bond of hydroxylherderite is bifurcated with O2 and O4 acceptors forming H bonds with O5⋯O2 = 3.163(1) Å, H⋯O2 = 2.544(2) Å and O5-H⋯O2 = 121.8(1)°; O5⋯O4 = 3.081(1) Å, H⋯O4 = 2.428(2) Å and O5-H⋯O4 = 124.4(1)°. The highly non-linear O-H⋯O hydrogen bonds in hydroxylherderite and in the isotypic datolite [ideally CaBSiO4(OH)] are constrained by the tetrahedral network topology. Two main O-H stretching modes were observed in the Raman spectra at 3565 and 3620 cm-1, which are attributed to the bifurcated H bond. Two additional weak bands at 3575 and 3610 cm-1 are attributed to Si-P disorder in the tetrahedral sites. Results of this study will contribute to the correlation of H-bonding geometry and O-H stretching frequencies of highly non-linear H bonds.